Alphabetic list of terms

Numeric

2DS

A variant of the standard G.703 framing required by most E1 DPNSS providers in the U.K. See also G.703.

2-wire continuity test

In a Signaling System 7 (SS7) network, a test in which a TAOS unit sends a 2010Hz tone (for a T1 line) or a 2000Hz tone (for an E1 line) and expects to receive a 1780Hz tone in order to verify that the physical link between the central office (CO) switch and the unit is available. The TAOS unit supports both incoming and outgoing 2-wire continuity checks for T1 and E1 lines. You can select the type of check to perform on a per-line basis. Both the native 2-wire continuity check (GR-246-CORE Section B.2) and 4-wire-to-2-wire emulation (GR-246-CORE Section B.3) are supported. Compare with 4-wire continuity test. See also 4-wire-to-2-wire continuity test, continuity test, SS7 network.

3DES-CBC

Triple data encryption standard-cypher block chaining. 3DES-CBC is a variant of DES-CBC. 3DES-CBC encrypts each block of data three times, using a different key each time. See also 40DES-CBC, DES-CBC.

3.1kHz audio-bearer service

A service that sends a data call over a voice trunk. Because echo cancellation corrupts data transmitted on voice trunks, each switch should turn off echo cancellation on the trunks handling 3.1kHz audio-bearer service. The 3.1kHz audio-bearer service is sometimes referred to as data over subscriber bearer service (DOSBS).

4-wire continuity test

In a Signaling System 7 (SS7) network, a test in which a TAOS unit verifies that the physical link between the central office (CO) switch and the unit is available. For a T1 line, the TAOS unit sends a 2010Hz tone and expects to receive a 2010Hz tone in return. For an E1 line, the TAOS unit sends a 2000Hz tone and expects to receive a 2000Hz tone in return.

The 4-wire continuity test requires one end of a line to place a channel into loopback state while the other end sends the tone. The check concludes successfully if the tone sent on the outgoing path is received on the return path within acceptable quality and timing limits. The 4-wire check procedure cannot detect potential inadvertent loops in the line path or in line facilities, and cannot be used when the other exchange is analog. For these reasons, the procedure known as a 2-wire continuity test is recommended by the International Telecommunications Union Telecommunication Standardization Sector (ITU-T).

Compare with 2-wire continuity test, 4-wire-to-2-wire continuity test. See also continuity test, SS7 network.

4-wire-to-2-wire continuity test

In a Signaling System 7 (SS7) network, a test in which a TAOS unit sends a 1780Hz tone and expects to receive a 2010Hz tone (for a T1 line) or a 2000Hz tone (for an E1 line) in order to verify that the physical link between the central office (CO) switch and the unit is available. Compare with 4-wire continuity test. See also 2-wire continuity test, continuity test, SS7 network.

7-bit mode

See ASCII mode.

8-bit binary mode

See binary mode.

10Base2

A 50-ohm coaxial RG-58u cable with a data rate of 10Mbps and a maximum length of 180 meters. 10Base2 does not include a built-in transceiver. It is also known as thin Ethernet or thinnet.

10Base5

A 50-ohm coaxial RG-6 cable with a data rate of 10Mbps and a maximum length of 500 meters. It is also known as thick Ethernet or thicknet.

10BaseT

The 802.3 IEEE standard for operating a 10Mbps Ethernet network with twisted-pair cabling and a wiring hub. 10BaseT is also known as UTP Ethernet and twisted-pair Ethernet. See also 10BaseT hub.

10BaseT hub

A hub providing a common termination point for hosts connected to 10BaseT wiring. See also 10BaseT.

40DES-CBC

40-bit data encryption standard-cypher block chaining. 40DES-CBC is the same algorithm as DES-CBC, but it uses a 40-bit key. See also 3DES-CBC, DES-CBC.

40-bit data encryption standard-cypher block chaining

See 40DES-CBC.

100BaseFX

A dual-fiber cable standard designed for 100Mbps Fiber Distributed Data Interface (FDDI). Compare with 100BaseT, 100BaseT4, 100BaseTX.

100BaseT

The 802.3 IEEE standard for operating a 100Mbps Ethernet network. 100BaseT differs from the 10BaseT standard by requiring higher-grade cable or more wiring pairs, with cable lengths that are only a tenth as long as 10BaseT cable lengths. Compare with 100BaseFX, 100BaseT4, 100BaseTX. See also 10BaseT.

100BaseT4

A 4-pair, category-3, half-duplex cable with a data rate of 33.3Mbps per pair, and a maximum length of 100 meters. Compare with 100BaseFX, 100BaseT, 100BaseTX.

100BaseTX

A 2-pair, category-5, half-duplex cable with a data rate of 33.3Mbps per pair, and a maximum length of 100 meters. The 100BaseTX standard was designed for 100Mbps Fiber Distributed Data Interface (FDDI) and Copper Distributed Data Interface (CDDI). Compare with 100BaseFX, 100BaseT, 100BaseT4. See also CDDI, FDDI.

802.2

An IEEE protocol specification for the media access control (MAC) header of an Internetwork Packet Exchange (IPX) frame in NetWare 3.12 or later. An 802.2 frame contains the Logical Link Control (LLC) protocol header in addition to the MAC header. Compare with 802.3, Ethernet II, SNAP. See also IPX frame, LLC protocol, MAC.

802.3

An IEEE protocol specification for the media access control (MAC) header of an Internetwork Packet Exchange (IPX) frame in NetWare 3.11 or earlier. An 802.3 frame does not contain the Logical Link Control (LLC) protocol header in addition to the MAC header. The 802.3 frame is also called Raw 802.3. Compare with 802.2, Ethernet II, SNAP. See also IPX frame, LLC protocol, MAC.

802.5

An IEEE protocol specification for the physical layer and media access control (MAC) sublayer of a token-ring topology. 802.5 implements token passing over shielded twisted pair (STP) cabling and offers data rates of 4 or 16Mbps. See also STP cable.

802.6

An IEEE media access control (MAC) standard used on local area networks (LANs). 802.6 is also known as the Distributed Queue Dual Bus (DQDB) protocol.

A

AAL

Asynchronous Transfer Mode (ATM) adaptation layer. The AAL is a protocol that translates higher-layer data from its native size and format to the size and format of an ATM cell, enabling engineers to adapt the ATM layer to particular services. The AAL consists of two sublayers: the convergence sublayer (CS) and the segmentation and reassembly (SAR) sublayer. See also ATM, ATM layer, convergence sublayer, SAR.

ABR

(1) Area border router. An ABR is an Open Shortest Path First (OSPF) router that belongs to both a regular area and the backbone area. See also area, backbone area, OSPF protocol.

(2) Available bit rate. ABR is an Asynchronous Transfer Mode (ATM) service class that handles bursty LAN traffic and data that is tolerant of delays and cell loss. ABR is a best-effort, managed service. Compare with CBR, UBR, VBR-nonreal time, VBR-real time. See also ATM.

absolute congestion

In a frame relay network, a congested condition that occurs when the queue length reaches the threshold of 64 buffers, and no room remains in the queue. When the average queue length (AQL) exceeds the threshold for absolute congestion, all incoming frames are discarded, and the forward explicit congestion notification (FECN) and backward explicit congestion notification (BECN) bits are set. Compare with mild congestion, severe congestion. See also BECN, congestion, FECN.

acceptance policy

A set of rules that determine the path and route information the TAOS unit accepts from a Border Gateway Protocol (BGP) peer for further processing. Compare with advertisement policy, injection policy. See also BGP, policy.

Accept Packet Pass-Through Call message

See ACCP message.

Accept Pass-Through Call message

See ACST message.

Access Accept packet

A packet sent by the RADIUS server to inform the TAOS unit that a client's request for access has been granted. See also RADIUS server.

Access Challenge packet

A request for the user to enter a password in a hand-held token card. The token-card server sends the Access Challenge packet through the RADIUS server and the TAOS unit to the user. See also RADIUS server, token card, token-card server.

access concentrator

A device that efficiently forwards data, handling incoming calls for a network point of presence (POP). In general, an access concentrator supports dial-in modem calls, ISDN connections, dedicated links, frame relay traffic, and multiprotocol routing. See also dedicated circuit, dial-in modem access, frame relay concentrator, ISDN, POP.

access link

See A-link.

Access Password Ack packet

A response from the RADIUS server, informing the TAOS unit that it has accepted a new password. See also RADIUS server.

Access Password Reject packet

A response from the RADIUS server, informing the TAOS unit that it has rejected a new password. See also RADIUS server.

Access Password Request packet

A password-change request that a TAOS unit sends to the RADIUS server. See also RADIUS server.

access rate

The data rate of the user access channel.

Access Reject packet

A packet the RADIUS server sends to inform the TAOS unit that it has not granted a client's request for access. The RADIUS server sends an Access Reject packet if the user enters an unknown username, fails to enter the correct password, or enters an expired password. See also RADIUS server.

Access Request packet

A packet that a TAOS unit sends to the RADIUS server on behalf of a client attempting to establish a connection. See also RADIUS server.

access router

A device that supports basic routing protocols and enables remote users to gain access to a corporate backbone network.

Access SS7 Gateway Control Protocol

See ASGCP.

access tandem switch

A tandem switch that provides equal-access connections for carriers by linking local end-office switches. An access tandem switch aggregates voice and data calls from several local switches, then connects to a tandem switch in another area's network to move calls from one region to another. Typically, the tandem switch receives toll traffic and uses its trunks to process and route the traffic to and from another service provider's end-office switch.

accounting

A way to log information in RADIUS about Start session, Stop session, and Failure-to-start session events. When a TAOS unit recognizes one of these events, it sends an accounting request to RADIUS. When the accounting server receives the request, it combines the information into a record and timestamps it. Each type of accounting record contains attributes associated with an event type, and can show the number of packets the TAOS unit transmitted and received, the protocol in use, the username and IP address of the client, and other session information. See also accounting server, Failure-to-start session, Start session, Stop session.

accounting checkpoint

A RADIUS feature that provides periodic session information to enable accurate session billing even if the RADIUS accounting server does not receive a Stop packet. Typically, when RADIUS accounting is enabled and a Point-to-Point Protocol (PPP) connection terminates, the TAOS unit sends a Stop packet to the RADIUS accounting server, which stores the packets for use in billing. If the checkpoint feature is also enabled and the RADIUS accounting server fails to receive a Stop packet for any reason, it can still close off the session billing on the basis of the last Checkpoint packet it received. See also accounting, Checkpoint packet, Checkpoint record.

Accounting Request packet

A request for accounting information. A TAOS unit sends an Accounting Request packet to the RADIUS accounting server. See also accounting server, RADIUS.

Accounting Response packet

A packet containing accounting information. A RADIUS accounting server sends an Accounting Response packet to the TAOS unit. See also accounting server, RADIUS.

accounting server

The RADIUS daemon with accounting enabled. See also accounting, RADIUS daemon.

ACCP message

Accept Packet Pass-Through Call message. A call-confirmation message sent by a TAOS unit to a Signaling System 7 (SS7) signaling gateway. An ACCP message verifies the call-setup information that the TAOS unit used for routing the call to its destination. Compare with RCCP message. See also signaling gateway, SS7.

ACD

Automatic call distributor. An ACD is a telephone service that handles incoming calls on the basis of the number called. Many companies offering sales and service support use ACD to validate callers, make outgoing responses or calls, forward calls to the correct party, enable callers to record messages, gather usage statistics, and balance the use of telephone lines.

ACE authentication

A form of token-card authentication in which RADIUS forwards a connection request to a Security Dynamics ACE/Server. The ACE/Server sends an Access Challenge packet back through the RADIUS server and the TAOS unit to the user dialing in. The user sees the challenge message, obtains the current token from his or her card, and enters the token.

The token travels back through the TAOS unit and the RADIUS server to the ACE/Server. The ACE/Server sends a response to the RADIUS server, specifying whether the user has entered the proper username and token. If the user enters an incorrect token, the ACE/Server returns another challenge, and the user can again attempt to enter the correct token. The server sends up to three challenges. After three incorrect tries, the TAOS unit terminates the call.

See also ACE token, authentication, RADIUS server, token, token card, token-card authentication, token-card server.

ACE token

A randomly generated access code that a user obtains from a SecurID token card. The code changes every 60 seconds. See also ACE authentication, token card.

ACF message

Admission Confirmation message. Upon successful call authentication, an H.323 registration, admission, and status (RAS) message sent by the MultiVoice® Access Manager (MVAM) device to a MultiVoice® gateway in response to an Admission Request (ARQ) message. An ACF message contains the network routing information that an ingress MultiVoice® gateway uses to connect the call. See also H.323, MultiVoice®, MVAM, RAS.

ACK

Acknowledgment. An ACK is a packet that the system uses to acknowledge a successful transmission. When a device receives a packet, it sends back either an ACK packet or a negative acknowledgment (NAK) packet to the sending device. If all the data arrived without corruption, the receiving device sends an ACK. If some of the data is missing or corrupted, the receiving device sends a NAK, which acts as a request that the sender retransmit the data.

Acknowledge Request Test Echo message

See ARTE message.

Acknowledge Send Tones message

See ASTN message.

acknowledgment

See ACK.

ACM

Address Complete message. An ACM is sent from a signaling gateway to the SS7 network to acknowledge that the gateway has received the information required to route the call. See also signaling gateway, SS7 network.

ACR

Allowed cell rate. In an Asynchronous Transfer Mode (ATM) configuration, an available bit rate (ABR) service value specifying the maximum rate (in cells per second) at which a device is allowed to send data. See also ABR, ATM.

ACR message

Release Channel Completed message. An IP Device Control (IPDC) protocol call-control message sent between the remote access server (RAS) and the signaling gateway, reporting that the call channel is closed. At a minimum, this message reports the disconnect-cause code that identifies how the call was terminated and the source port type, source module number, source line number, and source channel number assigned to that call. Compare with RCR message. See also IPDC, RAS, signaling gateway.

ACST message

Accept Pass-Through Call Setup message. An ACST message is a notification that the TAOS unit sends to the Softswitch to notify the device that two channels have been connected. Compare with RCST message. See also Softswitch.

active hub

A multiport device that amplifies local area network (LAN) transmission signals on a network, enabling them to be transmitted over a much greater distance than is possible with a passive hub. Compare with passive hub, smart hub. See also hub.

active open

A client-initiated operation that enables a device to establish a TCP link with a server at a fixed IP address.

A-D conversion

See analog-to-digital conversion.

add-on number

One or more numbers that a TAOS unit uses to build a multichannel MP, MP+, AIM, or BONDING call.

A multichannel call begins as a single-channel connection to one telephone number. The calling unit then requests additional numbers to connect additional channels, and stores the add-on numbers it receives from the answering unit. To add channels to the call, the calling unit integrates the add-on numbers with the telephone number it dialed initially. Typically, the numbers assigned to the channels share a group of leading digits. The add-on numbers are the rightmost digits identifying each telephone number, excluding the digit(s) that the telephone numbers have in common.

Address Complete message

See ACM.

address resolution

A method of mapping a logical address (such as an IP address) to a hardware address (such as a MAC address). See also ARP, hardware address, IP address, logical address, MAC address.

Address Resolution Protocol

See ARP.

ADI

Analog Devices, Inc. ADI is a company that provides the firmware version for the MultiDSP module See also MultiDSP.

adjacency

A relationship formed between neighboring Open Shortest Path First (OSPF) routers for the purpose of exchanging routing information. An OSPF router dynamically detects its neighboring routers by sending Hello packets to the multicast address AllSPFRouters. It then attempts to form adjacencies (Figure 1).

Figure 1. Adjacency between neighboring routers

Neighbors exchange databases and build a consistent, synchronized database between them. When an OSPF router detects a change on one of its interfaces, it modifies its link-state database and multicasts the change to its adjacent neighbor, which in turn propagates the change to its adjacent neighbor, until all routers within an area have synchronized link-state databases. This method of updating routing information results in quick convergence among routers.

See also area, convergence, link-state database, OSPF protocol, router.

admin cost

An Asynchronous Transfer Mode (ATM) routing metric that measures the administrative cost associated with the logical port. See also ATM, CDV, end-to-end delay, logical port.

administrative weight

In a Private Network-to-Network Interface (PNNI) configuration, a value used to specify preferential use of a link or node for a specific service category. Administrative weight is one of the elements of topology-state information exchanged among the nodes. Other such elements include a dynamic assessment of available bandwidth, assigned metrics, and other possible attribute values. All the elements affect how the most efficient link is chosen at a given time. See also PNNI protocol.

Admission Confirmation message

See ACF message.

Admission Reject message

See ARJ message.

Admission Request message

ARQ message.

ADSL

Asymmetric digital subscriber line. ADSL is a standard that enables devices attached to twisted-pair copper wiring to transmit data at rates from 1.5Mbps to 9Mbps downstream, and 16Kbps to 640Kbps upstream. ADSL devices can transmit data at distances of up to 18,000 feet. ADSL configurations can use more bandwidth in one direction than the other. Compare with HDSL, IDSL, RADSL, SDSL, SHDSL, VDSL. See also DSL.

ADSL transceiver unit

See ATU.

ADSL transceiver unit--central office

See ATU-C.

ADSL transceiver unit--remote

See ATU-R.

Advanced Mobile Phone Service

See AMPS.

advertisement policy

A set of rules that determines the path and route information the TAOS unit advertises to a Border Gateway Protocol (BGP) peer. Compare with acceptance policy, injection policy. See also BGP, policy.

AESA format

ATM end system address format. AESA format can be the format used for an ATM end-point address assigned to a TAOS unit's ATM interface. AESA addresses are required for IP over ATM.

An AESA address is a 20-byte, 40-digit hexadecimal number. The first 13 bytes are the address prefix, or network portion of the address. The last 7 bytes are the host portion of the address. Depending on the AESA format chosen, the content of each byte of the address varies, as shown in Figure 2. All of the supported AESA formats divide the address into the initial domain part (IDP) and the domain-specific part (DSP).

AESA addresses use one of the following formats:

Figure 2 shows how each format divides the 20-byte address into subfields. The shaded portion represents the address prefix, which is always the first 13 bytes.

Figure 2. Subfields in the AESA address formats

See also ATM, custom AESA format, DCC AESA format, E.164 AESA format, ICD AESA format.

AFI

Authority and format identifier. The AFI is part of the network-level address header of an Asynchronous Transfer Mode (ATM) cell. It is a subfield of the initial domain part (IDP) of an ATM end system address (AESA). The AFI identifies the type of AESA address in use: custom AESA, data country code (DCC) AESA, E.164 AESA, or international code designator (ICD) AESA. Compare with IDI. See also AESA format, ATM, custom AESA format, DCC AESA format, E.164 AESA format, ICD AESA format, IDP.

agent

A network device that provides Simple Network Management Protocol (SNMP) information to a manager application running on another computer. The agent and manager share a database of information, called the management information base (MIB). The manager polls the agent for information at regular intervals. When an unusual system event occurs, the agent can use a message called a traps-PDU to send unsolicited information to the manager. See also manager, MIB, SNMP, traps-PDU.

aggregation

The process of combining multiple prefixes from one or several routes so that a single prefix and route can be advertised. Route aggregation reduces the amount of information that a device running Border Gateway Protocol (BGP) must store and exchange with its BGP peers. See also BGP.

AH

Authentication Header. AH is an Internet Protocol Security (IPSec) protocol that uses a shared secret to run portions of a data packet through digest algorithms to create a digital fingerprint. The receiving system performs the same process and compares the fingerprints. Matching fingerprints verify that the packet was sent by the right source and was not altered in transit. AH works with the message-digest algorithm 5 (MD5), secure hash algorithm 1 (SHA1), message-digest algorithm 5--hash-based message authentication code (MD5-HMAC), and SHA1-HMAC authentication algorithms. Compare with ESP. See also IPSec protocol, MD5, MD5-HMAC, SHA1, SHA1-HMAC.

AIM

Ascend Inverse Multiplexing. AIM manages the connection of two remotely located inverse multiplexers. See also inverse multiplexer, inverse multiplexing.

AIM port

A port that supports Ascend Inverse Multiplexing (AIM) and BONDING (Bandwidth on Demand Interoperability Group) functionality. See also AIM, BONDING.

AIS

Alarm Indication signal. An AIS is an all-ones signal that a device sends when it detects an error condition or receives an error notification.

alarm

A signal that indicates that the system has detected a security violation or error. See also Blue Alarm signal, RAI signal, Red Alarm signal.

Alarm Indication signal

See AIS.

alarm relay

A mechanism whose contacts remain open on the back panel's terminal block during normal operation. If you enable them, the alarm relay contacts close during loss of power, hardware failure, or a system reset. You can also specify whether the contacts close when the bit error rate exceeds a certain value, or when all T1 PRI lines go out of service.

a-law

An International Telecommunication Union Telecommunication Standardization Sector (ITU-T) standard for sampling data by means of pulse code modulation (PCM). A-law is most commonly used outside of North America and Japan. Compare with mu-law. See also PCM.

A-link

Access link. In a Signaling System 7 (SS7) configuration, an A-link is a signaling link that does not carry any data traffic. See also SS7.

Allowed cell rate

See ACR.

Alternate mark inversion

See AMI.

alternate path

An Open Shortest Path First (OSPF) path on which the system reroutes a permanent virtual circuit (PVC) if a trunk fails.

ALTN message

Answer to LTN message. An ALTN message is sent by a TAOS unit to a Softswitch. It is a response to a Listen for Tones (LTN) message, and provides data about the status of the listening process, including information about whether the LTN request timed out and whether the maximum number of tones was received. See also LTN message, Softswitch.

ALU

Average line utilization. ALU is the average amount of bandwidth used on a line over a user-specified period of time. A TAOS unit uses ALU when determining whether to add or subtract bandwidth from a multichannel call. See also DBA protocol.

Always On/Dynamic ISDN

See AO/DI.

amber frame

A type of packet frame that enables you to identify packets passing through the frame relay network. The network forwards amber frames with the discard eligibility (DE) bit set, enabling the network to discard the packet if it passes through a congested node. Congested nodes that must discard packets use color designations to determine which frames to discard. Red frames are discarded first, followed by amber frames, and then green frames. Compare with green frame, red frame. See also congestion, DE, frame relay.

AMCP message

Accept Modify Packet Pass-Through Call message. On a MultiVoice® network, an AMCP message is sent by a TAOS unit to a Signaling System 7 (SS7) signaling gateway and specifies that the unit modifies one or more of the following values for a Voice over IP (VoIP) call:

Compare with RMCP message. See also MultiVoice®, SS7.

American National Standards Institute

See ANSI.

AMI

Alternate mark inversion. Used on T1 lines, AMI is a signaling method in which the 1 bits have alternating priority. See also T1 line.

AMPS

Advanced Mobile Phone Service. AMPS is a standard system for analog cellular telephone service. Introduced by American Telephone and Telegraph (AT&T) in 1983, AMPS is the most widely used cellular system in the United States. The service uses frequency ranges between 800MHz and 900MHz. Each provider can use half of the 824MHz to 849MHz range for receiving signals, and half the 869MHz to 894MHz range for transmission.

analog data

Data that can change continuously and have any value in a range. Examples of analog data are the time of day represented by clock hands and the temperature represented by a liquid thermometer. Compare with digital data. See also analog signal.

Analog Devices, Inc.

See ADI.

analog line

A line that transmits data by means of an analog signal. See also analog signal.

analog loopback

A test that checks whether the modem or data terminal equipment (DTE) is causing errors in data transmission. During an analog loopback, the system sends data between the local modem and the local DTE. Errors in transmission indicate a problem with the modem, DTE, or the interface between them. Compare with digital loopback. See also local loopback, loopback, remote loopback.

analog signal

A type of signal that encodes data transmitted over wire or through the air, commonly represented as an oscillating wave. An analog signal can transmit analog or digital data. It takes any value in a range, and changes smoothly between values. A radio station uses analog signals to send analog music data, while a modem uses analog signals to transmit digital data. Compare with digital signal. See also analog data.

analog-to-digital conversion

A process in which an analog signal is modified into a digital signal. Analog-to-digital conversion takes place, for example, when an analog modem call reaches a digital modem. Compare with digital-to-analog conversion. See also analog signal, digital modem, digital signal, modem.

ANI

Automatic number identification. ANI is a mechanism that informs the called party of the calling party's telephone number. See also caller identification, CLID.

ANM

Answer message. An ANM is sent between a signaling gateway and an SS7 network. For an incoming call, the signaling gateway sends an ANM to inform the SS7 network that the path is set up to divert the Internet call. For an outgoing call, the SS7 network send an ANM to the signaling gateway, indicating that the central office (CO) switch is ready to receive the call at the identified circuit. CO, signaling gateway, SS7 network.

Annex A

See frame relay Annex A.

Annex D

See frame relay Annex D.

ANSI

American National Standards Institute. ANSI creates standards for networking and communications. It is the U.S. representative to the International Organization for Standardization (ISO). See also ISO.

ANSI T1.617 Annex D

See frame relay Annex D.

Answer message

See ANM.

answer number

The telephone number used for routing incoming calls. See also call routing.

Answer to LTN message

See ALTN message.

AO/DI

Always On/Dynamic ISDN. AO/DI is a networking service that enables you to send and receive data through a dedicated X.25 connection over an ISDN D channel, ISDN B channel, or dedicated 56K line.

In a traditional ISDN environment, data moves across B channels, and signaling information moves across the D channel. Because signaling information uses a small percentage of available D-channel bandwidth, AO/DI was developed to maximize bandwidth usage while reducing the necessity that all data travel over the B channels.

A TAOS unit uses switched ISDN B channels only when required, on the basis of increased bandwidth use. Through its use of AO/DI, X.25, and Bandwidth Allocation Control Protocol (BACP), a TAOS unit avoids dial-up charges and the use of switched B channels whenever it sends or receives data over the X.25 connection. Among the functions that can use AO/DI are the following:

For TAOS units, AO/DI enables you to use X.25 bandwidth of up to 9600bps. If a data transfer requires more bandwidth, the TAOS unit adds dial-up B channels by means of BACP. See also BACP, B channel, D channel, X.25.

APP

Ascend Password Protocol. APP is a User Datagram Protocol (UDP) that enables a user to respond to password challenges received from an authentication server.

applet

A small software module that runs on a Java Virtual Machine (JVM) inside a Web browser. See also JVM.

Application layer

The highest layer of the OSI Reference Model. The Application layer provides applications with access to the network. File transfer, email, and network management software are examples of Application-layer programs. Protocols such as File Transfer Protocol (FTP), Rlogin, Simple Network Management Protocol (SNMP), and Telnet provide Application-layer services. See also FTP, OSI Reference Model, Rlogin, SNMP, Telnet.

APS

Automatic protection switching. On a Stinger® unit, a feature that provides line redundancy for a trunk aggregation module. With APS, a working channel has a dedicated standby channel. If the working channel fails, the standby channel is activated.See also Stinger®, trunk aggregation module.

APX 8000

A carrier-class remote access server (RAS), a unit that combines a fault-tolerant design with high port density.

AQL

Average queue length. AQL is a time-average algorithm that the frame relay switch executes each time it queues a frame for transmission. The AQL value is compared against a precalculated threshold. When the AQL is less than or equal to the threshold, maximum throughput and minimum delay occur. See also absolute congestion, congestion, congestion management, mild congestion, severe congestion.

ARCnet

Attached Resource Computer Network. ARCnet is a baseband network architecture with a transmission rate of up to 2.5Mbps. Because it is relatively inexpensive and easy to set up, ARCnet is typically used for smaller networks.

area

A portion of an Open Shortest Path First (OSPF) autonomous system. An area acts as its own network. All area-specific routing information stays within the area, all routers within an area have a synchronized link-state database, and each database within an area is unique. On a TAOS unit, an area number uses dotted decimal notation, but it is not an IP address. To tie the areas together, some routers belong to a backbone area and one other type of area. These routers are called area border routers (ABRs). In Figure 3, all of the routers are ABRs.

Figure 3. Dividing an autonomous system into areas

See also ABR, autonomous system, backbone area, link-state database, normal area, NSSA, OSPF protocol, router, stub area.

area border router

See ABR.

area ID

See area number.

area number

A portion of a Switched Multimegabit Data Service (SMDS) address, or a number denoting an Open Shortest Path First (OSPF) area. In an SMDS address, the area number can be four bytes long, and is sometimes referred to as an area ID. An OSPF area number is expressed in dotted decimal notation, but it is not an IP address. See also OSPF protocol, SMDS.

ARJ message

Admission Reject message. An H.323 registration, admission, and status (RAS) message sent by the MultiVoice® Access Manager (MVAM) device to a MultiVoice® gateway in response to an Admission Request (ARQ) message if the call was not authenticated. Compare with ACF message, ARQ message. See also H.323, MultiVoice®, MVAM, RAS.

ARP

Address Resolution Protocol. ARP is a protocol in the TCP/IP protocol suite. By mapping an IP address to a physical hardware address, ARP enables a unit to identify hosts on an Ethernet LAN. See also Ethernet, Proxy ARP, TCP/IP.

ARQ message

Admission Request message. An H.323 registration, admission, and status (RAS) message sent from a MultiVoice® gateway to a MultiVoice® Access Manager (MVAM) device, requesting authorization for a calling end point. When the multiple logical gateway feature is enabled on a MultiVoice® gateway, an incoming call request causes the gateway to send an ARQ message that includes:

Compare with ACF message, ARJ message. See also H.323, MultiVoice®, MVAM, RAS.

ARTE message

Acknowledge Request Test Echo message. An IP Device Control (IPDC) protocol heartbeat message sent by a signaling gateway in response to a Request Test Echo (RTE) message from a TAOS unit. See also IPDC, RTE message.

AS

See autonomous system.

ASBR

Autonomous system border router. An ASBR is an Open Shortest Path First (OSPF) router that handles communication between autonomous systems by using an Exterior Gateway Protocol (EGP), as shown in Figure 4.

Figure 4. Autonomous system border routers (ASBRs)

ASBRs perform calculations related to external routes. A TAOS unit imports external routes by means of Routing Information Protocol (RIP) when it establishes a WAN link with a caller that does not support OSPF, and the ASBR calculations are always performed.

Compare with ABR. See also autonomous system, EGP, external route, OSPF protocol.

Ascend Access Event Request packet

A packet containing either a notification that the TAOS unit has started up or a request for the RADIUS server to record the number of open sessions. See also RADIUS server.

Ascend Access Event Response packet

A response from the RADIUS server, either reporting that the TAOS unit has started up or specifying the number of open sessions and informing the TAOS unit that the server has received and recorded the unit's ID. See also RADIUS server.

Ascend Access New Pin packet

A response from the RADIUS server, informing the TAOS unit that it should request access again, but with the next personal identification number (PIN) in the sequence. See also RADIUS server.

Ascend Access Next Code packet

A response from the RADIUS server, informing the TAOS unit that it should request access again, but with the next password in the sequence. See also RADIUS server.

Ascend callback

A callback method in which a TAOS unit uses a username and password to detect callback during the authentication phase (after going off hook). The originating caller is charged for the initial call. Compare with CBCP callback, CLID callback, DNIS callback. See also callback.

Ascend Inverse Multiplexing

See AIM.

Ascend Password Expired packet

A response from RADIUS server to the TAOS unit, indicating that the password the user entered matches the one in the user profile, but has expired. (That is, the Access Request packet sent a valid but expired password.) See also RADIUS server.

Ascend Terminate Session packet

A response from the RADIUS server, informing the TAOS unit that it should terminate the session and display the message sent in the packet. See also RADIUS server.

Ascend Tunnel Management Protocol

See ATMP.

ASCII

American Standard Code for Information Interchange. ASCII is a character-encoding system used on local area networks (LANs). The 128 standard ASCII characters are composed of seven bits and have the values 0--127. The extended ASCII character set contains another 128 values.

ASCII mode

A Telnet mode for terminal-server users. In ASCII mode, bit 8 is set to 0 (zero). ASCII mode is also called standard 7-bit mode or network virtual terminal (NVT) ASCII. This mode is the default if no other mode is specified. Compare with binary mode, transparent mode. See also Telnet, Telnet mode.

ASCII text file

A file that contains only letters, numbers, and punctuation symbols. An ASCII text file cannot include hidden text-formatting codes. See also ASCII.

ASE

Autonomous system external. A TAOS unit uses the term ASE to denote external routes it imports into its Open Shortest Path First (OSPF) database. The TAOS unit redistributes these routes by means of OSPF ASE advertisements, and propagates its OSPF routes to remote WAN routers running Routing Information Protocol (RIP). See also external route, OSPF protocol, RIP, router.

ASE type 5

Autonomous system external type 5. ASE type 5 is an external route originated by an area border router (ABR) as a link state advertisement (LSA). An Open Shortest Path First (OSPF) normal area allows type 5 LSAs to be transmitted throughout it. A not-so-stubby area (NSSA) and a stub area do not receive or originate type 5 LSAs. However, for NSSAs, all routes imported to OSPF have the P-bit set (P stands for propagate). When the P-bit is enabled, ABRs translate type 7 LSAs to type 5 LSAs, which can then be transmitted to the backbone. These external routes are considered type 7 LSAs. Compare with ASE type 7. See also ABR, ASE, autonomous system, external route, LSA, normal area, NSSA, OSPF protocol, stub area.

ASE type 7

Autonomous system external type 7. ASE type 7 is a type of link state advertisement (LSA) defined for not-so-stubby areas (NSSAs) in Open Shortest Path First (OSPF) version 2. For NSSAs, all routes imported to OSPF have the P-bit set (P stands for propagate). When the P-bit is enabled, ABRs translate type 7 LSAs to type 5 LSAs, which can then be transmitted to the backbone. These external routes are considered type 7 LSAs. Compare with ASE type 5. See also ASE, autonomous system, LSA, NSSA, OSPF protocol, stub area.

ASGCP

Access SS7 Gateway Control Protocol. ASGCP enables you to integrate a TAOS unit into a Signaling System 7 (SS7) network. With an ASGCP-Q.931+ license, a TAOS unit can decrease congestion on the Public Switched Telephone Network (PSTN) caused by users connecting to the Internet. Compare with IPDC. See also PSTN, SS7, SS7 network.

ASN.1

Abstract Syntax Notation One. In the OSI Reference Model, ASN.1 is a notation for describing data structures on a network. It provides a consistent syntax when transferring data between different systems. See also OSI Reference Model.

AS path list

See autonomous system path list.

ASTN message

Acknowledge Send Tones message. A message sent by a TAOS unit to a Signaling System 7 (SS7) signaling gateway to confirm that the call-progress tones or voice announcements requested by a Send Tones (STN) message have been played. Compare with STN message. See also signaling gateway, SS7, VoIP.

asymmetric digital subscriber line

See ADSL.

asynchronous communications server

A LAN server that enables a network user to dial out of the network and into the Public Switched Telephone Network (PSTN), or to access dedicated lines for asynchronous communications. An asynchronous communications server is also called a dial-in/dial-out server or a modem server. See also asynchronous transmission.

asynchronous PPP

A mode for sending Point-to-Point Protocol (PPP) packets. In asynchronous mode, the characters that form the data packets are sent at irregular intervals, without a clocking signal. Figure 5 illustrates a single-channel asynchronous PPP call in which the calling device is a modem.

Figure 5. Asynchronous PPP connection

Asynchronous PPP is commonly used in lower-speed transmission and less-expensive transmission systems. See also asynchronous transmission, PPP.

Asynchronous Transfer Mode

See ATM.

Asynchronous Transfer Mode (ATM) adaptation layer

See AAL.

asynchronous transmission

A mode in which the sending and receiving serial hosts know where a character begins and ends because each byte is framed with additional bits, called a start bit and a stop bit. A start bit indicates the beginning of a new character. It is always 0 (zero). A stop bit marks the end of the character. It appears after the parity bit if parity bits are in use.

An asynchronous link uses the type of serial communication provided by a PC COM port. A dial-in modem or V.120 terminal adapter (TA) initiates an asynchronous host-to-network or host-to-host connection. The call can use Point-to-Point Protocol (PPP) encapsulation, V.120 encapsulation, or raw (unencapsulated) Transport Control Protocol (TCP).

A TAOS unit routes an asynchronous call to a digital modem as a voice call, and then to the terminal-server software. If the terminal server does not detect a PPP packet, it begins a login sequence. If the terminal server detects a PPP packet, it passes the call on to the router, where it is handled as a regular PPP connection. The caller never sees the terminal-server interface.

See also asynchronous PPP, digital modem, PPP, TCP, terminal server, V.120, V.120 TA.

async PPP

See asynchronous PPP.

AT command set

A set of standard instructions used to activate functions on a modem. Originally developed by Hayes Microcomputer Products, the AT command set is now used by almost all modem manufacturers. See also modem.

ATM

Asynchronous Transfer Mode. ATM is a packet-switched, broadband network architecture central to broadband ISDN (B-ISDN). It ensures reliable delivery of packets and provides very high bandwidth, enabling data, voice, and video transmissions to occupy the same line.

ATM is based on connections, not channels. The term asynchronous refers to the way in which ATM achieves its unchannelized bandwidth allocation. ATM sends data associated with a connection only when there is actual data to send. This functionality is in contrast to that found in channelized or time-division multiplexing (TDM) networks, in which a special bit pattern must be sent in every time slot representing a channel, even when the connection is idle.

In the past, companies built large voice, data, and television networks to accommodate each specific kind of data transmission. Duplication of effort and tremendous cost outlays resulted. Because many of these networks were built for peak load conditions, the average usage was typically very low, leading to excessive costs. Many organizations needed to find ways to use a single network infrastructure and assign bandwidth on an as-needed basis.

ATM lets both private corporations and public service providers build unchannelized networks to make more efficient use of the underlying bandwidth on the network. By offering scalable rates from 1.5Mbps to 155Mbps or higher, ATM services can make the WAN transparent for applications. And unlike frame relay or other data services, ATM can easily accommodate delay-sensitive traffic such as voice and video.

ATM uses very short, fixed-length packets called cells. The ATM cell is 53 bytes long, consisting of a 5-byte header containing an address, and a fixed 48-byte information field. To handle the various data types on a network, ATM supports five service classes: constant bit rate (CBR), variable bit rate (VBR)-real time, variable bit rate (VBR)-nonreal time, available bit rate (ABR), and unspecified bit rate (UBR).

ATM is also known as cell relay. See also ABR, ATM, B-ISDN, broadband, CBR, cell, packet-switched network, packet switching, UBR, VBR-nonreal time, VBR-real time.

ATM cell

A 53-byte, fixed-length Asynchronous Transfer Mode (ATM) data packet that contains the following fields:

See also ATM, CLP, GFC, HEC, payload, PT field, VCI, VPI.

ATM circuit

A point-to-point permanent virtual circuit (PVC) established on the TAOS unit in an Asynchronous Transfer Mode (ATM) configuration. An ATM circuit can make use of any two physical interfaces in the system.

ATM contract

See QoS contract.

ATM direct

A feature that enables a TAOS unit to concentrate incoming Point-to-Point Protocol (PPP) calls onto an Asynchronous Transfer Mode (ATM) interface. ATM-direct aggregates multiple PPP connections and forwards them as a combined data stream. An upstream device then examines the packets and routes them appropriately. In Figure 6, the TAOS unit forwards the data stream from two PPP dial-in hosts across the same ATM link.

Figure 6. ATM-direct concentrating PPP calls to an ATM interface

An ATM-direct connection is not a full-duplex tunnel between a PPP dial-in user and a remote device. Although the TAOS unit does not route the packets onto the ATM link, it must use the router to send packets received across ATM back to the appropriate PPP caller. For this reason, ATM-direct connections must enable IP routing.

See also ATM, IP routing, PPP.

ATM direct trunk

A logical-port configuration that enables you to make a direct trunk connection between two Asynchronous Transfer Mode (ATM) switches. See also ATM.

ATM end system address format

See AESA format.

ATM Flow-Control processor

An Asynchronous Transfer Mode (ATM) network-management system that uses binary, hop-by-hop, closed-loop flow-control algorithms that shift network congestion to the edge of the network. See also ATM.

ATM framer

A device that multiplexes Asynchronous Transfer Mode (ATM) cells into the SONET payload and extracts cells from the SONET payload for reassembly into packets. See also SONET.

ATM IISP-DCE

ATM Interim Interswitch Signaling Protocol for data circuit-terminating equipment. A logical-port configuration that enables you to connect two Asynchronous Transfer Modem (ATM) switches by means of IISP DCE ports. This service routes switched virtual circuits (SVCs) through a mixed-vendor switch network. Compare with ATM IISP-DTE. See also ATM, logical port, SVC.

ATM IISP-DTE

ATM Interim Interswitch Signaling Protocol for data terminal equipment. A logical-port configuration that enables you to connect two Asynchronous Transfer Mode (ATM) switches by means of IISP DTE ports. This service routes switched virtual circuits (SVCs) through a mixed-vendor switch network. Compare with ATM IISP-DCE. See also ATM, logical port, SVC.

ATM layer

The core layer of the Asynchronous Transfer Mode (ATM) standard. The ATM layer routes the cells across the network, performing both multiplexing and demultiplexing functions. See also AAL, ATM.

ATMP

Ascend Tunnel Management Protocol. ATMP provides a tunneling mechanism between two TAOS units across the Internet or a frame relay network. The protocol uses standard Generic Routing Encapsulation (GRE) protocol and is based on the User Datagram Protocol (UDP) and Internet Protocol (IP). ATMP provides a virtual private network (VPN) solution over the backbone resources of Internet service providers (ISPs) and carriers. Without ATMP, each mobile client and remote user has to dial directly into the network, resulting in long-distance charges. With ATMP, users can make a local call and have the transmission securely tunneled.

Figure 7 shows an ATMP tunnel between two TAOS units. A mobile client, such as a traveling salesperson, initiates the connection. The unit that authenticates the mobile client is the ATMP Foreign Agent. The unit that gains access to the home network is the ATMP Home Agent. The home network is the destination network for mobile clients. In Figure 7, the mobile client is a salesperson who logs in to an ISP (the Foreign Agent) to access her home network.

Figure 7. ATMP tunnel across the Internet

As described in RFC 1701, GRE hides packet contents and enables transmission of packets that the Internet would otherwise not accept. When you use ATMP with a TAOS unit, you can transmit either IP packets that use unregistered addresses or Internetwork Packet Exchange (IPX) packets from roaming clients. See also Foreign Agent, frame relay, GRE protocol, Home Agent, home network, IP, IPX protocol, ISP, mobile client, UDP, VPN.

ATM service class

A method of designating the type of Asynchronous Transfer Mode (ATM) service in use. ATM supports five service classes to handle the various data types on a network. Each service class ensures optimal network usage and guaranteed end-to-end delivery. The five ATM service classes are constant bit rate (CBR), variable bit rate (VBR)-real time, variable bit rate (VBR)-nonreal time, available bit rate (ABR), and unspecified bit rate (UBR). See also ABR, ATM, CBR, UBR, VBR-nonreal time, VBR-real time.

ATM service interworking feeder

A feature that enables the system to feed frame relay network traffic into an Asynchronous Transfer Mode (ATM) network so that a frame relay end user can communicate with an ATM end user. See also ATM, frame relay network.

ATM Setup message

An Asynchronous Transfer Mode (ATM) signaling message that enables a device to select the desired bandwidth and quality of service (QoS) levels when establishing a connection. The Setup message is sent by the calling user to the network and by the network to the called user. Key information elements of the Setup message include the following:

See also ATM, called-party number, CLID, QoS.

ATM SVC

ATM switched virtual circuit. An ATM SVC is a point-to-point switched connection between Asynchronous Transfer Mode (ATM) interfaces. An ATM SVC provides a lower-cost, usage-based alternative to an ATM permanent virtual circuit (PVC). Like other types of switched connections, SVCs can be initiated by a dial-in or a dial-out call.

A dial-in ATM SVC terminates locally. The TAOS unit receives the call on an ATM interface. An example of a terminating SVC is shown in Figure 8.

Figure 8. Terminating SVC on an ATM interface

A dial-out ATM SVC is initiated as an outgoing call on an ATM interface. Dial-out can be initiated explicitly, or it can take place on the basis of IP routing. Figure 9 shows a Pipeline® unit using Point-to-Point Protocol (PPP) to dial in to a MAX TNT® unit. The MAX TNT® unit establishes the incoming call and then dials out on an ATM interface on the basis of IP routing, just as it would for another type of switched dial-out call.

Figure 9. Dial-out SVC on an ATM interface

Unlike PVCs, which require dedicated connections, SVCs are on-demand connections and must use ATM end-point addresses to identify the interface and route to it. See also ATM, PPP, PVC, SVC.

ATM switched virtual circuit

See ATM SVC.

ATM UNI-DCE interface

ATM User-to-Network Interface for data circuit-terminating equipment. On an ATM UNI-DCE interface, the TAOS unit acts as the network side communicating with the user side (UNI-DTE) of an Asynchronous Transfer Mode (ATM) device. The logical port communicates with ATM customer premises equipment (CPE) over permanent virtual circuits (PVCs) and switched virtual circuits (SVCs). Compare with ATM UNI-DTE interface. See also ATM, CPE, PVC, SVC.

ATM UNI-DTE interface

ATM User-to-Network Interface for data terminal equipment. On an ATM UNI-DTE interface, the TAOS unit acts as the user side communicating with the network-side DCE switch. The logical port communicates with an Asynchronous Transfer Mode (ATM) switch over permanent virtual circuits (PVCs) and switched virtual circuits (SVCs). Compare with ATM UNI-DCE interface. See also ATM, CPE, PVC, SVC.

AT switch

See access tandem switch.

Attached Resource Computer Network

See ARCnet.

attenuation

The reduction in the strength of a signal over distance, expressed in decibels per kilometer (dB/km) or per 100 feet. Factors affecting attenuation are the frequency range of the signal, wire shielding, and cable type. Unshielded twisted pair (UTP) cable suffers from the most attenuation, while fiberoptic cable has very little attenuation. See also attenuator, UTP cable.

attenuator

A device that reduces the amplitude of a signal. See also attenuation.

attribute

A characteristic, similar to a parameter, in a RADIUS user profile or pseudo-user profile. RADIUS attributes can be assigned values to specify usernames and passwords and to configure routing, call management, and usage restrictions. See also pseudo-user profile, RADIUS, RADIUS server, user profile.

attribute set

In a transparent tunneling configuration, a set of RADIUS attributes that share the same tag number. A user's profile includes a primary attribute set, which specifies all of the values required to set up the tunnel, and additional attribute sets that can be used to establish a tunnel if the primary server is unavailable. See also tag.

attribute-value pair

See AVP.

ATU

ADSL transceiver unit. An ATU is a hardware device used with ADSL service. It connects to an Ethernet interface on one end and a telephone jack on the other. See also ADSL, ATU-C, ATU-R.

ATU-C

ADSL transceiver unit--central office. An ATU device for the central office (CO) side. Compare with ATU-R. See also ADSL, ATU, CO.

ATU-R

ADSL transceiver unit--remote. An ATU customer premises equipment (CPE) device. Compare with ATU-C. See also ADSL, ATU, CPE.

audio codec

Audio coder-decoder. A device that encodes analog voice data into a digital signal for transmission over a digital medium. See also codec, G.711 audio codec, G.728 codec, RT-24 codec.

audio coder-decoder

See audio codec.

AUI

Auxiliary unit interface. An AUI is a 15-pin D-type connector for Ethernet connections. It typically links a cable to a network interface card (NIC). An AUI is also known as a Digital, Intel, Xerox (DIX) connector. See also Ethernet, NIC.

authentication

A method of identifying the users permitted to gain access to network resources. Authentication is the first line of defense against unauthorized access to your network. Each TAOS unit supports a variety of authentication methods. You can use

When a TAOS unit is shipped from the factory, it is set to not require any authentication.

See also cache-token authentication, called-number authentication, CHAP, CLID authentication, expect-send script, PAP, PAP-Token authentication, PAP-Token-CHAP authentication, token card, token-card authentication, token-card server.

Authentication Header

See AH.

authentication key

A shared secret passed between a TAOS unit and an authentication server. An authentication key can be one of the following types:

See also authentication server, OSPF protocol, RADIUS, TACACS protocol, TACACS+ protocol.

authentication request

A request that a TAOS unit sends to an authentication server on behalf of a client requesting access. See also authentication response, authentication server.

authentication response

A response from an authentication server, notifying the TAOS unit that a user's request for access has been either granted or denied. See also authentication request, authentication server.

authentication server

An external server, such as a RADIUS, TACACS, TACACS+, or token-card server, that verifies whether a user requesting access to the network has permission to use network resources. See also RADIUS, RADIUS server, TACACS protocol, TACACS+ protocol, token-card server.

authentication timeout

The number of seconds between retries to an authentication server. If the TAOS unit is acting as a RADIUS, TACACS, or TACACS+ client, the unit waits the specified number of seconds for a response to an authentication request. If it does not receive a response within that time, it times out and sends the authentication request to the next authentication server. If the TAOS unit is acting as a Defender or SecurID client (both of which support only one server address), it waits the specified number of seconds before assuming that the server is unavailable. See also authentication server, RADIUS, RADIUS server, TACACS protocol, TACACS+ protocol.

authenticator field

In a RADIUS packet, a field that enables the system to authenticate transmissions between the TAOS unit and the authentication server. See also authentication, authentication server, CHAP, DES, encryption, RADIUS, RC4.

authority and format identifier

See AFI.

authority zone

A portion of the domain-name hierarchy associated with a name server.

authorization

Permission for a user to carry out tasks after he or she has access to the LAN. Authorization occurs after authentication is complete. See also authentication.

autobaud

A method of training up to a set modem data rate. If a DSL remote transceiver unit cannot train to this data rate, it will connect at the closest rate to which it can train (the modem's ceiling rate). See also modem rate control.

auto-BERT

Automatic bit-error-rate test. During an auto-BERT, a TAOS unit monitors the entire data stream between coder-decoders (codecs). At the end of the time period, if any channels have failed, the TAOS unit clears them, redials, and repeats the test. The maximum number of errors that can accumulate per channel is 65,000. The TAOS unit reports the total number of errors for each channel during the current call, but not the error rate. The unit resets the error display for the current call to 0 (zero) when the call disconnects.

automatic-at-startup rate adaptation

A type of rate adaptation that specifies that the customer premises equipment (CPE) initializes at a minimum specified bit rate and target noise margin. If the CPE fails to achieve the minimum bit rate in either direction, it cannot initialize, and it sends a message that the requested bit rate was too high. If the CPE can support a higher bit rate than the specified minimum, it can train up to a higher rate within the acceptable noise margin. Each direction can have a different minimum and maximum bit rate for the fast or interleaved asymmetric digital subscriber line (ADSL) channel. Compare with operator-controlled rate adaptation. See also ADSL, CPE.

automatic bit-error-rate test

See auto-BERT.

automatic call distributor

See ACD.

automatic LIM port redundancy

Automatic line interface module port redundancy. A feature that enables a Stinger® unit to detect a line interface module (LIM) port failure and automatically transfer the port connection to the same port on the spare LIM. When automatic LIM port redundancy is activated, the primary LIM port is monitored. If modem errors exceed the specified thresholds, the port connection to the primary LIM is transferred to the spare (secondary) LIM. Monitoring continues on the secondary LIM port. If modem errors again exceed thresholds, the connection is transferred back to the primary LIM port and the automatic redundancy process stops. See also LIM, LIM port redundancy.

automatic LIM redundancy

Automatic line interface module redundancy. A feature that enables a Stinger® unit to detect a line interface module (LIM) failure and automatically set up all the virtual channels of that LIM on the spare. When automatic LIM redundancy is activated, the primary LIM is monitored. If modem errors exceed the specified thresholds, all connections to the primary LIM are transferred to the spare (secondary) LIM. Monitoring continues on the secondary LIM. If modem errors exceed thresholds, the connections are transferred back to the primary LIM and the automatic redundancy process stops. See also LIM, LIM redundancy.

automatic line interface module port redundancy

See automatic LIM port redundancy.

automatic line interface module redundancy

See automatic LIM redundancy.

automatic number identification

See ANI.

automatic protection switching

See APS.

autonomous system

A group of Open Shortest Path First (OSPF) routers that exchange information, typically under the control of one company. An autonomous system can include a large number of networks, all of which share the same autonomous-system number. All information exchanged within the autonomous system is interior. Exterior protocols, such as Border Gateway Protocol (BGP), exchange routing information between one autonomous system and another. See also BGP, external route, OSPF protocol, router.

autonomous system border router

See ASBR.

autonomous system path list

In Border Gateway Protocol (BGP), the list of autonomous systems that a packet must traverse to reach a given set of IP address destinations located within a single autonomous system destination. The list can consist of sequences (which are series of autonomous systems that must be traversed in the order specified) and sets (which are collections of autonomous systems, one of more of which must be traversed in any order to the destination). See also autonomous system, BGP.

autoranging

The power supply's ability to detect the correct voltage received from the power source.

autosensing

A feature that enables you to change the device attached to an Ethernet port without reconfiguring the TAOS unit.

auxiliary unit interface

See AUI.

available bit rate

See ABR.

available cell rate

See AvCR.

AvCR

Available cell rate. The available capacity for constant bit rate (CBR), variable bit rate (VBR)-real time, and variable bit rate (VBR)-nonreal time services. For available bit rate (ABR) service, AvCR specifies the capacity available for minimum cell rate (MCR) reservation. See also ABR, CBR, MCR, PNNI protocol, VBR-nonreal time, VBR-real time.

average line utilization

See ALU.

average queue length

See AQL.

AVP

Attribute-value pair. (1) A RADIUS attribute and its specified value. Packets sent between a RADIUS server and a network access server (NAS) consist of AVPs, such as password="s64bigE&rt". See also attribute, NAS, RADIUS.

(2) In a Layer 2 Tunneling Protocol (L2TP) configuration, a variable-length string consisting of the name of a unique attribute and the attribute's value. AVPs make up the control messages used for establishing, maintaining, and disconnecting L2TP tunnels. See also L2TP.

B

B8ZS

Bipolar 8-zero substitution. B8ZS is an encoding method in which an alternating positive and negative voltage represents a 1 (one), no voltage represents a 0 (zero), and at least one bit out of every eight bits must be a 1 (one).

backbone

The part of the communications network designed to carry the bulk of the traffic. The backbone provides connectivity between subnets in an enterprise-wide network. See also enterprise-wide network, IP subnet.

backbone area

An Open Shortest Path First (OSPF) area that connects routers for the purpose of hierarchical routing. The backbone area is special and always has the area number 0.0.0.0. To tie areas together, some routers belong to the backbone area and one other area. These routers are called area border routers (ABRs). See also ABR, area, OSPF protocol, router.

backbone network

A network with a central cabling scheme linking it to other networks. Hosts on networks linked to the backbone can communicate with one another. See also backbone router.

backbone router

A router attached to a backbone network by dedicated lines. Usually, a backbone router does not have built-in digital dial-up WAN interfaces. Manufacturers of backbone routers include Cisco, Wellfleet, 3Com, and CrossCom. See also backbone network, router.

backoff queue

A file in which the RADIUS accounting server stores unacknowledged records. See also accounting server, RADIUS.

backplane

A circuit board assembly that provides a means of transferring signals between other circuit board assemblies connected to it.

back-to-back connection

A link in which the output of a sending device is connected directly to the input of a receiving device.

backup

The ability of the system to establish and use a temporary, alternative connection to a destination when the primary connection becomes unavailable. A backup connection replaces the primary connection, which must be a dedicated connection. The backup interface can be dedicated or switched.

When the system detects that the primary interface is unavailable, it puts the primary interface in a Backup Active state. It does not remove the routes to the primary interface. The system then diverts traffic from the primary to the backup interface. When the system detects that the primary interface is available again, it diverts traffic back to the primary interface. It also disconnects the backup interface if that interface has a switched connection. But if the backup interface is a dedicated link, the routes to the backup interface remain active.

Nested backups are not supported. The profile for a backup interface cannot specify another backup interface. The profile for a backup interface does not inherit attributes, such as filters or firewalls, from the profile for the primary dedicated connection.

See also dedicated circuit, switched circuit.

backup and overflow

See FT1-B&O.

backup designated router

See BDR.

backward explicit congestion notification

See BECN.

BACP

Bandwidth Allocation Control Protocol. BACP provides dynamic bandwidth allocation for Multilink PPP (MP) connections. With criteria very similar to those for the bandwidth-on-demand feature in Multilink Protocol Plus (MP+), BACP can be used with digital or analog links. BACP is described in RFC 2125. See also MP, MP+.

balun

A small device that connects a balanced line (such as a twisted-pair cable) to an unbalanced line (such as a coaxial cable). See also coaxial cable, twisted-pair cable.

bandwidth

The amount of data a link can carry, measured in bits per second (bps) for digital signals and in hertz (Hz) for analog signals. See also analog signal, digital signal.

Bandwidth Allocation Control Protocol

See BACP.

Bandwidth Allocation Protocol

See BAP.

bandwidth-on-demand

A WAN feature that enables a user to add bandwidth as required. See also bandwidth.

Bandwidth on Demand Interoperability Group

See BONDING.

banner

The text that first appears when a user logs in to the terminal server.

BAP

Bandwidth Allocation Protocol. BAP is a PPP protocol for managing bandwidth between two peers. Using BAP, the peers coordinate the process of adding and removing bandwidth. See also BACP, PPP.

base channel count

The initial number of channels to use for a Multilink PPP (MP) or Multilink Protocol Plus (MP+) connection. See also MP, MP+.

Basic Rate Interface line

See ISDN BRI line.

baud rate

The number of times a signal can switch from one state to another within 1 second. The more times a switch can occur, the higher the baud rate.

Bayonet-Neill-Concelman connector

BNC connector.

Bc

Committed burst. Bc is the maximum number of data bits that the network agrees to transfer, under normal conditions, during the time interval specified by Tc. The Bc value is defined for each permanent virtual circuit (PVC). See also PVC, Tc.

B channel

A 64Kbps channel that carries user data. A B channel is a bearer channel, one of the fundamental components of the ISDN interface. See also E1 PRI line, ISDN, ISDN BRI line, T1 PRI line.

B-channel bundling

A technique for putting multiple voice conversations on a single line. Speech is divided so that bits are transmitted only when someone is speaking. In T1 multiplexing, bundles consist of four bits, represent 11 channels carrying 32Kbps of compressed data, and have an associated signaling Delta channel. See also B channel.

BDR

Backup designated router. A BDR is the router that an Open Shortest Path First (OSPF) area uses in the event that the designated router goes out of service. To prevent the designated router from becoming a serious liability to the network if it fails, OSPF elects a BDR. Other routers maintain adjacencies with both the designated router and BDR, but the backup router leaves as many processing tasks as possible to the designated router. If the designated router fails, the BDR immediately becomes the designated router, and a new BDR is elected.

A TAOS unit can function as either a designated router or a BDR. However, to dedicate the TAOS unit to WAN processing, many sites choose to assign both these functions to LAN-based routers. See also adjacency, area, designated router, OSPF protocol, router.

Be

Excess burst. Be is the maximum number of bits of uncommitted data, in excess of the committed burst (Bc) size, that a frame relay network attempts to deliver during the time interval specified by Tc. In general, Be is delivered with a lower probability than Bc and is considered eligible for discard. See also Bc, Tc.

bearer channel

See B channel.

bearer service

An ISDN service for transmitting information from one device to another. Common bearer services are circuit-switched and frame relay services. See also circuit switching, frame relay.

BECN

Backward explicit congestion notification. BECN is a bit used in a frame relay header to notify data terminal equipment (DTE) that there is traffic congestion on the network and that the sending device should begin congestion-avoidance procedures. Compare with FECN. See also congestion, congestion management, frame relay.

Bell 103

A carrier standard created by Bell Labs in the 1960s and 1970s. Bell 103 accommodates modem-to-modem speeds of up to 300bps and is equivalent to the International Telecommunication Union Telecommunication Standardization Sector (ITU-T) V.21 standard. See also ITU-T, V.21.

Bell 212A

A carrier standard created by Bell Labs in the 1960s and 1970s. Bell 212A accommodates modem-to-modem speeds of up to 1200bps, and is equivalent to the International Telecommunication Union Telecommunication Standardization Sector (ITU-T) V.22 standard. See also ITU-T, V.22.

BER

Bit-error rate. The BER is the number of received bits with errors as a percentage of the total number of bits received. It is commonly expressed as a number to the power of 10. See also BERT.

BERT

Bit-error-rate test. The BERT calculates the number of received bits with errors as a percentage of the total number of bits received. See also BER.

best effort

A bit in an Asynchronous Transfer Mode (ATM) cell header, specifying that the network attempts to deliver traffic in excess of the limits of the quality of service (QoS) contract. However, there are no guarantees that traffic will be delivered. See also ATM, QoS contract.

best-effort packets

Packets delivered to the best of the network's ability, after the requirements for delivering the guaranteed packets have been met. Compare with guaranteed packets.

BGP

Border Gateway Protocol. BGP is a routing protocol for exchanging network information between one autonomous system and another. A routing device can use this information to construct a "map" of autonomous system connectivity. BGP uses TCP as its transport protocol. BGP is known as an Exterior Gateway Protocol (EGP). See also autonomous system, BGP-4, EGP.

BGP-4

Border Gateway Protocol Version 4. BGP-4 is the predominant routing protocol used to propagate routes between one autonomous system and another on the Internet. It supports Classless Inter-Domain Routing (CIDR) and route aggregation. See also autonomous system, BGP, CIDR.

bidirectional Challenge Handshake Authentication Protocol

See bidirectional CHAP.

bidirectional CHAP

An implementation of the Challenge Handshake Authentication Protocol (CHAP) that enables the called PPP device and the calling PPP device to authenticate each other. The called device determines whether the caller is permitted to access its resources, and the caller determines whether the remote site is the correct one. Bidirectional CHAP increases compliance with the RFC 1994 standard for PPP CHAP authentication. Note that the feature is not implemented for PAP-based authentication (PAP, PAP-TOKEN, or PAP-TOKEN-CHAP). Bidirectional CHAP is supported locally and through RADIUS.

Figure 10 shows a configuration in which a TAOS unit and its dial-in clients authenticate each other by means of bidirectional CHAP. One or more clients can dial in to the TAOS unit. The TAOS unit authenticates each calling device by means of a Connection profile, and each dial-in client authenticates the TAOS unit by means of a password value.

Figure 10. Bidirectional CHAP for all incoming calls to a TAOS unit

Note As noted in RFC 1994, a security hole can occur when you use bidirectional authentication for an incoming call if the secrets used in each of the two directions are identical. Bidirectional authentication on a TAOS unit has been developed to avoid the security hole, even if the secrets are identical. For best results, however, Lucent Technologies recommends that you specify a different secret for each authentication direction.

See also CHAP.

binary data

Data in the form of zeroes and ones.

binary mode

The Telnet 8-bit binary option. You can run XModem and other 8-bit file transfer protocols with binary mode. However, in binary mode, the Telnet escape sequence does not operate. The Telnet session can close only if one end of the connection quits the session. If you are a local user not connected through a digital modem, the remote-end user must quit. A user can override the binary setting on the Telnet command line. Compare with ASCII mode, transparent mode. See also Telnet, Telnet mode.

binary local mode

A data-transfer mode for X.25 T3POS calls. Binary local mode specifies that there is no error recovery between the T3POS packet assembler/disassembler (PAD) and the host, but that error recovery is in place between the PAD and the DTE. Like blind mode, binary local mode passes data between the DTE and the host without reference to the protocol being used. Unlike blind mode, binary local specifies that the system continues to use the T3POS protocol between the DTE and the PAD. Compare with blind mode, local mode, transparent mode. See also DTE, PAD, X.25 T3POS.

BIP

Bit interleaved parity. BIP is an error-detection method that uses odd or even parity to verify the accuracy of a transmission.

bipolar 8-zero substitution

See B8ZS.

B-ISDN

Broadband Integrated Services Digital Network. B-ISDN is a very high-speed data service, providing data transmission over fiberoptic media at a rate of 155Mbps and higher. See also broadband, broadband network, E1 line, ISDN, T1 line.

bit

Binary digit, the smallest unit of information a computer can process, representing one of two states (indicated by 1 and 0).

bit-error-rate

See BER.

bit-error-rate test

See BERT.

bit interleaved parity

See BIP.

bit inversion

A method of turning data 1s into 0s and data 0s into 1s. Bit inversion applies only to calls between coder-decoders (codecs). In some connections, you need to invert the data to avoid transmitting a pattern that the connection cannot handle. If you apply bit inversion, you should do so on both sides of the connection. See also codec.

bit rate

The number of bits that travel over a connection per second. See also bps.

bits per second

See bps.

blackhole interface

An interface that enables a router to handle packets whose IP address matches an unused IP address in a summarized address pool. The blackhole interface has an IP address of 127.0.0.3. When you specify this address as the router to the destination pool network, the TAOS unit silently discards packets to an invalid host on that network. See also pool summary.

blind mode

A data-transfer mode for X.25 T3POS calls. Blind mode provides a method of passing raw binary data between a data terminal equipment (DTE) device and the host system without reference to the protocol being used. In addition, blind mode specifies that the T3POS packet assembler/disassembler (PAD) does not provide any error recovery. In this mode, the DTE device and the host system provide error recovery for the connection. Note that the T3POS PAD does not clear a call when it receives a clear-request command from the DTE device. The PAD or the host system must clear the call. Compare with binary local mode, local mode, transparent mode. See also DTE, PAD, X.25 T3POS.

Blue Alarm signal

An alarm signal indicating that the unit is receiving all 1s. Compare with RAI signal, Red Alarm signal.

BNC connector

Bayonet-Neill-Concelman connector. A small connector with a half-turn locking shell. A BNC connector is commonly used with 10Base2 cabling.

BONDING

Bandwidth on Demand Interoperability Group. BONDING is a consortium of over 40 data-communications-equipment vendors and service providers who joined together to create a standardized inverse-multiplexing protocol. The BONDING protocol enables inverse multiplexers from different vendors to interoperate. BONDING also refers to the resultant specification, sometimes known as the BONDING specification. See also inverse multiplexer, inverse multiplexing.

BOOTP

Bootstrap Protocol. BOOTP starts up a network device by using information from a server. A TAOS unit can use BOOTP to get settings and check for a new software load. In addition, you can enable the terminal server to respond to BOOTP within a Serial Line Internet Protocol (SLIP) session. An interactive user who initiates a SLIP session can get an IP address from a designated IP address pool by means of BOOTP. See also BOOTP relay, BOOTP request, BOOTP server, DHCP, IP address, IP address pool, SLIP, terminal server.

BOOTP relay

A method of sending (relaying) Bootstrap Protocol (BOOTP) requests to other networks. On a TAOS unit, you specify the IP address of a BOOTP server for handling BOOTP requests. If a server is on the same LAN as the TAOS unit, BOOTP requests from other networks are relayed to the server. If a server is on another network, BOOTP requests from clients on the same LAN as the TAOS unit are relayed to the remote server. If you specify two BOOTP servers, the TAOS unit that relays the BOOTP request determines when each server is used. See also BOOTP, BOOTP request, BOOTP server.

BOOTP request

A request a client makes to a Bootstrap Protocol (BOOTP) server in order to receive an IP address or start the operating system of a network workstation. See also BOOTP, BOOTP relay, BOOTP server.

boot programmable read-only memory

See boot PROM.

boot PROM

Boot programmable read-only memory. A boot PROM is a chip mounted on a printed circuit board. The chip provides executable boot instructions to a computing device. See also PROM.

BOOTP server

A server that handles Bootstrap Protocol (BOOTP) requests from network clients. See also BOOTP, BOOTP relay, BOOTP request.

Border Gateway Protocol

See BGP.

Border Gateway Protocol Version 4

See BGP-4.

Bootstrap Protocol

See BOOTP.

Bootstrap Protocol relay

See BOOTP relay.

Bootstrap Protocol request

See BOOTP request.

Bootstrap Protocol server

See BOOTP server.

bps

Bits per second. A measure of the capacity of a line.

branding

A feature that enables you to select a set of voice announcements for playback from multiple sets of voice announcements stored on a MultiVoice® gateway. This feature is useful when you enable multiple logical gateways. See also logical gateway, MultiVoice®.

break-in voice announcement

A feature that enables a voice announcement to be played while a packet call is in progress. When a break-in voice announcement is playing, the Real-Time Transport Protocol (RTP) flow to the called party is suspended, the calling party hears the voice announcement, and the called party hears silence. While a break-in announcement is typically played to the calling party, it could be played out to the called party as well. See also voice announcement file.

bridge

A hardware device that transmits packets between networks. A bridge forwards packets from one network to another and ignores packets destined for hosts on the sending network. Operating at the Data Link layer, a bridge makes multiple networks look like a single network to higher-level protocols and software. See also Data Link layer.

bridge entry

An entry in a bridging table. See also bridge, bridging table.

bridging

A method of moving packets between networks by means of a device called a bridge, which operates at the Data Link layer. In Figure 11, the TAOS unit at Site A acts as a bridge between the Ethernet network and Site B.

Figure 11. Bridging configuration

The TAOS unit at Site A gradually learns addresses on both networks by examining each packet's source address, and it develops a bridging table that includes the following entries:

If the TAOS unit receives a packet whose destination MAC address is not on the local network, it first checks its internal bridging table. If it finds the packet's destination MAC address, the unit dials the connection and bridges the packet. If it does not find the address, the unit checks for active sessions that have bridging enabled. If one or more active bridging links are up, the unit forwards the packet across all active sessions that have bridging enabled.

See also bridge, bridge entry, bridging table, Data Link layer.

bridging table

A table that contains entries pairing up a host's media access control (MAC) address with a particular Ethernet interface. If a TAOS unit receives a packet whose destination MAC address is not on the local network, it first checks its bridging table. If it finds the packet's destination MAC address, the unit dials the connection and bridges the packet. If it does not find the address, the unit checks for active sessions that have bridging enabled. If one or more active bridging links are up, the unit forwards the packet across all active sessions that have bridging enabled. See also bridge, bridge entry, Ethernet, MAC.

BRI line

See ISDN BRI line.

broadband

A data communications technology that transmits data in channels to simultaneously carry several services, such as voice, video, and data, at rates greater than the T1 PRI maximum. See also B-ISDN, broadband network.

broadband Integrated Services Digital Network

See B-ISDN.

broadband ISDN

See B-ISDN.

broadband network

A network that enables a device to transmit a large amount of voice, data, and video information on the same cable over long distances. See also B-ISDN, broadband.

broadband service unit

See BSU.

broadband wireless LAN

Technology that provides higher data rates than either cellular or packet-radio communication. Broadband wireless LAN products operate at a much higher frequency (up to 2.4MHz) than cellular or packet radio products. However, the range of broadband systems is restricted to approximately 300 yards. Compare with cellular communication, packet-radio communication.

broadcast

To send a message to all users currently logged in to the network. Compare with multicast. See also broadcast address, broadcast network, broadcast packet.

broadcast address

A special address reserved for sending a message to all stations. Generally, a broadcast address is a media access control (MAC) destination address of all 1s (ones). See also broadcast, broadcast network, broadcast packet, MAC address.

broadcast network

A network in which the router sends packets to all users, whether they appear on subscription lists or not. In an Open Shortest Path First (OSPF) topology, a broadcast network is any network that has more than two OSPF routers attached and can address a single physical message to all of them. Compare with multicast network, unicast network. See also broadcast, broadcast address, broadcast packet, OSPF protocol, router.

broadcast packet

A packet containing a broadcast address, which indicates that all connected hosts receive the message. See also broadcast, broadcast address, broadcast network.

brouter

A networking device that combines the functionality of a bridge and a router.

browser

A software program for navigating and viewing the World Wide Web.

BRQ message

Bandwidth Request message. An H.323 registration, admission, and status (RAS) message sent from a MultiVoice® gateway to a MultiVoice® Access Manager (MVAM) device, requesting a change in bandwidth. See also H.323, MultiVoice®, MVAM, RAS.

BSU

Broadband service unit. A BSU is a broadband WAN device that consolidates wide-area Asynchronous Transfer Mode (ATM) access for a combination of video, voice, and LAN-based data traffic. See also ATM, broadband.

build

The name of the software binary file you install on a TAOS unit. If possible, stay with the same build when upgrading. Loading a different build can cause your unit to lose all or part of its configuration. If this situation occurs, you must restore your configuration from a backup.

buildout

For a T1 line with an internal channel service unit (CSU), the amount of attenuation the TAOS unit should apply to the line's network interface. The value depends on the cable length from the TAOS unit to the next repeater, because a repeater boosts the signal. If the TAOS unit is too close to a repeater, you need to add some attenuation. Obtain the value from your T1 provider. See also attenuation, attenuator, CSU, repeater, T1 line.

bundle

A group of physical links (such as multiple asynchronous lines) or multiplexed links (such as MP, MP+, X.25, or frame relay connections). The links in a bundle can be of different types (for example, dial-up asynchronous and dedicated synchronous connections). See also bundle owner.

bundle owner

In a stack of TAOS units, the unit that establishes the initial link. The bundle owner manages the connection's traffic across the stack. Stack peers forward incoming traffic from the WAN to the bundle owner, and the bundle owner receives outgoing traffic destined for the remote end and distributes it to bundled links. See also bundle, MP, MP+, stack.

burstiness

On a frame relay network, a characteristic of data that does not use the total bandwidth of a connection at all times. See also frame relay network.

burst mode

A method of transmitting data by collecting and sending it in a single high-speed transmission, rather than one character at a time.

bus

A path for signals transmitted between a computer's CPU and other hardware devices.

byte

Eight bits of data, also called an octet.

byte offset

See offset.

C

cable modem

A device that delivers high-speed data throughput over the coaxial cables used by the cable TV industry. Cable modems translate radio frequency signals to and from the cable plant into Internet Protocol (IP) signals. Compare with digital modem, modem.

CAC

Connection admission control. In an Asynchronous Transfer Mode (ATM) transmission, CAC consists of tasks performed by the network in order to determine whether to accept or reject a request for a connection or for reallocation of bandwidth. See also ATM.

cached token

A password dynamically generated on a token card, transmitted by Challenge Handshake Authentication Protocol (CHAP), and then cached for reuse. When a TAOS unit needs to add channels or make a new call, it uses the cached token to authenticate the additional bandwidth. You can specify a timeout value for the cached token, or configure the system to maintain the token throughout the session. See also cache-token authentication, CHAP, token, token card, token-card authentication, token-card server.

cache-token authentication

An authentication method that uses Challenge Handshake Authentication Protocol (CHAP) to transmit the initial token, and then caches the token for reuse. The system later uses the cached token when adding new channels or making a new call. See also ACE authentication, cached token, SafeWord authentication, token, token card, token-card authentication, token-card server.

call

A single session in which a calling device and an answering device connect over the WAN.

callback

A type of security in which you instruct a TAOS unit to hang up and call back when it receives an incoming call. You can require callback to ensure that the unit makes a connection with a known device. Hanging up and calling back adds a level of certainty that the connection is with a trusted user, especially because the TAOS unit calls back immediately after verifying the user's name and password. For a TAOS unit to use callback, it must be configured to both receive and initiate calls. Callback security applies only to switched lines. See also Ascend callback, authentication, CBCP callback, CLID callback, DNIS callback, switched line.

Callback Control Protocol callback

See CBCP callback.

call blocking

A feature that enables you to automatically stop the unit from attempting to place an outgoing call on a connection that repeatedly fails. Successive retries can cause excessive charges, congestion, and performance problems. Using call blocking, you can prohibit additional retries after a specified number of failed connection attempts. You can also control the length of time call blocking is in effect.

Call Connected packet

A packet sent by a remote data terminal equipment (DTE) device when it accepts a call from a unit on an X.25 network. See also DTE, X.25.

call-countdown timer

A value that specifies the maximum amount of time that a call can remain connected before the MultiVoice® gateway disconnects the link. See also call-disconnect warning timer, MultiVoice®.

call detail recording

See CDR.

call-disconnect warning timer

A value that specifies the amount of time that must elapse before the MultiVoice® gateway plays a call-disconnect warning announcement for the caller. This warning announcement alerts the caller to the time remaining before the call is terminated. See also call-countdown timer, MultiVoice®.

called-number authentication

A form of authentication in which a TAOS unit uses the called-party number to authenticate the connection. The remote end uses this form of authentication to verify that the call goes to a known destination. When the profile requires called-number authentication, the number called must match a specified telephone number. The TAOS unit also uses the called number to direct incoming calls to a particular device. See also called-party number, user profile.

called-party number

An information element of the Q.931 ISDN signaling protocol. The called-party number is the telephone number the remote device calls to connect to the TAOS unit, but without a trunk group or dialing prefix specification. The called-party number is always available if specified in a profile. See also called-number authentication, Q.931.

caller ID

See caller identification, CLID.

caller identification

A telephone service that enables the called party's equipment to display the telephone number of the caller. See also caller identification restriction.

caller identification restriction

A telephone service that enables the caller to prevent his or her telephone number from being displayed to the called party. See also caller identification.

call filter

A packet filter that defines which packets can initiate a connection, or reset the idle timer for an established link (Figure 12). A call filter prevents unnecessary connections and helps a TAOS unit distinguish active traffic from "noise."

Figure 12. Call filters can prevent certain packets from resetting the timer.

By default, any traffic to a remote site triggers a call, and any traffic across an active connection resets the connection's idle timer. When a session's idle timer expires, the TAOS unit terminates the session.

When you apply a call filter, its forwarding action does not affect which packets are sent across an active connection. For a call filter to prevent an interface from remaining active unnecessarily, you must define rules for both incoming and outgoing packets. Otherwise, if only input rules are defined, outgoing packets keep a connection active. If only output rules are defined, incoming packets keep a connection active.

Compare with data filter. See also input filter, output filter, packet filter.

calling line ID

See CLID.

calling line ID authentication

See CLID authentication.

calling line ID callback

See CLID callback.

calling line ID substitution

See CLID substitution.

call logging

A method of logging call information from a TAOS unit. Call logging enables you to keep records for resource management or troubleshooting. It permits NavisAccess to maintain information about the real-time state of the TAOS unit without having to perform an SNMP query for retrieving the information. In general, SNMP is not an optimal means for tracking the state of a network access server (NAS) in real time. Because call-logging data is acknowledged, it is also much more reliable than SNMP.

You can use call logging with all versions of NavisAccess later than 4.0. See also call-log host, NAS, SNMP.

call-log host

A NavisAccess host that supports the call-logging accounting protocol and is configured to communicate with the TAOS unit. See also accounting, call logging.

call-proceeding timer

In an Asynchronous Transfer Mode (ATM) configuration, a timer that starts when a Setup message is received. The call-proceeding timer is also called the T310 timer. See also ATM.

call-progress tones

Dual tone multifrequency (DTMF) tones that report call states. Ringback and busy signals are common call-progress tones.

call request

On an X.25 network, a request made by the calling party, asking the data terminal equipment (DTE) to accept the call. See also Call Request packet, call-request timer, DTE, X.25.

Call Request packet

A packet sent by a device when it makes an outgoing call on an X.25 network. See also call request, call-request timer, X.25.

call-request timer

A value that specifies the number of 10-second ticks that a TAOS unit waits before clearing an outgoing call that the remote data terminal equipment (DTE) has not accepted. When a device makes an outgoing call, it sends a Call Request packet. If the remote DTE accepts the call, it sends back a Call Connected packet. If the DTE refuses the call, it sends back a Clear Request packet. See also Call Connected packet, call request, Call Request packet, Clear Request packet, DTE, X.25.

call routing

The process of routing incoming and outgoing calls to the proper modules on a TAOS unit.

When a TAOS unit receives a call on a WAN line, it performs calling line ID (CLID) or called-number authentication (if appropriate), answers the call, and determines which slot should receive the call. It then finds the caller's profile, authenticates the call, builds a session, and passes the data stream to the appropriate module or host.

When a TAOS unit dials an outgoing call, it routes the call from the originating slot to a WAN channel. It first looks for channels associated with the trunk group specified by the dialed number (if any) and the port that originated the call. If no trunks have available channels, the TAOS unit does not place the call.

See also called-number authentication, CLID authentication.

Call Setup message

See ISDN Call Setup message.

call spanning

A method of enabling a Multilink PPP (MP) or Multilink Protocol Plus (MP+) call to span the TAOS units in the stack. Call spanning using a stack configuration can be effective when:

Call spanning is protocol independent. See also hunt group, MP, MP+, stack.

call-user data

See CUD.

CAP

(1) Carrierless amplitude phase. CAP is a modulation method used with some DSL technologies. It uses both amplitude and phase to create signals for data transmission over twisted-pair lines. CAP stores different parts of a message in memory and then reassembles those parts in the modulated wave. CAP uses the frequency range from 4kHz to 1.1MHz. See also DSL, RADSL.

(2) Competitive access provider. A CAP is a business that competes with the local telephone company in providing clients with access to services. For example, a cable company that offers high-speed data communications services is a CAP.

Carrier Detect signal

See CD signal.

carrierless amplitude phase

See CAP.

Carrier Sense Multiple Access/Collision Detect protocol

See CSMA/CD protocol.

carrier services

Telecommunications services provided to the public for a fee (for example, ISDN lines and frame relay services).

CAS

Channel-associated signaling. CAS is a carrier switch type in New Zealand. When you use CAS, the circuit state is indicated by one or more signaling bits sent in a repetitive manner.

cause element

See disconnect-cause code.

cause field

A field that indicates an event that triggered an X.25 Clear Request,
Reset Request, or Restart Request packet. Values for the cause field can vary, depending on the packet type. See also Clear Request packet, diagnostic field, Reset Request packet, Restart Request packet, X.25.

CBCP callback

Callback Control Protocol callback. Microsoft's CBCP callback is a Link Control Protocol (LCP) option negotiated at the beginning of Point-to-Point Protocol (PPP) sessions. CBCP authenticates a caller by means of a username and password.

Microsoft developed CBCP callback to address a need for greater security when establishing PPP connections. The standard callback option defined in RFC 1570 has a potential security risk because the authentication is performed after the callback. CBCP callback, like Ascend callback, occurs after authentication, leaving no potential security hole. CBCP also offers features not available with standard callback. The client side supports a configurable time delay, enabling users to initialize modems or startup software before the TAOS unit calls the client. You can configure the TAOS unit with a telephone number to use for the callback, or you can allow the client to specify the telephone number.

Compare with Ascend callback, CLID callback, DNIS callback. See also callback, LCP, PPP.

c-bit parity error

See CPERR.

CBR

Constant bit rate. CBR is a quality of service (QoS) class defined by the Asynchronous Transfer Mode (ATM) forum for ATM networks. CBR is used for connections that depend on precise clocking to ensure undistorted delivery of bits. Compare with ABR, UBR, VBR-nonreal time, VBR-real time. See also ATM, QoS.

CCITT

Consultative Committee for International Telegraph and Telephone. The CCITT is a disbanded organization whose standards were moved to the UN-sanctioned International Telecommunication Union Telecommunication Standardization Sector (ITU-T) on March 1, 1993. See ITU-T.

CCP

Compression Control Protocol. CCP enables both ends of a Point-to-Point Protocol (PPP) connection to negotiate whether to use data compression, and if so, which algorithm to use.

CD signal

Carrier Detect signal. CD is a signal sent from a modem to a host, indicating that the modem is online.

CDDI

Copper Distributed Data Interface. CDDI is the copper equivalent of FDDI. It is a network architecture that supports transmission rates of up to 100Mbps over unshielded twisted pair (UTP) cable, with a maximum length of 100 meters. See also FDDI, UTP cable.

CDMA

Code division multiple access. CDMA is a digital wireless transmission technique that uses mathematical codes, instead of frequencies or time slots, to transmit information. CDMA is a leading digital standard. See also AMPS, CDPD, cellular communication, cellular modem, cellular network, wireless technology.

CDPD

Cellular digital packet data. CDPD is a digital wireless transmission technique that uses idle voice channels on the existing Advanced Mobile Phone Service (AMPS) cellular telephone network. CDPD transmits data packets at a raw data rate of 19.2Kbps, using channel hopping to move data packets through unused spaces across different frequencies. Because data is not as time-sensitive a service as voice, data can be fragmented and then reassembled at the receiving end. CDPD is particularly suited to sending small messages and transactions. It is not appropriate for transmitting multimegabit files. See also AMPS, CDMA, cellular communication, cellular modem, cellular network, wireless technology.

CDR

Call detail recording. CDR is a feature that provides a database record of information about each call, including date, time, duration, called number, calling number, call direction, service type, and associated inverse-multiplexing session and port. Because the network carrier charges for bandwidth on an as-used basis, and bills each connection in an inverse-multiplexed call as a separate charge, you can use CDR to understand and manage bandwidth usage and the cost of each inverse-multiplexed session.

CDV

Cell delay variation. In an Asynchronous Transfer Mode (ATM) configuration, CDV is a routing metric that measures the average variation in delay between one cell and the next, expressed in fractions of a second. CDV measurements enable the network to determine whether cells are arriving too quickly or too slowly. See also admin cost, ATM, CDVT, end-to-end delay.

CDVT

Cell delay variation tolerance. In an Asynchronous Transfer Mode (ATM) configuration, CDVT is the maximum number of microseconds of delay between one cell and the next. See also ATM, CDV.

cell

In Asynchronous Transfer Mode (ATM), a 53-byte fixed-length data packet consisting of a 5-byte header and a 48-byte payload; in cellular communication, a portion of a city or county. In cellular communication, each cell contains the transmitters and receivers that provide the telephone service. The frequencies assigned to one cell are limited to the boundaries of that cell. When a cellular telephone moves from one cell toward another, a computer at the switch monitors the motion and hands off the telephone call to the new cell, which uses another radio frequency. The transfer is not noticeable to the user. See also ATM, ATM cell, cellular modem, cellular network.

cell delay variation

See CDV.

cell delay variation tolerance

See CDVT.

cell loss priority

See CLP.

cell relay

See ATM.

cell switching

In a cellular network, a feature that enables a caller to move from one location to another without losing the connection. The cellular system is designed to switch calls to a new cell without a noticeable drop in the connection. Although not noticeable in voice communications, the 300 milliseconds (ms) required for cell switching can cause problems in data transmission. Cell switching is sometimes referred to as handing off. See also cellular communication, cellular network.

cell transfer delay

See CTD.

cellular communication

A type of wireless communication first available in 1981. To implement this technology, a carrier divides a city or county into units called cells. Each cell contains the transmitters and receivers that provide the telephone service. The frequencies assigned to one cell are limited to the boundaries of that cell. When a cellular telephone moves from one cell toward another, a computer at the switch monitors the motion and hands off the telephone call to the new cell, which uses another radio frequency. The transfer is not noticeable to the user. Compare with landline telephone communication. See also CDMA, CDPD, cellular modem, cellular network, wireless technology.

cellular digital packet data

See CDPD.

cellular modem

A modem that uses cellular technology to transmit data between remote locations. See also CDMA, CDPD, cellular communication, cellular network.

cellular network

A network that enables cellular subscribers to travel anywhere in the country and remain connected to the Public Switched Telephone Network (PSTN) by means of their mobile telephones. See also CDMA, CDPD, cellular communication, cellular modem.

central office

See CO.

central processing unit

See CPU.

central site

A data-location point for telecommuters, branch offices, and remote users.

central site modem

See CSM.

Centrex

Business telephone service that a local exchange carrier (LEC) offers from a central office (CO). Centrex services include call forwarding, call transfer, call restrictions, and call hold. Centrex service is an alternative to buying or leasing a private branch exchange (PBX). See also LEC, PBX.

Challenge Handshake Authentication Protocol

See CHAP.

Change Filter Request ACKed packet

A message a TAOS unit sends if it finds at least one routing session for which it could change packet filters. Compare with Change Filter Request NAKed packet. See also Change Filter Request packet.

Change Filter Request NAKed packet

A message a TAOS unit sends if it could not find a routing session for which it could change packet filters. Compare with Change Filter Request ACKed packet. See also Change Filter Request packet.

Change Filter Request packet

A request to change the packet filters for a routing session. See also Change Filter Request ACKed packet, Change Filter Request NAKed packet.

channel

A portion of a line's bandwidth. A line contains a fixed number of channels. Each line can contain switched channels only, dedicated channels only, or a combination of switched and dedicated channels. See also bandwidth, dedicated channel, line, switched channel.

channel-associated signaling

See CAS.

channel bank

Equipment that converts multiple voice signals to time-division multiplexing (TDM) signals for transmission on a T1 or E1 line. A channel bank connects a T1 or E1 line to a PBX or a central office (CO) switch. The channel bank takes in the analog signals from the PBX and uses pulse code modulation (PCM) to convert the signals to digital. At the receiving end of the connection, the channel bank converts the digital signals back to analog. Using a channel bank enables an organization to use the T1 or E1 circuit for voice, video, fax, and data.

Any digital PBX that terminates T1 or E1 lines at a customer site is capable of terminating them directly without use of a channel bank. The CO portion of the T1 or E1 circuit, however, still requires a channel bank, because telephone lines are primarily analog. See also CO, D4-framed T1 line, ESF, G.703, PBX, TDM.

channelized T1 PRI or E1 PRI

A T1 PRI or E1 PRI line divided into individual 64Kbps channels, or into channels whose data rate is a multiple of 64Kbps (for example, a 256Kbps channel made of four 64Kbps channels). Channelized T1 PRI or E1 PRI lines can be switched or dedicated. For example, a dedicated line can run from the central office (CO) to the corporate headquarters as a single, unchannelized T1 PRI line, and can then be divided into channels when it runs to remote sites from the corporate headquarters. Compare with unchannelized T1 PRI or E1 PRI. See also dedicated line, E1 line, E1 PRI line, inband signaling, switched line, T1 line, T1 PRI line.

channel service unit

See CSU.

CHAP

Challenge Handshake Authentication Protocol. CHAP authentication verifies the caller's identity by using a three-way handshake upon initial link establishment, and then by repeating the handshake any number of times. In CHAP authentication, the authentication server sends a challenge to the caller. The caller responds with an MD5 digest calculated from the password. The authentication server then checks the digest against its own calculation of the expected hash value to authenticate the call. The server can send a new challenge at random intervals.

CHAP is a stronger authentication method than Password Authentication Protocol (PAP), because the password does not travel across the line as plain text. In addition, the use of repeated challenges limits the time of exposure to any single attempt to break the encryption code. The server is in control of how often it sends challenges.

Using bidirectional CHAP, you can enable the calling device and the called device to authenticate each other. See also bidirectional CHAP, encryption, hash value, PAP.

char-to-char timer

For an X.25 T3POS connection, a value that specifies the maximum amount of time permitted between characters sent from the data terminal equipment (DTE) to the packet assembler/disassembler (PAD). The char-to-char timer is also called the T1 timer. See also DTE, PAD, X.25 T3POS.

CHCS errors

Correctable header check sequence errors. CHCS errors are header check sequence (HCS) errors that the unit can correct. Compare with UCHCS errors. See also HCS.

check item

A component of a RADIUS user profile that must be matched in an Access Request packet for the access to succeed. See also Access Reject packet, RADIUS, reply item.

Checkpoint packet

A RADIUS accounting (session-in-progress) packet that is identical to a Stop packet, except that Acct-Status-Type=3 (instead of 2) and the packet does not include the Ascend-Disconnect-Cause (195) attribute. See also accounting, accounting checkpoint, Checkpoint record.

Checkpoint record

A RADIUS accounting record that enables you to retrieve information about each user session in the event of network disruption. By default, RADIUS accounting logs a Start and Stop record for each user session. If a disruption in service causes a connection to fail before the TAOS unit receives a RADIUS Stop record, you can use the Checkpoint records to reconstruct usage.

In the RADIUS detail file, a Checkpoint record contains the same group of attributes as a RADIUS Stop record. However, the value for the Acct-Status-Type attribute in a Checkpoint record is the number 3. When queuing RADIUS accounting records, the TAOS unit prioritizes Start and Stop records ahead of Checkpoint records. See also accounting, accounting checkpoint, Start record, Stop record.

checksum

A count of the number of bits in a transmission unit. A checksum enables the receiving device to determine whether the same number of bits arrived as were sent. If the counts match, the receiver can assume that the transmission arrived intact. See also CRC.

CIC

Circuit identification code. In a Signaling System 7 (SS7) network, the CIC specifies a port or time slot on a T1 or E1 interface. See also SS7 network.

CIDR

Classless Inter-Domain Routing. CIDR is a technique supported by Border Gateway Protocol Version 4 (BGP-4) that eliminates the necessity for network address classes by explicitly advertising the subnet mask associated with each prefix. See also BGP, BGP-4.

CIR

Committed information rate. The rate (in bits per second) at which the network agrees to transfer information under normal conditions. The rate is averaged over the minimum increment of time specified by Tc. See also Tc.

circuit

A connection between end points over a physical medium.

circuit connection

A connection that follows a specified path through the frame relay switch. By linking two data link connection identifier (DLCI) end points, a TAOS unit creates a permanent virtual circuit (PVC). The two DLCI end points act as a tunnel. Figure 13 illustrates a circuit connection.

Figure 13. Circuit connection

Data that the TAOS unit receives on one DLCI bypasses the router and goes out on the other DLCI. If any one of the DLCIs in a PVC becomes inactive because of a disconnect or failure, the PVC using that DLCI becomes inactive. A physical line can carry multiple DLCIs, and the failure of the line causes the failure of all the DLCIs it carries. Compare with frame relay direct. See also DLCI, frame relay switch, PVC, router.

circuit identification code

See CIC.

circuit-level inverse multiplexing

A method of inverse multiplexing in which the inverse multiplexer slices the data stream into equal portions and transmits each portion over an available circuit. The receiving end adjusts for network-induced delay and reassembles the data packets into their proper order. The AIM and BONDING protocols define how circuit-level inverse multiplexing works. Applications that require transparent digital circuits (for example, videoconferencing, dedicated backup and overflow, and bulk file transfer) use circuit-level multiplexing. Compare with packet-level inverse multiplexing. See also AIM, BONDING, inverse multiplexer, inverse multiplexing.

circuit-switched cellular data

See CSCD.

circuit-switched line

A temporary connection, like a telephone call. A temporary connection can be made to a variety of sites to handle occasional data-transfer needs or to provide additional bandwidth.

circuit-switched network

A type of network that uses a continuous link between a sender and a receiver. Voice and video applications use circuit switching to ensure that all the parts of a signal arrive in the proper order. Compare with packet-switched network.

circuit-switched public data network

See CSPDN.

circuit switching

A mode of data transfer in which a dedicated connection is busy for the duration of the call. Compare with packet switching.

CIR guaranteed bandwidth TOS routing

A type of Open Shortest Path First (OSPF) routing that guarantees that if multiple paths have sufficient bandwidth to meet committed information rate (CIR) type of service (TOS) requirements to the destination, the system chooses the path with the lowest administrative cost. If multiple paths exist with sufficient available bandwidth and the same administrative cost, the system chooses the path with the largest available bandwidth. If no path to the destination has sufficient available bandwidth, the system reestablishes the circuit, using the path with the lowest administrative cost. See also CIR, OSPF protocol.

Classless Inter-Domain Routing

See CIDR.

Clear Confirmation packet

On an X.25 network, a packet that the data terminal equipment (DTE) or data circuit-terminating equipment (DCE) device receives in response to its request to clear a call. When the device receives a Clear Confirmation packet from the remote end, the call is cleared and the logical channel is available for other calls. See also Clear Indication packet, Clear Request packet, DCE, DTE, X.25.

Clear Indication packet

On an X.25 network, a packet that the data circuit-terminating equipment (DCE) sends when it refuses an incoming call or when it clears a call upon completion of the data exchange. See also Clear Confirmation packet, Clear Request packet, DCE, X.25.

Clear Request packet

On an X.25 network, a packet that the data terminal equipment (DTE) device sends when it refuses an incoming call or when it clears a call upon completion of the data exchange. See also Clear Confirmation packet, Clear Indication packet, DTE, X.25.

Clear-Request retries

The number of times a TAOS unit sends a Clear Request packet on an X.25 network before waiting indefinitely for a response. See also Clear Request packet, X.25.

clear-request timer

A value that specifies the number of 10-second ticks that a TAOS unit waits before retransmitting a Clear Request packet. See also Clear Request packet, X.25.

Clear To Send signal

See CTS signal.

CLEC

Competitive local exchange carrier. A CLEC is a company that competes with the established local telephone company by providing its own network and switching services. See also LEC.

CLID

Calling line ID. The CLID is the telephone number of a calling device that attempts to connect to a TAOS unit. The telephone company provides the telephone number. A CLID is also known as a caller ID. See also CLID authentication.

CLID authentication

A method a TAOS unit uses to authenticate incoming calls by checking the calling party's telephone number (as received from the telephone company). The calling line ID (CLID) is the telephone number of the calling device. A TAOS unit performs CLID authentication before enabling itself to answer an incoming call. When a profile requires CLID authentication, the caller's telephone number must match a specified telephone number. You can thereby ensure that the call comes from a known source.

You can use CLID authentication only when the call information is available end-to-end and automatic number identification (ANI) applies to the call. In some areas, the WAN provider might not be able to deliver CLIDs, or a caller might keep a CLID private. Typically, a site uses CLID authentication to protect against a situation in which an unauthorized user obtains the name, password, and IP address of an authorized user and then calls the TAOS unit from another location.

See also CLID, RADIUS, user profile.

CLID callback

The method by which a TAOS unit uses the calling line ID (CLID) information element to detect callback during the ringing state of an incoming call. The TAOS unit does not answer the call (go off hook), and the originating caller is not charged for the call. Compare with Ascend callback, CBCP callback, DNIS callback. See also callback.

CLID substitution

Substitution that occurs when a MultiVoice® gateway connects a Voice over IP (VoIP) call and transmits a calling line ID (CLID) generated by the MultiVoice® Access Manager (MVAM) software on the gatekeeper instead of the PSTN-generated CLID collected on the trunk line. When MVAM receives the CLID from a gateway, it translates the CLID to the appropriate dial string, adding or removing country codes and area codes as appropriate. The gatekeeper then reports the revised CLID to both gateways as part of the Admission Confirmation (ACF) message.

CLID substitution enables the MultiVoice® network to provide the appropriate E.164 addresses for the called and calling telephone numbers to the appropriate Public Switched Telephone Networks (PSTNs). When the gateways connecting the call reside in different area codes or countries, the CLID received from the PSTN must be changed to provide the appropriate calling number information to the local carrier, to call management applications, and to billing software. See also CLID, MultiVoice®, MVAM, PSTN, VoIP.

client

A user or device that requires services from another unit or program. For example, a user requesting access is a client of the TAOS unit, and a TAOS unit making a RADIUS authentication request is a client of the RADIUS server. See also RADIUS server.

client DNS

A configuration that enables a TAOS unit to direct incoming connections to a Domain Name System (DNS) server belonging to a particular client or location, thereby preventing WAN users access to a local DNS server. Client DNS has two levels: a global configuration that applies to all PPP connections, and a connection-specific configuration. The addresses configured for client DNS servers are presented to WAN connections during IPCP negotiation. You can also choose to present your local DNS servers in the event that no client servers are defined or available. See also DNS, IPCP.

CLNP

Connectionless Network Protocol. CLNP is the Open Systems Interconnection (OSI) protocol for datagram service. It is the OSI equivalent of the Internet Protocol (IP). See also IP, SDTN.

clock

A timing mechanism for synchronizing data communication and processing tasks. A clock divides time into very short intervals. See also clock speed.

clock speed

The number of intervals per second that a clock uses for synchronizing data communication and processing. See also clock.

closed user group

See CUG.

CLP

Cell loss priority. A field in an Asynchronous Transfer Mode (ATM) cell header, CLP indicates the eligibility of the cell for discard under congested conditions. See also ATM, congestion, GFC, HEC, payload, PT field, VCI, See also ATM, VPI, VPL..

cluster

A group of internal Border Gateway Protocol (BGP) peers that share a common set of route reflectors. See also BGP, route reflector.

cluster ID

An identifier that uniquely identifies a Border Gateway Protocol (BGP) route reflection cluster within an autonomous system. All route reflectors within the cluster must be configured with the same cluster ID. Internal peers that are not reflectors within the cluster must not be configured with a cluster ID. The cluster ID is typically set to the BGP router ID of one of the route reflectors within the cluster. See also autonomous system, BGP, cluster, internal peer, route reflector.

CLT module

Copper loop test module. A Stinger® module that consists of an integrated test head installed on a path selector module (PSM). The test head is used to perform a full suite of technical tests on copper loops associated with digital subscriber line (DSL) connections. A CLT module can also be used as a PSM to support line interface module (LIM) and LIM port redundancy. See also DSL, LIM, PSM.

CM

See control module.

CMAS

Confederation member autonomous system. A CMAS is a subdivision of an autonomous system that is recognized only by other peers within the confederation. Within the confederation, each Border Gateway Protocol (BGP) peer treats only the peers in its own CMAS as internal peers. Peers in different CMASs are treated as external peers. See also BGP, confederation, external peer, internal peer.

CMF-R2

Compelled multiple frequency R2. A type of signaling in which an R2 tone is sent until a reply tone is received, however long that process takes.

CO

Central office. The CO is the telephone switching office to which a customer directly connects. It links the customer to other portions of the telephone network.

coaxial cable

A data-transmission cable consisting of a braided outer shield surrounding an insulated core.

codec

Coder-decoder. A codec is a device that encodes analog video or voice data into a digital signal for transmission over a digital medium. Codecs are often used for videoconferencing. See also analog data, audio codec, digital signal.

code division multiple access

See CDMA.

coder-decoder

See codec.

coldboot

A reboot that enables the user to restart the switch as if it were powered off and then on again. Compare with warmboot.

coldstart

The process by which a TAOS unit reinitializes itself in a way that might alter the configuration of the SNMP manager or the system. Compare with warmstart.

coldstart notification

In a RADIUS accounting Stop record, a value that informs the accounting server that the TAOS unit has started up. See also accounting, accounting server, Stop record.

collision detection

See CSMA/CD protocol.

command mode

A terminal-server mode in which you can enter commands at the terminal-server prompt. Compare with immediate mode, menu mode.

comment line

A line, in a RADIUS user profile or pseudo-user profile, that describes the purpose of one or more lines of the profile. Beginning with the # character at column one, the comment line consists of text that extends to the end of the line. You can embed a comment line anywhere in a profile. See also pseudo-user profile, user profile.

committed burst

See Bc.

committed information rate

See CIR.

committed rate measurement interval

See Tc.

common part

The portion of the signaling ATM adaptation layer (SAAL) that represents the functionality common to all users requiring a connection-oriented, variable bit-rate information transfer. It provides unguaranteed information transfer but includes a mechanism for detecting corruption of information carried in the SAAL frames. Compare with SSP. See also SAAL.

community

A label that identifies a group of Border Gateway Protocol (BGP) destinations for the purpose of policy enforcement. Assembling destinations into identifiable communities lets BGP peers base policy decisions on the identity of the group rather than on individual destinations. The community identifier is advertised in update messages between BGP peers. See also BGP.

community name

A password that a TAOS unit sends to the Simple Network Management Protocol (SNMP) manager when an SNMP trap event occurs, and that the manager sends to the TAOS unit with each polling request. The password authenticates the sender. The default is public. See also agent, manager, SNMP.

compelled multiple frequency R2

See CMF-R2.

competitive access provider

See CAP.

competitive local exchange carrier

See CLEC.

Compressed Serial Line Internet Protocol

See CSLIP.

compression

A process that reduces the quantity of bandwidth or storage space required to encode a block of information. See also VJ compression.

communications protocol

A standard method of communicating between networked devices. A hardware interface standard (for example, RS-232C). See also protocol.

concentrator

A repeater or hub that joins communications channels from several network nodes and provides bridging, routing, and management functions. See also hub, repeater.

confederation

In Border Gateway Protocol (BGP), an autonomous system that has been subdivided into smaller confederation member autonomous systems (CMASs). A confederation appears like a single autonomous system to other autonomous systems and is recognized only by other confederation members. Subdivision of an autonomous system into a confederation changes the peer relationships of confederation members in different CMASs from internal to external. Use of confederations in an autonomous system requires that all routers in the autonomous system belong to a CMAS. Confederations are one method for avoiding the overhead of having all peers within an autonomous system fully communicate with each other. Route reflection clusters provide an easier method, but require the use of identical policies on all peers within the autonomous system. See also autonomous system, BGP, CMAS.

confederation member

Any router running Border Gateway Protocol (BGP) and recognizing that its autonomous system is subdivided into smaller confederation member autonomous systems (CMASs). The CMASs are recognized only by confederation members and not by peers external to the confederation. Subdivision of an autonomous system into a confederation changes the peer relationships of confederation members in different CMASs from internal to external. See also autonomous system, BGP, CMAS.

confederation member autonomous system

See CMAS.

congestion

The point at which a network device is operating at its highest degree of utilization. See also absolute congestion, congestion avoidance, congestion management, mild congestion, severe congestion.

congestion avoidance

A method of notifying the edge nodes of congestion in a frame relay network. The header of each frame includes a field for the forward explicit congestion notification (FECN) bit and the backward explicit congestion notification (BECN) bit. FECN informs the destination device of congestion for destination-controlled protocol suites. BECN informs the source device of congestion for source-controlled protocol suites.

FECN and BECN bits indicate the existence of network congestion to the higher-layer protocols. The system can then reduce the rate of information flow into the network until the congestion clears. However, many frame relay customer premises equipment (CPE) devices cannot use this information. Both the FECN and BECN bits are set by the network, not by the user. Therefore, end-point nodes are not required to do anything about them. The FECN and BECN bits are often ignored, or are simply counted by internetworking devices, such as routers, to provide an indication of congestion in the network.

See also congestion, congestion management.

congestion management

A method for monitoring congestion on a frame relay network. As data travels through the frame relay network and is queued for transmission, the state of each queue is checked for pending congestion. Each switch executes a time-average algorithm, average queue length (AQL), each time it queues a frame for transmission. It then compares the AQL value to a precalculated threshold. When the AQL is less than or equal to the lowest threshold, maximum throughput and minimum delay occur.

The switch uses three thresholds to determine congestion:

If the AQL exceeds the threshold for mild congestion but is less than the threshold for severe congestion, the link is considered mildly congested. If the AQL exceeds the threshold for severe congestion but is less than the threshold for absolute congestion, the link is considered severely congested. If the AQL exceeds the threshold for absolute congestion, the link is considered absolutely congested. When the link is in this state, there is no room in the queue for any packets. See also absolute congestion, congestion, congestion avoidance, mild congestion, severe congestion.

connection admission control

See CAC.

connectivity

The degree to which a given computer or application can interoperate with other network components.

connect-request timer

In an Asynchronous Transfer Mode (ATM) configuration, a value that specifies the maximum number of seconds that can elapse between the transmission of a Connect message and the receipt of a Connect Acknowledge message. The connect-request timer is also known as the T313 timer. Compare with restart-request timer. See also ATM.

constant bit rate

See CBR.

Consultative Committee for International Telegraph and Telephone

See CCITT.

continuity test

In a Signaling System 7 (SS7) network, a test to verify that the physical link between the central office (CO) switch and the TAOS unit is available. The CO switch informs the signaling gateway, which then informs the TAOS unit that it will conduct a continuity test on the circuit. During a call continuity test, the CO switch sends a tone through the physical path to the TAOS unit and receives back from the TAOS unit a tone indicating the continuity of the path. The TAOS unit supports a 4-wire-only continuity test (as defined in Q.724 Sections 7 and 8, ANSI T1.113.4 Annex B, GR-246-CORE Annex B), a 2-wire continuity test (as defined in GR-246-CORE Section B.2), and 4-wire-to-2-wire emulation (as defined in GR-246-CORE Section B.3). See also 2-wire continuity test, 4-wire continuity test, 4-wire-to-2-wire continuity test, CO, signaling gateway, SS7 network.

control frame

In an X.25 T3POS network, a supervisory frame of the following format:

where:

Control frames are in use only when a call is being established, not during data transfer. See also binary local mode, blind mode, CRC, DTE, local mode, LRC, transparent mode, X.25 T3POS.

control-lead signaling

A method of toggling one or more leads within a cable in order to initiate a dialed call.

control-line state

A state that results when a device sends a signal through a pin and over the line to another device. The signal being sent determines the control-line state. For example, a device can send a signal to inform another party that it is ready to receive data. In this case, the control-line state is Data Transmit Ready (DTR). The process of sending control signals is called handshaking. See also DTR signal, handshaking.

control module

A Stinger® controller designed for functional redundancy. Although the unit can operate with a single control module, redundant operation is recommended. When two control modules are installed and the system is powered on, the unit elects one of the modules as the primary control module. The other becomes the secondary control module. As soon as the election process is complete, the status lights (LEDs) on the control module's front panel indicate which of the modules is primary.

The primary and secondary control modules monitor each other, using a heartbeat protocol, and maintain synchronized repositories of the system configuration stored in the primary control module's flash memory. If the primary control module resets, the system switches over to the secondary control module. The mechanism for switchover is built into the control module hardware, and allows switchover to occur instantaneously. To maintain full functional redundancy, the primary and secondary control modules have separate paths to each line interface module (LIM) and trunk module, as shown in Figure 14.

Figure 14. Redundant paths from each control module

The primary control module manages the LIMs, and assumes all the normal controller responsibilities of managing the unit, including the call-control and circuit-management functions. In the event of a switchover, the LIMs are not hardware reset (avoiding the need to retrain the DSL remote transceiver units). However, all connections are dropped. Connections are subsequently rebuilt after the new primary control module completes its initialization. Log messages notify the user of the following significant events related to control module redundancy:

All configuration must take place on the primary control module. The primary control module configuration repository regularly overwrites that of the secondary control module. In addition, the primary control module always overwrites the secondary control module's repository immediately following a configuration change. See also LIM, Stinger®, trunk module.

convergence

The time it takes all routers to receive information about a change to the network topology. Slow convergence can result in routing loops and errors. A Routing Information Protocol (RIP) router broadcasts its entire routing table every 30 seconds. On a 15-hop network, convergence can be as high as 7.5 minutes. In contrast, Open Shortest Path First (OSPF) uses a link-state database of the network, and propagates only changes to the database, resulting in faster convergence. See also link-state database, OSPF protocol, RIP.

convergence sublayer

A sublayer of the Asynchronous Transfer Mode (ATM) adaptation layer (AAL). Its primary purpose is to convert data and protocols between ATM and non-ATM formats. See also AAL, ATM.

Coordinated Universal Time

See UTC.

Copper Distributed Data Interface

See CDDI.

copper loop test module

See CLT module.

correctable header check sequence errors

See CHCS errors.

cost

An Open Shortest Path First (OSPF) value you assign to the output side of each router interface. The cost indicates the likelihood that the TAOS unit will use the interface to transmit data. The lower the cost, the more likely is the TAOS unit to use the interface.

Figure 15 shows how costs are used to direct traffic over high-speed links. For example, if Router-2 in Figure 15 receives packets destined for Host B, it will route them through Router-1 across two T1 links (Cost=20), rather than across one 56Kbps B channel to Router-3 (Cost=240).

Figure 15. OSPF costs for different types of links

You can use the cost value to perform preferred path selection. If two paths to a destination have equal costs, you can assign a higher cost to one of the paths, making it a backup when the primary path is not available. In addition, you might want to reflect the bandwidth of a connection when assigning costs. As in Figure 15, the cost of a single B-channel connection could be 24 times greater than the cost of a T1 link.

A TAOS unit has a default cost of 1 for a connected route (Ethernet) and 10 for a WAN link. Be careful when assigning costs. Incorrect cost metrics can cause delays and congestion on the network. See also OSPF protocol, route, router.

coverage area

A group of telephone numbers that the system can use to dial and receive calls through a particular MultiVoice® gateway. You define coverage areas for each MultiVoice® gateway by assigning dial strings to a database on the gatekeeper. A dial string can be a country code, an area code, a country code/area code combination, an area code/exchange combination, or a complete telephone number. See also gatekeeper, gateway, inclusion area, MultiVoice®.

CP

See common part.

CPE

Customer premises equipment. CPE is equipment connected to the telephone network and located at the customer's site. The equipment can be owned or leased.

CPERR

C-bit parity error. A CPERR indicates that the c-bit parity check failed on the DS3 line. Compare with FERR, PERR. See also DS3 line.

CPU

Central processing unit. The CPU is the computer's main processor.

crankback

In a Private Network-to-Network Interface (PNNI) configuration, the ability to reroute a call on an alternative path in case of failure. See also PNNI protocol.

CRC

Cyclic redundancy check. CRC is an error-detection method that uses a mathematical divisor to check the integrity of the data in a transmitted packet. See also checksum, CRC error.

CRC-4

Cyclic redundancy check 4. A four-bit error-checking technique that calculates a numeric value to check the integrity of transmitted data and to detect errors in the data stream.

CRC error

Cyclic redundancy check error. A CRC error is a condition that occurs when the CRC in a frame does not match the CRC received from the network. See also CRC.

cross-connect

An Asynchronous Transfer Mode (ATM) function that enables each switch along the path of an end-to-end permanent virtual circuit (PVC) to receive the cell stream on one interface and transmit it on another. Figure 16 shows an end-to-end PVC connection across two switches. Switch A has an ATM circuit configuration between its interfaces, labeled A and B, cross-connecting two virtual links. Switch B has a similar ATM circuit configuration between its interfaces, labeled C and D.

Figure 16. End-to-end PVC cross-connected in each switch

See also ATM, ATM circuit, PVC.

crossover cable

A cable with wires that cross over so that the terminating ends of the cable have opposite wire assignments. A crossover cable is sometimes referred to as a null modem. Compare with straight-through cable.

CS

See convergence sublayer.

CSCD

Circuit-switched cellular data. CSCD is a wireless transmission technology that supports sending large files and faxes. CSCD uses switches to set up connections in analog cellular networks, and is also used in conjunction with such digital packet technologies as cellular digital packet data (CDPD). See also CDPD, cellular communication, wireless technology.

CSLIP

Compressed Serial Line Internet Protocol. CSLIP is a form of the Serial Line Internet Protocol (SLIP). Both SLIP and CSLIP enable you to transmit IP packets over serial connections, but CSLIP uses a compressed packet header and involves less overhead than SLIP. See also SLIP.

CSM

Central site modem. A CSM is a digital modem whose digital signal is converted to analog format before it reaches the subscriber's 56Kbps modem. This single digital-to-analog conversion increases throughput in the CSM-to-subscriber direction. See also digital modem.

CSMA/CD protocol

Carrier Sense Multiple Access/Collision Detect protocol. CSMA/CD is a media-access mechanism in which a device ready to transmit data checks the channel for the presence of a carrier. If it does not sense a carrier for a specific period of time, the device can transmit its data. If two devices transmit at the same time, a collision occurs and all transmitting devices detect it. The collision delays retransmissions from the devices for random lengths of time. Ethernet and IEEE 802.3 use CSMA/CD. See also 802.3, Ethernet.

CSPDN

Circuit-switched public data network. A CSPDN is a communications network that uses circuit-switched digital data circuits and is available to the public. See also circuit switching, digital signal.

CSU

Channel service unit. Along with a data service unit (DSU), a CSU is a component of data circuit-terminating equipment (DCE). A CSU connects a digital telephone line to a customer's network-access equipment. It can be built into the network interface of the network-access equipment, or it can be a separate device. The CSU terminates the connection at the user's end and processes digital signals. It also prevents a faulty DSU from interfering with data transmissions on the digital line.

On a frame relay circuit, the CSU converts the signals between the V.35 interface on data terminal equipment (DTE) and the T1 or E1 interfaces on a frame relay switch.

See also DCE, digital signal, DSU.

CTD

Cell transfer delay. The elapsed time between a cell exit event at the source and the corresponding cell entry event at the destination for a particular Asynchronous Transfer Mode (ATM) connection. See also ATM.

CTS signal

Clear To Send signal. CTS is a signal sent from a receiving device to a transmitting device, indicating that the transmitter can begin sending data. A CTS signal is generally a response to a transmitter's Request To Send (RTS) signal. See also RTS signal.

CUD

Call-user data. The CUD field in a control frame identifies the encapsulation in use over an X.25 virtual circuit. See also encapsulation, virtual circuit, X.25.

CUG

Closed user group. A CUG is a calling group to which access is restricted. A user can be a member of more than one CUG. In general, members of a specific CUG can communicate among themselves, but not with users outside the group. In some cases, however, specific CUG members can originate calls to destinations outside the group, or receive calls from outside the group. The network service provider (NSP) can determine the maximum number of CUGs a user can belong to. See also CUG index, NSP.

CUG index

Closed user group index. On an X.25 PAD call, the CUG index indicates to the called switch the CUG selected for a virtual call. See also CUG, X.25 PAD.

custom AESA format

Custom ATM end system address format. The custom AESA format uses the custom authority and format identifier (AFI) and byte order. Compare with DCC AESA format, E.164 AESA format, ICD AESA format. See also AESA format, AFI.

customer premises equipment

See CPE.

cyclic redundancy check

See CRC.

cyclic redundancy check 4

See CRC-4.

D

D4-framed T1 line

A T1 line that uses the D4 format, also known as the Superframe format, to frame data at the physical layer. The D4 format consists of 12 consecutive frames, each one delimited by framing bits. T1 lines that do not use ISDN D-channel signaling use the D4 format. See also T1 line.

DAC

Digital access cross-connect A DAC is a device used to split and switch channels between incoming and outgoing circuits. This capability, referred to as fanout, provides for distribution of these channels to accommodate changes in traffic patterns and site relocations. In some cases, a DAC reallocates channels to support peak traffic overflow and provide redundancy in fault-tolerant configurations. See also drop-and-insert multiplexer.

D-A conversion

See digital-to-analog conversion.

daemon

A type of program that, once activated, carries out a specific task without user intervention. A daemon typically handles a task that runs repeatedly (for example, a printing, mail, or communications service). See also RADIPAD, RADIUS.

DARPANet

A network created in 1969 by the Defense Advanced Research Projects Agency (DARPA). The DARPANet provided an efficient way to exchange military information between scientists in various locations. Originally consisting of a 4-computer network, the DARPANet expanded into a network of 37 computers and was later renamed ARPANet. ARPANet was the precursor of the present-day Internet. See also Internet.

DASS-2

Digital Access Signaling System 2. DASS-2 is a signaling protocol used by British Telecom on ISDN links. DASS-2 specifies the signaling that occurs on the D channel. Although DASS-2 is widely available, many new installations use Q.931. See also Q.931.

Database Description packet

A Type-2 Open Shortest Path First (OSPF) packet. OSPF routers exchange Database Description packets when an adjacency is being initialized. Each packet describes the contents of the link-state database. The routers use a poll-response procedure. One of the routers is the master, and the other a slave. The master sends Database Description poll packets, and the slave sends Database Description response packets. OSPF links the responses to the polls by means of a sequence number in each packet. See also adjacency, link-state database, OSPF protocol.

data bits

In asynchronous transmission, the bits that contain the data being sent. Data bits are sometimes referred to as a payload. See also asynchronous transmission.

data bus connector

See DB connector.

Data Carrier Detect signal

See DCD signal.

data circuit-terminating equipment

See DCE.

data compression

A method for current modem standards and protocols to attain higher rates of speed. Compression algorithms, such as those in the V.42bis standard, take advantage of redundancies in data files by substituting a few characters for many. Compression is especially effective with text files and certain graphic-file formats, and has become an important topic as multimedia, video, document imaging, and other technologies emerge.

data country code

See DCC.

data country code ATM end system address format

See DCC AESA format.

data delivery layer

See DDL.

data encryption standard

See DES.

data encryption standard-cypher block chaining

See DES-CBC.

Data Exchange Interface

See DXI.

data filter

A packet filter that defines which packets a TAOS unit can transmit on a connection. When you apply a data filter, its forward or drop action affects the actual data stream by preventing certain packets from reaching the Ethernet network from the WAN, or vice versa (Figure 17).

Figure 17. Data filters can drop or forward certain packets.

Many sites apply data filters for security purposes, but you can apply them for any purpose that requires the TAOS unit to drop or forward only specific packets. For example, you can use data filters to drop packets addressed to particular hosts, to prevent broadcasts from going across the WAN, or to allow users to access only specific devices across the WAN. Compare with call filter. See also packet filter.

data frame

See general frame.

datagram

A message unit that contains a source address, destination address, and data. A datagram is routed through a packet-switched network. See also packet-switched network.

data link

The link interface to a frame relay device. The data link refers to specific dedicated bandwidth on the TAOS unit and defines the operations and link-management functions that the TAOS unit performs on the interface. See also frame relay, frame relay network.

data link connection identifier

See DLCI.

Data Link layer

The second layer of the OSI Reference Model. The Data Link layer creates, sends, and receives data packets appropriate for the type of network in use. Data-Link-layer protocols include High-Level Data Link Control (HDLC), Link Access Procedure, Balanced (LAPB), Link Access Procedure on the D channel (LAPD), Point-to-Point Protocol (PPP), and Serial Line Internet Protocol (SLIP). See also HDLC, LAPD, OSI Reference Model, PPP, SLIP.

data over subscriber bearer service

See 3.1kHz audio-bearer service.

data-over-voice

A method of sending digital data over telephone trunks by means of either voice-bearer service or 3.1kHz audio-bearer service. See also 3.1kHz audio-bearer service.

data packet

See packet.

data rate

The transmission speed of data over a line, generally expressed as thousands of bits per second (Kbps).

data service

A service provided over a WAN line and characterized by the unit measure of its bandwidth. A data service can transmit either data or digitized voice. The following types of data services are available:

See also GloBanD, MultiRate, Switched-56, Switched-64, Switched-384, Switched-1536.

data service unit

See DSU.

Data Set Ready signal

See DSR signal.

data terminal equipment

See DTE.

data-transfer mode

For an X.25 T3POS connection, the method used for error recovery and data transmission. A TAOS unit enables you to specify local, transparent, blind, or binary local. See also binary local mode, blind mode, local mode, transparent mode.

data transfer rate

The speed at which data is transferred, usually measured in megabits per second (Mbps).

Data Transmit Ready signal

See DTR signal.

DB-9

A 9-pin serial port. See also DB connector.

DB-25 pin connector

A 25-pin connector on which the RS-232C standard is based. Ten connections are commonly used. On a standard DB-25 pin connector, the pin designations and names of the signals are:

See also CD signal, CTS signal, DSR signal, DTR signal, protective ground, RD signal, RI signal, RS-232C, RTS signal, SG, TD signal.

DBA protocol

Dynamic Bandwidth Allocation protocol. DBA denotes the process of adding or subtracting bandwidth from a switched connection in real time without terminating the link. Multilink Protocol Plus (MP+) supports DBA governed by a set of parameters you specify. To add bandwidth, the TAOS unit dials additional connections.

The TAOS unit can reject a request to add bandwidth if no more channels are available or if the network is congested. Under either of these conditions, the two ends enter bandwidth-addition-lockout mode, in which neither side can request bandwidth. The lockout prevents both ends from continually trying to add new channels unsuccessfully. Both ends automatically remove the lockout restriction when the conditions that caused the lockout change. When the lockout ends, each end is free to add bandwidth.

If you use a circuit between two locations to capacity 24 hours per day, using a dedicated line is more cost-effective than using a switched line. However, if you need the circuit only sporadically, or if the circuit is sometimes underutilized, it often makes more sense to lease a smaller amount of dedicated bandwidth and then supplement it with additional switched bandwidth as traffic requirements dictate.

For example, you might establish some connections only when you need to transfer data, and a single circuit can accommodate low traffic levels. However, if traffic levels grow beyond the capacity of the circuit (such as during a large file transfer), DBA automatically adds additional switched channels. When traffic levels subside, DBA automatically removes the channels from the connection. The bandwidth and connection costs are thereby reduced. You pay for bandwidth only when you need it. See also bandwidth, circuit, dedicated line, MP+, switched line.

DB connector

Data bus connector. A DB connector is a cable connector for parallel or serial ports. The number following DB indicates the number of pins on the connector. For example, a DB-25 connector has 25 pins. See also DB-9, DB-25 pin connector.

DCA

Defense Communications Agency. The DCA is an agency responsible for installing the Defense Data Network.

DCC

Data country code. The DCC is a 2-byte portion of an ATM end system address (AESA) and identifies the country in which the address is registered. Country codes are standardized and defined in ISO Reference 3166. See also AESA format.

DCC AESA format

In the data country code (DCC) ATM end system address (AESA) format, the DCC is specified in the address, identifying the country in which the address is registered. Compare with custom AESA format, E.164 AESA format, ICD AESA format. See also AESA format.

DCD signal

Data Carrier Detect signal. DCD is a hardware signal defined by the RS-232C standard. It indicates that the device is online and ready to receive a transmission.

DCE

Data circuit-terminating equipment. A DCE device connects data terminal equipment (DTE) to a communications channel, such as a telephone line. DTE refers to a device that an operator uses (for example, a computer or a terminal). A DCE device converts the format of the data coming from the DTE into a signal suitable to the communications channel. An example of a DCE device is a modem, which converts digital data from a computer to analog signals suitable for sending over a telephone line. See also analog signal, digital data, DTE, modem.

DCE interface

An interface that provides AIM/BONDING inverse multiplexing services to a device connected to it.

DCF message

Disengage Confirmation message. An H.323 registration, admission, and status (RAS) message sent by the MultiVoice® Access Manager (MVAM) device to a MultiVoice® gateway in response to a Disengage Request (DRQ) message. Compare with ACF message. See also DRQ message, H.323, MultiVoice®, MVAM, RAS.

D channel

A channel that carries WAN synchronization and signaling information on a T1 PRI or E1 PRI line. See also E1 PRI line, T1 PRI line.

DCS 1800

Digital Cellular System working at 1800 MHz. DCS 1800 is a European mobile-telephone service based on European Telecommunication Standards Institute (ETSI) standards. See also ETSI.

DCS 1900

See GSM 1900.

DDL

Data delivery layer. In a Signaling System 7 (SS7) configuration, the DDL uses a TCP/IP socket on both the signaling gateway and the TAOS unit. On the signaling gateway side, the DDL is the server that listens for the socket connection and keeps track of the mapping between a TAOS unit and its socket. On the TAOS unit side, the DDL is the client that initiates a socket connection and handles connection establishment, connection recovery, and link selection. See also SS7.

DE

Discard eligibility. On a frame relay network, DE is a bit that you set in a frame to indicate that the network can discard the frame in favor of others in order to maintain quality of service (QoS). Frames that have the DE bit set are Be excess data. See also Be, frame relay network.

dedicated channel

A channel on a line rented from the telephone company for exclusive use, 24 hours per day, 7 days per week. A dedicated channel is also called a leased channel, a nailed channel, or a nailed-up channel. See also dedicated circuit, dedicated line.

dedicated circuit

A permanent connection between end points, over which two parties exchange data. The number of dedicated channels must be the same at both ends of the connection. For example, if there are five dedicated channels at the local end, there must be five dedicated channels at the remote end. However, channel assignments do not have to match. For example, channel 1 can be switched at the local end and dedicated at the remote end. A dedicated circuit is also known as a leased circuit, a nailed circuit, a nailed-up circuit, or a private circuit. See also dedicated channel, dedicated line.

dedicated line

A line rented from the telephone company for exclusive use, 24 hours per day, 7 days per week. The connection exists between two predetermined points and cannot be switched to other locations. A dedicated line is also called a leased line, a nailed line, or a nailed-up line. See also dedicated channel, dedicated circuit.

dedicated server

A network device that functions only as a server, performing specific network tasks. See also server.

default gateway

The default router that a TAOS unit uses for traffic from a specific connection if it finds no explicit route in the IP routing table. See also IP router, IP routing table.

default route

The route that a TAOS unit uses if it does not find a match for a packet's destination address. The default route has the destination address 0.0.0.0. If the TAOS unit finds a default route, it establishes the required connection (if necessary) and forwards the packet.

Figure 18 shows a router, on a local subnet, configured as the default route in a TAOS unit. This type of configuration enables the TAOS unit to turn off Routing Information Protocol (RIP) on its local interfaces and forward all local packets to the default route.

Figure 18. Default route to a local IP router

If the routing table has no default route and no route that matches a packet's destination address, the TAOS unit drops the packet. See also IP route, IP router, IP routing table.

Defender authentication

A form of token-card authentication that makes use of the AssureNet Pathways Defender authentication server. When you configure Defender authentication, the TAOS unit forwards any call not authenticated by a local Connection profile to the Defender server. Defender authentication proceeds in three stages:

You can use a Defender server with or without RADIUS authentication. The Defender server does not provide per-user control, such as enforcing a maximum number of channels. It provides only per-user authentication. If you need both per-user control and authentication, use RADIUS. See also authentication server, RADIUS, token-card server.

Defense Communications Agency

See DCA.

define path

A function that enables you to define a manual path for the permanent virtual circuit (PVC), bypassing the Open Shortest Path First (OSPF) algorithm for PVC routing decisions. See also OSPF protocol, PVC.

degree of preference

In Border Gateway Protocol (BGP), an arbitrary rating number that the unit assigns to every route it receives from a BGP peer. A higher number indicates a greater preference for a route when more than one exists to a destination. A route from an internal peer is assigned the local preference number that the unit learned with the route. For a route learned from an external peer, the unit calculates a number based on the autonomous system path length. The shortest path is preferred. See also autonomous system, BGP, external peer, internal peer, local preference.

denial of service attack

See DoS attack.

density enforcement

For AMI-encoded T1 lines, requirements that restrict the system from transmitting 16 consecutive zeroes. See also AMI, T1 line.

DES

Data encryption standard. DES is the U.S. encryption standard for nonclassified documents. This standard uses a 64-bit key and private-key encryption. In private-key encryption, only the sender and receiver know the key for encrypting the data. DES cannot ensure that the sender and receiver are legitimate. A sender who has learned the key can fraudulently use it. See also DES-CBC, encryption, private-key encryption.

DES-CBC

Data encryption standard-cypher block chaining. DES-CBC is an Internet Protocol Security (IPSec) protocol encryption algorithm. When you use DES-CBC, message text and signatures are encrypted by means of the DES algorithm in CBC mode. See also 3DES-CBC, 40DES-CBC, DES, IPSec protocol.

designated router

The router with which all other Open Shortest Path First (OSPF) routers in a broadcast network establish adjacencies. Figure 19 illustrates a configuration with both a designated router and a backup designated router (BDR).

Figure 19. Designated and backup designated routers

To reduce the number of adjacencies each router must form, OSPF designates one of the routers as the designated router. Doing so simplifies the routing table update procedure and reduces the number of link-state records in the database. The designated router plays other important roles in reducing the overhead of OSPF link-state procedures. For example, other routers send link state advertisements (LSAs) to the designated router by using the "all-designated-routers" multicast address of 224.0.0.6.

The administrator chooses the designated router on the basis of the processing power, speed, and memory of the system, and then assigns priorities to other routers in case the BDR is down at the same time. A TAOS unit can function as a designated router or BDR. However, many sites choose to assign a LAN-based router as the designated router or BDR in order to dedicate the TAOS unit to WAN processing.

See also adjacency, BDR, LSA, OSPF protocol, router.

DeskDial client

Lucent Technologies client software for network modem-pool access.

destination

In Border Gateway Protocol (BGP), the final autonomous system in the autonomous system path whose IP address prefixes and associated subnet masks are reported in the network layer reachability information (NLRI) field of an update message. A destination and its path comprise a BGP route. See also autonomous system, autonomous system path list, BGP, update message.

destination address

In a frame, packet, or message sent over a bridged or routed connection, the Internet Protocol (IP), Internetwork Packet Exchange (IPX), or hardware address of the intended recipient of the transmission. Compare with source address.

destination point code

See DPC.

destination port

The port to use to communicate with the destination machine. The port might be, for example, a User Datagram Protocol (UDP) port on an authentication server or a Simple Mail Transfer Protocol (SMTP) port on a mail server. Compare with source port. See also SMTP, UDP, UDP port.

destination service access point

See DSAP.

destination transit list

See DTL.

device integration

In ISDN technology, the ability to carry digital signals from various devices over a single network interface. Communication that once required numerous wire pairs can now take place over a single wire pair. The system passes digital signals through local B channels. You can connect a variety of devices to an ISDN line by means of an ISDN network termination 1 (NT1) device or an ISDN integrated access device (IAD). See also IAD, ISDN, ISDN line, NT1 device.

DHCP

Dynamic Host Configuration Protocol. DHCP is a TCP/IP protocol that enables a client to obtain a temporary IP address from a central server (known as a DHCP server). See also DHCP server.

DHCP server

A server that uses Dynamic Host Configuration Protocol (DHCP) to assign a temporary IP address to a client that requests it. See also DHCP, DHCP spoofing, IP address.

DHCP spoofing

A process that enables a local device to receive an IP address from a Dynamic Host Configuration Protocol (DHCP) server across a slow WAN link. When you set up a DHCP connection, the TAOS unit can assign a dynamic IP address to a remote DHCP client over a bridged connection. The TAOS unit becomes a DHCP server.

For example, suppose a group of DHCP clients resides on a LAN connected to a Pipeline® unit, and the Pipeline® unit connects to a MAX unit over a bridged PPP connection (Figure 20). The MAX unit can assign dynamic IP addresses to any of the DHCP clients on the remote LAN.

Figure 20. Pipeline® unit connected to DHCP clients

The RADIUS server holds the configuration information the TAOS unit uses to identify and authenticate each DHCP client. When a PC sends a broadcast DHCP request, the following events take place:

1. Acting as a DHCP server, the Pipeline® unit receives the DHCP request and sends the PC a temporary IP address. The address can be static or dynamic. It has a very short time-to-live (TTL) value.
2. The Pipeline® unit dials the remote side, passing along the original DHCP request.
3. The DHCP server sends back a server-assigned IP address.
4. When the Pipeline® unit receives the address from the remote side, it passes the address to the PC.
5. The PC changes its IP address to the server-assigned address.

Typically, a device requesting an IP address from a DHCP server waits a limited amount of time before timing out the request. For complex WAN links with authentication processes, there might not be enough time to complete the process.

The DHCP server allocates an IP address from one of its IP address pools and assigns it to the client for 30 minutes. The client must renew the IP address assignment before the 30-minute period expires. The DHCP server uses its local memory to keep track of all IP addresses it has assigned. Therefore, if you reset it, the server loses the entries for current unexpired IP address assignments.

A client can keep an unexpired IP address assignment if you reset the DHCP server. But after the reset, the server might assign that address to a new client. The duplicate IP addresses cause network problems until the first assignment expires or one of the two clients reboots.

See also DHCP, DHCP server, IP address.

diagnostic field

On an X.25 network, an optional field in a Clear-Request, Reset-Request, or Restart Request packet. Each packet has a required cause field and an optional diagnostic field. If the cause field indicates that the remote data terminal equipment (DTE) did not request the clear, reset, or restart, the diagnostic field has standard values. If the cause field indicates that the remote DTE requested the clear, reset, or restart, the diagnostic field contains information specified in the cause field by the remote DTE. See also cause field, Clear Request packet, DTE, Reset Request packet, Restart Request packet, X.25.

Dialed Number Information Service

See DNIS.

Dialed Number Information Service callback

See DNIS callback.

dial-in/dial-out server

See asynchronous communications server.

dial-in modem access

The ability of a remote worker using a modem with a PC or other computer device to dial in to a remote access server (RAS) at the corporate office, a central-site location, or a service provider's Point-of-Presence (POP). Compare with dial-out modem access, LAN-to-LAN modem access.

dial-in user

A remote user or device that calls a TAOS unit over a switched circuit and requests a connection.

dial-out modem access

The ability of a user at a workstation on a corporate LAN to dial out through a shared modem to a service provider's Point-of-Presence (POP) or branch-office location. Compare with dial-in modem access, LAN-to-LAN modem access.

dial-out timer

A value that specifies the maximum number of seconds the system waits for a Call Setup Complete message from the remote side when dialing out.

dial query

A feature designed for sites that support many clients and connections to only a few remote Internetwork Packet Exchange (IPX) networks. When a TAOS unit receives a Service Advertising Protocol (SAP) query for a file server (service type 0x04) and its SAP table has no entry for that service type, the unit establishes all connections that enable dial query. See also SAP.

dial-up line

A connection or circuit between two sites through a switched telephone network. A dial-up is most commonly associated with a voice telephone call between two locations. For modem access, a dial-up line forms a link between two distant pieces of equipment, such as computers or LANs. Not restricted to landline connections, a dial-up circuit can also be established through a circuit-switched cellular network, or through a combination of landline and cellular media. See also cellular communication, circuit-switched line, landline telephone communication, wireless technology.

DID

Direct inward dialing. A feature that enables a caller outside a company to dial an internal extension without having to go through an operator or attendant.

differentiated services

A method of providing distinct quality of service (QoS) classes for Internet applications by using a six-bit differentiated services (DS) field in the IPv4 TOS octet or the IPv6 Traffic Class octet of an IP packet. The DS field determines the way in which a packet is forwarded at each network node. See also DSCP, DS field, QoS.

differentiated services code point

See DSCP.

differentiated services field

See DS field.

digital access cross-connect

See DAC.

Digital Access Signaling System 2

See DASS-2.

digital cellular

See PCS.

digital data

Data that can have only a limited number of separate values. The time of day represented by a digital clock and the temperature represented by a digital thermometer are examples of digital data. The digital values do not change continuously, but remain at one discrete value and then change to another discrete value. Compare with analog data. See also digital signal.

Digital Identification signal

See DIS.

digital line

A line that transmits data by means of a digital signal. See also digital signal.

digital loopback

A procedure that tests the digital processing for a communications device. Compare with analog loopback. See also local loopback, loopback, remote loopback.

digital loop carrier

See DLC.

digital milliwatt tone

See DMW tone.

digital modem

A device in a TAOS unit that enables the unit to communicate over a digital line with a station connected to an analog line. Incoming modem calls and incoming digital calls come over the same digital line. The TAOS unit can accept an incoming call from the network either as a pure digital stream or as a digital stream encoded by pulse code modulation (PCM). A PCM-encoded digital stream contains a digitized version of the analog waveform sent by a device attached to a modem.

A TAOS unit can also convert outgoing data into analog waveforms, convert these waveforms to a PCM-encoded digital stream, and send them to the network over a digital line. The network presents the data to the receiving modem in analog form over an analog line. The data is in exactly the same form as it would be if sent by an analog-based modem. See also analog line, digital line, modem.

digital multimeter

See DMM.

Digital Private Network Signaling System

See DPNSS.

digital signal

A type of signal that uses a limited number of discrete values to encode data transmitted over a wire.

The value of the data encoded in a digital signal depends on the state of the signal during a particular time period. Therefore, the sender and the receiver must synchronize their clocks. Each clock runs at a baud rate, the number of times per second the state of the signal is read or set. Several clocking schemes are available, and digital signals often include clock-timing cues.

A digital signal can transmit analog or digital data. For example, a Compact Disc (CD) encodes analog music data into digital signals, while the wires between computers transmit digital data in digital signals. Compare with analog signal. See also analog data, clock, digital data.

digital signal cross-connect

See DSX.

digital signal level

See DSx.

Digital Signal Processor

See DSP.

digital subscriber line

See DSL.

digital subscriber line access multiplexer

See DSLAM.

digital-to-analog conversion

A process in which a digital signal is modified into an analog signal. Digital-to-analog conversion takes place, for example, when digital data reaches an analog modem. Compare with analog-to-digital conversion. See also analog signal, digital signal, modem.

DIMM

Dual inline memory module. A DIMM is a small assembly of memory chips on a board that contains parallel rows of connectors or pins.

DIP switch

Dual inline package switch. A DIP switch is a small switch that you use to select the operating mode of a device.

direct-access dial-out

A feature that enables terminal-server users to have direct access to a particular Telnet port for modem dial-out. See also modem dial-out.

direct FRAD

See FRAD.

direct frame relay access device

See FRAD.

direct inward dialing

See DID.

direct route

A route that can reach a destination without going through any intervening routers. See also route, router.

DIS

Digital Identification signal. A signal sent at 300 baud by a fax device when it answers a call. A fax device uses the DIS to specify the features that it supports.

discard eligibility

See DE.

disconnect-cause code

A numerical diagnostic code sent from an ISDN switch to data terminal equipment (DTE). A disconnect-cause code indicates why call establishment failed, or why a call was terminated. The disconnect-cause codes are part of ISDN D-channel signaling communications supported by the Signaling System 7 (SS7) supervisory network. When you dial a call from a TAOS unit with ISDN access, the TAOS unit reports the disconnect-cause codes. When the TAOS unit clears the call, it reports a disconnect-cause code, even when inband signaling is in use. A disconnect-cause code is also called a cause element. See also DTE, ISDN, SS7.

Disconnect message

See ISDN Disconnect message.

Disconnect Request ACKed packet

A message that a TAOS unit sends to a client if it finds at least one session to disconnect. Compare with Disconnect Request NAKed packet. See also Disconnect Request packet.

Disconnect Request NAKed packet

A message that a TAOS unit sends to a client if it could not find a session to disconnect. Compare with Disconnect Request ACKed packet. See also Disconnect Request packet.

Disconnect Request packet

A message from a client of a TAOS unit, asking the unit to disconnect the session. See also Disconnect Request ACKed packet, Disconnect Request NAKed packet.

discrete multitone

See DMT.

Disengage Confirmation message

See DCF message.

Disengage Request message

DRQ message.

disk capture feature

A feature that enables your terminal emulator to capture to disk the ASCII characters it receives at its serial port. See also serial port, terminal emulator.

distance-vector metric

A metric that uses a hop count to select the shortest route to a destination network. Routing Information Protocol (RIP) always uses the lowest hop count, regardless of the speed or reliability of a link. Compare with link-state metric. See also RIP.

distinct secret

In a Layer 2 Forwarding (L2F) configuration, a feature that enables you to specify one password to authenticate the network access server (NAS) to the home gateway, and another password to authenticate the home gateway to the NAS. The following sequence of events describes how a TAOS unit uses distinct secrets to authenticate L2F tunnels:

1. A client requests access and the TAOS unit looks up the username and password in the associated Connection profile or RADIUS profile.
2. If an L2F tunnel is specified in the Connection profile or RADIUS profile, the TAOS unit either adds the client connection to an existing tunnel or creates a new tunnel to the specified server end point.
3. If a password is present in the Connection profile or RADIUS profile, the TAOS unit uses this password to authenticate the NAS to the home gateway.
4. The TAOS unit authenticates the home gateway by locating a profile that matches the name provided by the home gateway.
5. The TAOS unit establishes the tunnel between itself and the home gateway.

Compare with shared secret. See also L2F, NAS.

Distributed Queue Dual Bus protocol

See 802.6.

DIX connector

See AUI.

DLC

Digital loop carrier. DLC is equipment that concentrates analog local loop lines by digitizing and multiplexing calls for transmission to the central office (CO). See also CO, local loop, multiplexing.

DLCI

Data link connection identifier. A unique number that the administrator assigns to a permanent virtual circuit (PVC) in a frame relay network. See also frame relay network.

DLCI bundling

A multilink frame relay (MFR) feature that creates data link connection identifier (DLCI) bundles to enable a more flexible use of physical lines. When you use DLCI bundling, a single line can support both bundled and nonbundled connections. In each of the MFR peers, the bandwidth used by the bundled connections must reside on the same card.

In the following example, a company has three offices connected by means of frame relay. Office A receives heavy traffic from Office B, and Office C receives very little traffic from Office B. Office B has two T1 lines to Office A. As shown in Figure 21, instead of installing a third fractional T1 line from Office B to Office C, you can include traffic destined for Office C on one of the existing T1 lines and define a bundle of two DLCIs to Office A.

Figure 21. Example of MFR on per-DLCI basis

Because very little traffic is sent to Office C, most of the bandwidth of the second T1 line is available for traffic to Office A.

See also DLCI, frame relay network, MFR, MFR bundle.

DLE, EOT command

A clear-request signal from the data terminal equipment (DTE) on a T3POS PAD connection. The X.25 T3POS PAD clears the call when it receives a DLE, EOT command. See also DTE, X.25 T3POS.

DLE, EOT timer

A value that specifies the maximum idle time the PAD allows for a T3POS call. The DLE, EOT timer applies only to transparent and blind mode. It is disabled in both local and binary local mode. The DLE, EOT timer is also called the T5 timer. See also binary local mode, blind mode, local mode, transparent mode, X.25 T3POS.

DMM

Digital multimeter. An instrument used for measuring voltage, current, and resistance. Values are displayed digitally rather than by an analog dial.

DMT

Discrete multitone. A method of sending data over copper telephone wires by dividing the frequency range into 256 subfrequencies, from 64kHz to 1.1MHz. Each subfrequency is an independent channel with its own signals.

DMW tone

Digital milliwatt tone. A 1000Hz tone that a Signaling System 7 (SS7) switch sends to a TAOS unit over an IP Device Control (IPDC) protocol link. The DMW tone requests that the unit generate special test calls that measure the line distortion and attenuation in the telephone network. See also IPDC, SS7 network.

DNIS

Dialed Number Information Service. DNIS is a telephone company service that provides information about the called number (for example, the name and location of the target user or device). See also DNIS callback, DNIS fallback.

DNIS callback

The method by which a TAOS unit uses the Dialed Number Information Service (DNIS) information element to detect callback during the ringing state of an incoming call. The unit does not answer the call (go off hook), and the originating caller is not charged for the call. Compare with Ascend callback, CBCP callback, CLID callback. See also callback, DNIS.

DNIS fallback

A feature that enables a TAOS unit to accept a call even after Dialed Number Information Service (DNIS) authentication has failed because of a timeout from the RADIUS server. The TAOS unit takes a given action when it encounters one of the following situations:

The DNIS fallback feature does not apply to DNIS authentication using local profiles. See also DNIS.

DNS

Domain Name System. DNS is a TCP/IP service for centralized management of address resolution. Using DNS, you can specify a symbolic name instead of an IP address. DNS maintains a database of network numbers and corresponding domain names. When you use a symbolic name, DNS translates the domain name into an IP address and sends it over the network. When the Internet service provider receives the message, it uses its own database to look up the symbolic name corresponding to the IP address. See also address resolution, host number, IP address, IP network number, local DNS table, symbolic name.

DNS list attempt

A feature that enables a TAOS unit to avoid disconnecting physical links when a host is unavailable. Domain Name System (DNS) can return multiple addresses for a hostname in response to a DNS query, but it does not include information about availability of the hosts. A user typically attempts to connect to the first address in the list. If that host is unavailable, the user must try the next host, and so forth. However, if the access attempt occurs automatically as part of immediate services, the physical connection is broken when the initial connection attempt fails. When you enable the DNS list attempt feature, the TAOS unit tries one entry in the DNS list of hosts, and if that connection fails, tries the next entry, and so on, without closing the WAN session. See also DNS, L2TP list attempt.

domain identifier

The portion of a domain name that appears last and specifies the type of organization to which the host belongs. The Internet Network Information Center (InterNIC) provides the following domain identifiers:

domain name

The portion of a symbolic name that corresponds to the network number in the IP address. In the symbolic name steve@abc.com, the domain name is abc.com. See also IP address, IP network number.

Domain Name System

See DNS.

domain-specific part

See DSP.

DoS attack

Denial of service attack. A DoS attack is a deliberate attempt to interfere with network performance by directing forged Internet Control Message Protocol (ICMP) Echo Request packets to IP broadcast addresses.

Under ordinary circumstances, to determine whether a machine on the Internet is connected and responding, a host sends an ICMP Echo Request packet. If a machine receives the packet, it returns an ICMP Echo Reply packet. In a DoS attack, however, an attacker directs ICMP Echo Request packets to IP broadcast addresses from one or more remote locations. An intermediary receives an ICMP Echo Request packet directed to the IP broadcast address of its network. If the intermediary does not filter ICMP traffic directed to IP broadcast addresses, the machines on the network receive the ICMP Echo Request packet, and each sends an ICMP Echo Reply packet in return.

The packets from the attacker do not use the IP address of the source machine as the source address. Instead, they contain the spoofed source address of the intended victim. When all the machines at the intermediary's site respond to the ICMP Echo Requests, they send replies to the victim's device. An attacker can send DoS attacks to multiple intermediaries at the same time, causing all of the intermediaries to direct responses to the same victim. Both the intermediary and victim of a DoS attack can suffer severely degraded network performance.

To protect against DoS attacks, you should disable IP-directed broadcasts on the TAOS unit. By disabling these broadcasts, you deny an attacker the ability to direct IP broadcast traffic onto your network. In addition, you should prevent the TAOS unit from responding to ICMP packets sent to IP broadcast addresses. If someone compromises a machine on your network, he or she might try to launch an attack using the TAOS unit as an intermediary, sending the ICMP Echo Request packet to the IP broadcast address of the local network. Because this traffic does not travel through a router to reach the machines on the local network, disabling IP-directed broadcasts on the TAOS unit is not sufficient to prevent a DoS attack. You must also prevent the TAOS unit from responding to ICMP packets sent to the local broadcast address. See also Echo signal, ICMP.

dotted decimal notation

A system for specifying an IP address or subnet mask. In dotted decimal notation, each of the four portions of the IP address or mask is separated from the others by a decimal point, as in the address 200.10.5.1. See also IP address, subnet mask.

downstream path

The path a call takes from a carrier's central office (CO) to the end user's home.

DPC

Destination point code. In a Signaling System 7 (SS7) network, the DPC identifies the destination of the signaling message. See also SS7 network.

DPNSS

Digital Private Network Signaling System. DPNSS is a standard that defines how different private branch exchange (PBX) systems can interoperate to produce a single virtual PBX. See also PBX.

DQDB protocol

See 802.6.

DR

See designated router.

DRAM

Dynamic RAM. DRAM is a kind of memory whose information resides in capacitors. The charge of each capacitor must be periodically refreshed. Compare with EEPROM, NVRAM, RAM.

drop-and-insert

A feature that enables a single T1 line to carry both data and voice traffic. The TAOS unit specifies a preallocated portion of the T1 line to use both dedicated and switched circuits for LAN internetworking. The remaining portion of the line can go to a PBX with a T1 interface. The PBX can access both dedicated and switched circuits for voice purposes. You can also use drop-and-insert to share access-line bandwidth between the TAOS unit and equipment other than a PBX (for example, a channel bank or T1 multiplexer). See also channel bank, dedicated circuit, multiplexer, PCM, switched circuit, T1 line.

drop-and-insert multiplexer

A T1 or E1 multiplexer that enables organizations to bypass the PBX for data communications. The Drop-and Insert-configuration is used to allocate channels to different equipment, such as channel banks and digital access cross-connects (DACs). The drop-and-insert multiplexer drops off some of the channels to the data devices. It then stuffs bits into the channels it drops off, so that a full T1 or E1 transmission is sent to the PBX's T1 or E1 multiplexing equipment. See also channel bank, DAC, multiplexer, PBX.

DRQ message

Disengage Request message. An H.323 registration, admission, and status (RAS) message sent from a MultiVoice® gateway to a MultiVoice® Access Manager (MVAM) device when a call ends. Compare with ARQ message. See also DCF message, H.323, MultiVoice®, MVAM, RAS.

DS0 channel

A 64Kbps D channel on a digital line. See also DS1 channel.

DS0 minute

The online usage of a single 56Kbps or 64Kbps switched channel for one minute. For example, a 5-minute, six-channel call uses 30 DS0 minutes.

DS1 channel

For a T1 line, a 1.544Mbps channel that consists of 24 DS0 channels and an extra framing bit; for an E1 line, a 2.048Mbps channel that consists of 32 DS0 channels. On a T1 line, a DS1 channel uses either the D4 or ESF method of framing. See also D4-framed T1 line, DS0 channel, DS3 line, E1 line, ESF, T1 line.

DS2 channel

For a T1 line, a 6.312Mbps channel that consists of four DS1 channels; for an E1 line, an 8.45Mbps channel that consists of four DS1 channels. See also DS1 channel.

DS3 line

A 44.736Mbps line consisting of seven DS2 channels. A DS3 line is also called a T3 line. See also DS2 channel, T3 line.

DSAP

Destination service access point. A DSAP is the service access point (SAP) address at which the Logical Link Control (LLC) protocol layer passes information to a Network-layer process. Compare with SSAP. See also SAP.

DSCP

Differentiated services code point. A DSCP is composed of six bits in the DS field of an IP packet's IPv4 TOS octet or IPv6 Traffic Class octet. The DSCP is used to specify the way that a packet is forwarded at each node. In a DSCP, the leftmost bit signifies bit 0 (zero) of the DS field, and the rightmost bit signifies bit 5. See also differentiated services, DSCP.

DS field

Differentiated services field. A field in an IP packet's IPv4 TOS octet or IPv6 Traffic Class octet. The bit pattern of the DS field specifies the way in which a packet is forwarded at each network node. The DS field is described in RFC2474 and RFC 2475. See also differentiated services, DSCP.

DSL

Digital subscriber line. DSL is a technology that provides high bandwidth over the conventional copper wiring that makes up the local loop of the Public Switched Telephone Network (PSTN). DSL links bypass the network's circuit-switched lines and provide much faster data-transmission rates than analog modem connections. See also ADSL, HDSL, IDSL, PSTN, RADSL, SDSL, SHDSL, VDSL.

DSLAM

Digital subscriber line access multiplexer. Located at the telephone company's central office (CO), a DSLAM concentrates the data traffic from multiple loops onto a single Asynchronous Transfer Mode (ATM) line. Some DSLAMs provide added features such as extensive call routing and comprehensive network management. See also ATM, CO, DSL, MultiDSL, NSP, T1 line, T3 line.

DSLMAX

A comprehensive digital subscriber line access multiplexer (DSLAM) providing digital subscriber line (DSL) access concentration and circuit termination. Designed for high-speed low-density WAN access, the DSLMAX unit is an integrated Layer-2 (multiplexing) and Layer-3 (routing) device that uses TAOS. See also DSL, DSLAM, TAOS.

DSL modem

See DSL remote transceiver unit.

DSLPipe

A Lucent Technologies product that enables you to take advantage of high-speed SDSL services. This powerful router lets you access multiple destinations simultaneously to download high-resolution graphics, gain access to multimedia applications, or transmit large files from a corporate intranet. See also DSL, DSLPipe plug-and-play.

DSLPipe plug-and-play

A feature that enables a DSLPipe to obtain its configuration through a TAOS unit by using the Dynamic Host Configuration Protocol (DHCP) and Trivial File Transfer Protocol (TFTP). DSLPipe units ship with the plug-and-play feature enabled, so they require absolutely no configuration (provided that the TAOS unit and servers have been configured properly).

When a DSLPipe unit starts up, it uses factory default settings that enable it to forward a DHCP request to a TAOS unit. The TAOS unit sends the request to a DHCP server. The DHCP server returns an IP address, subnet mask, the path to a more detailed configuration file, and a TFTP server hostname. The TAOS unit forwards the DHCP response to the requesting DHCP client.

Figure 22 illustrates how plug-and-play works.

Figure 22. DSLPipe unit obtaining its configuration (plug-and-play)

To gain access to the specified TFTP server and its configuration file, the DSLPipe unit uses the minimal configuration obtained by means of DHCP. After downloading the file, the unit begins using the configuration. See also DHCP, DHCP server, DSL, DSLPipe, TFTP.

DSL remote transceiver unit

A device that bridges or routes data between two LANs across a DSL connection. With router-based support for IP traffic, a DSL remote transceiver unit can create and maintain subnets to allow segmentation of the LAN. Multicast and unicast traffic recognition is also supported. In addition, router support can provide privacy, security, and protection against such LAN problems as broadcast storms.

Typically, a DSL remote transceiver unit consists of one DSL port, one Ethernet port, and a console port used for management. Units operate in pairs across the local loop. The type of unit used by the subscriber must match the one used at the central office (CO).

A DSL remote transceiver unit is also called a DSL modem. See also CO, DSL, local loop.

DSL Terminator

A unit that receives and routes the data on the virtual circuits provided by the digital subscriber line access multiplexers (DSLAMs) that physically terminate digital subscriber line (DSL) copper wiring. As a Layer-3 (routing) device, a DSL Terminator can be placed in remote locations or in central data centers. See also DSL, DSLAM, virtual circuit.

DSP

(1) Digital Signal Processor. A DSP analyzes and processes analog signals, converting them to digital format. See also analog signal, digital signal.

(2) Domain-specific part. The DSP is a portion of an Asynchronous Transfer Mode (ATM) address in ATM end system address (AESA) format. The DSP specifies the high-order domain-specific part (HO-DSP), end system identifier (ESI), and selector (SEL) subfields. Compare with IDP. See also AESA format, ESI, HO-DSP, SEL subfield.

DSR signal

Data Set Ready signal. DSR is a signal that a modem transmits when it is ready to send and receive data. See also modem.

DSU

Data service unit. Along with a channel service unit (CSU), a DSU is a component of data circuit-terminating equipment (DCE). The DSU connects to data terminal equipment (DTE) by means of a synchronous serial interface, such as a V.35, RS-422, or RS-423 connection. The DSU formats and controls the flow of digital data between the network and the CSU. See also CSU, DCE, digital data, DTE, RS-422, RS-423, V.35.

DSx

Digital signal level. DSx is a physical-layer designation associated with T-carrier and E-carrier circuits. Many people use the T1 or E1 and physical-layer designations interchangeably. However, DSx is a service designation that refers to the actual speed of the connection, while the T or E designation refers to the carrier type. See also DS0 channel, DS1 channel, DS2 channel, DS3 line, E-carrier circuit, T-carrier circuit.

DSX

Digital signal cross-connect. DSX is a method of connecting DS1 and DS3 signals by linking T1 and T3 lines. See also DS1 channel, DS3 line, T1 line, T3 line.

DTE

Data terminal equipment. DTE refers to a device that an operator uses (for example, a computer or a terminal). Compare with DCE.

DTE--DTE aggregation

A frame relay configuration that enables two data terminal equipment (DTE) devices to use the aggregate bandwidth of a multilink frame relay (MFR) bundle across a regular frame relay (non-MFR) network. The fact that aggregate bandwidth of multiple links is in use is transparent to the frame relay switching equipment that resides between MFR peers.

Figure 23 shows two DTE devices using an MFR bundle of three data links through a frame relay network.

Figure 23. MFR DTE-DTE aggregation

To aggregate the bandwidth, a TAOS unit uses a segmentation-sequencing-reassembly protocol described in the frame relay fragmentation implementation agreement FRF.12, which is based on the Multilink PPP (MP) described in RFC 1990. See also DTE, frame relay, MFR, MFR bundle, MP.

DTL

Destination transit list. A list of nodes and link IDs that indicate a path across a Private Network-to-Network Interface (PNNI) peer group. See also PNNI protocol, PNNI peer group.

DTMF

Dual tone multifrequency. DTMF is a technology that enables a touch-tone telephone to create 16 tones that use only eight frequencies.

DTMF-R2 signaling

Dual tone multifrequency R2 signaling. The dual-tone signaling standard used for European digital trunk signaling. Compare with MFC-R2 signaling.

DTR signal

Data Transmit Ready signal. DTR is a signal indicating that a device is ready to transmit and receive data.

dual-capacity card

A slot card that performs the functions of both a host card and a network card. See also host card, network card, slot card.

dual inline memory module

See DIMM.

dual inline package switch

See DIP switch.

dual IP

A method of assigning a second IP address to the Ethernet interface in order to give the TAOS unit a logical interface on two networks (or subnets) on the same backbone. See also IP, IP address.

dual latency

On a Stinger® unit, a feature that enables you to split a digital subscriber line (DSL) data stream into multiple subchannels for parallel data transport. See also DSL.

dual-port call

A call in which a coder-decoder (codec) performs inverse multiplexing on two channels in order to achieve twice the bandwidth of a single channel. The codec provides two ports, one for each channel. Two AIM ports on the TAOS unit connect a dual-port call to the codec. These ports are the primary port and the secondary port. Because the TAOS unit places the two calls in tandem and clears the calls in tandem, it considers them a single call. See also AIM port, codec, inverse multiplexing.

dual tone multifrequency

See DTMF.

dual tone multifrequency R2 signaling

See DTMF-R2 signaling.

DXI

Data Exchange Interface. Described in RFC 1483, DXI defines how a network device can convert data for transmission between different network services. For Asynchronous Transfer Mode (ATM) configurations, DXI is an interface between data terminal equipment (DTE) and an ATM channel service unit/data service unit (CSU/DSU). The ATM CSU/DSU carries out the process of conversion between variable-length DXI frames and fixed-length ATM cells. See also ATM, ATM cell, CSU, DSU, DTE.

Dynamic Bandwidth Allocation protocol

See DBA protocol.

dynamic bandwidth overflow

A method of supplementing bandwidth during periods of peak demand. Through the mechanism of inverse multiplexing, a TAOS unit adds bandwidth when traffic reaches a specified level. See also DBA protocol, inverse multiplexing.

Dynamic Host Configuration Protocol

See DHCP.

dynamic IP

The process of assigning an IP address to a dial-in caller from an IP address pool. Figure 24 shows a TAOS unit assigning an address from one of its defined pools.

Figure 24. Dial-in host requiring assigned IP address

See also IP address, IP address pool.

dynamic RAM

See DRAM.

dynamic rate adaptation

A process in which a line adjusts its bit rate dynamically on the basis of specified noise margins and noise-level intervals. The line upshifts to increase its bit rate or downshifts to reduce it.

dynamic route

A path that a router learns by means of dynamic updates from other routers, rather than by means of a static specification in a configured profile. Routers that use Routing Information Protocol (RIP) broadcast their entire routing tables every 30 seconds, updating other routers about which routes are usable. Hosts that run Internet Control Message Protocol (ICMP) can also send ICMP Redirect packets to offer a better path to a destination network. To update their routing tables, Open Shortest Path First (OSPF) routers propagate link-state changes as they occur. Compare with multipath route, static IP route. See also IP route, IPX route, route.

dynamic routing

A routing technique that enables a message's route to change as the message proceeds along the network.

E

E.164

A standard for public-network addressing, using up to 15 digits. See also E.164 AESA format.

E.164 AESA format

E.164 ATM end system address format. In E.164 AESA format, the E.164 address is specified in international format. Compare with custom AESA format, DCC AESA format, E.164, ICD AESA format. See also AESA format.

E.164 address

See native E.164 address.

E1 line

A 2.048Mbps line that supports 32 64Kbps channels, each of which can transmit and receive data or digitized voice. The line uses framing and signaling to achieve synchronous and reliable transmission. The most common configurations for E1 lines are E1 PRI and unchannelized E1. See also channelized T1 PRI or E1 PRI, E1 PRI line, unchannelized T1 PRI or E1 PRI.

E1 PRI line

E1 Primary Rate Interface line. An E1 PRI line consists of 32 64Kbps channels. It uses 30 B channels for user data, 1 64Kbps D channel for ISDN D-channel signaling, and 1 framing channel. The B channels can be all switched, all dedicated, or a combination of switched and dedicated. The E1 PRI line is a standard in Europe and Asia, but is called CEPT G.703 on those continents. Compare with ISDN BRI line, T1 PRI line. See also B channel, channelized T1 PRI or E1 PRI, D channel, dedicated channel, E1 line, ISDN D-channel signaling, switched channel, unchannelized T1 PRI or E1 PRI.

E1 Primary Rate Interface line

See E1 PRI line.

E1-R2 Israeli signaling

See R2 signaling.

E2 line

An 8.45Mbps line that supports four 2.048Mbps E1 channels.

E3 line

A 34Mbps line that supports 16 2.048Mbps E1 channels.

early packet discard

See EPD.

early ringback

In a MultiVoice® environment, a ringback tone that the local gateway sends to the caller as soon as a call is started on the remote gateway. Early ringback is intended only for networks with long call setup times, such as satellite IP networks, wireless networks, and networks using channel-associated signaling (CAS). See also CAS, MultiVoice®, ringback tone.

EAS

External authentication server. See authentication server.

EAZ

Endgeraete Auswahl Ziffer. For calls over German 1TR6 lines, EAZ is a one-digit string appended to the telephone number.

EBGP

Exterior Border Gateway Protocol. EBGP is the Border Gateway Protocol (BGP) used between peers in different autonomous systems, or, when confederations are in use, between peers in different confederation member autonomous systems (CMASs). Unlike internal BGP peers, EBGP peers need not have full connectivity with one another. See also autonomous system, BGP, CMAS.

E-carrier circuit

An E1 or E3 circuit. In Europe, the Pacific Rim, the Middle East, and Latin America, E-carrier circuits are the most widely used digital communications circuits. An E1 circuit has a total capacity of 2.048Mbps. Each E1 circuit can be divided into 32 channels of 64Kbps each. An E3 circuit is equivalent to 16 E1 circuits and has a total capacity of 34.368Mbps. E1 and E3 circuits are commonly used in central sites to support network services and applications similar to those handled by T1 and T3 circuits. Compare with T-carrier circuit. See also E1 line, E3 line.

Echo signal

A signal that determines whether a node can receive and acknowledge data transmissions. A host sends an Echo Request packet. If the destination is properly connected and receives the Echo Request packet, it sends back an Echo Reply packet.

echo cancellation

A method that the telephone company uses to remove echoes from an analog line. See also analog line, echo tail.

echo tail

The amount of hybrid-line echo that the G.165 echo canceller can eliminate in Voice over IP (VoIP) calls. The echo canceller is designed to eliminate the echo generated when a voice call is transmitted across a two-wire/four-wire boundary. The echo canceller is applied to the speech on the trunk side of a TAOS unit. It does not suppress the acoustic echo that is normally generated at the calling end point (receiver/transmitter echo). When the length of the hybrid-line echo exceeds 32 milliseconds, the echo will be detected at the distant end point. See also echo cancellation.

echo test

A diagnostic test, used to verify network reachability, in which an Internet Control Message Protocol (ICMP) Echo Request packet or Simple Network Management Protocol (SNMP) test packet is sent to elicit a standard response. See also ICMP, SNMP.

ECM

Error correction mode. In the event that a frame is not received correctly during a real-time fax call, ECM enables fax frames to be retransmitted. ECM frames are relayed end to end between terminals. See also real-time fax over IP.

ECN

Explicit congestion notification. ECN is a method of informing frame relay nodes that there is traffic congestion on the network. The frame relay header can use a backward explicit congestion notification (BECN) bit or a forward explicit congestion notification (FECN) bit to notify nodes of traffic congestion. See also BECN, congestion, congestion management, FECN, frame relay.

EDAC

Error detection and correction. EDAC is a method of determining whether transmission errors have occurred. In the event of an error, EDAC makes the necessary corrections. See also error correction.

EDT

External diagnostic test. On a Stinger® unit, an EDT puts the DSL remote transceiver unit into loopback mode. If a complete circuit is established, you can then send a data stream from an external device (such as a network traffic generator) through the trunk to the modem. The modem loops back the data stream to the external device, where it can be analyzed. Compare with IDT. See also DSL remote transceiver unit, Stinger®.

EEPROM

Electrically erasable programmable read-only memory. EEPROM is a type of programmable read-only memory (PROM) that can be erased by exposing it to an electrical charge. EEPROM retains its contents across resets and power cycles. Data is written or erased one byte at a time. See also PROM.

EFCI

Explicit forward congestion indication. In Asynchronous Transfer Mode (ATM), EFCI is a field in a cell header. The EFCI field is set to notify the destination device of a nearly congested or fully congested network condition. See also ATM, congestion.

EFCI marking

A type of marking that signals to the next Asynchronous Transfer Mode (ATM) switch that the TAOS unit's buffer thresholds are exceeded. The unit sets the explicit forward congestion indication (EFCI) bit in the header of transmitted data cells. The destination system stores the EFCI state and includes it in the congestion-indication bit of its resource management (RM) cells. The source system then uses the information it obtains from the RM cells to adjust its cell transmission rate.

Figure 25 shows a TAOS unit setting the EFCI bit to 1 in its transmitted data cells. The destination system returns RM cells that will cause the source system to slow down its transmissions.

Figure 25. Congestion management with EFCI

See also RM cell.

EGP

Exterior Gateway Protocol. EGP is a type of protocol used to exchange routing information between one Open Shortest Path First (OSPF) autonomous system and another. The autonomous-system number can be used by area border routers (ABRs) to filter out certain EGP routing information. OSPF can make use of EGP data generated by other border routers and added to the OSPF system as autonomous system externals (ASEs). See also ABR, ASE, autonomous system, OSPF protocol.

EIA

Electronic Industries Association. The EIA is a group that determines standards for electrical transmission.

EIA-449

A Physical-layer standard also known as RS-449. See also RS-449.

EIA-530

A way of referring to RS-422 and RS-423. See also RS-422, RS-423.

EIA/TIA-232

A Physical-layer standard nearly identical to V.24. EIA/TIA-232 is also known as RS-232. See also RS-232.

electrically erasable programmable read-only memory

See EEPROM.

Electronic Industries Association

See EIA.

embedded operations channel

See EOC.

en-bloc dialing

The process of sending all dialed digits to a TAOS unit in one block. When a TAOS unit uses Q.931 en-bloc dialing, the Setup message sent by the dialing unit to the network switch contains all information required to process the call. Therefore, the TAOS unit can obtain the dial number from a called-number Information Element, instead of from the keypad-facility Information Element used by overlap-dialing devices. See also en-bloc receiving, Q.931.

en-bloc receiving

A feature that affects the procedure of establishing an incoming call received on a T1 PRI or E1 PRI line on a TAOS unit. When a TAOS unit uses en-bloc receiving, the Setup message received from the network switch must contain all information required to process the call. See also en-bloc dialing. Compare with overlap receiving.

Encapsulating Security Payload

See ESP.

encapsulation

A technique used by layered protocols. A low-level protocol accepts a message from a higher-level protocol and places the message in the data portion of the lower-level frame. The logistics of encapsulation require that packets traveling over a physical network contain a sequence of headers. Encapsulation enables the transmission of data over networks that use differing protocols. See also protocol.

encryption

A process that takes ordinary data and converts it into a format unreadable to anyone without a decryption key. Authorized personnel with access to this key can unscramble the information. Data encryption is a useful tool against network snoopers. See also private-key encryption, public-key encryption.

end-of-pulse signaling

See EOP signaling.

end point

When a tunneling protocol is in use, the system that encapsulates the packets (the Foreign Agent) or the system that decapsulates the packets (the tunnel server). Examples of tunneling protocols are Ascend Tunnel Management Protocol (ATMP), Layer 2 Tunneling Protocol (L2TP), and Point-to-Point Tunneling Protocol (PPTP). See also ATMP, L2TP, PPTP.

end point discriminator option

On a Multilink PPP (MP) or Multilink Protocol Plus (MP+) link, a method of identifying the system transmitting the packet. The end point discriminator option indicates to a system whether the peer on the link could be the same as the peer on another connection. If the option distinguishes the peer from all others, the system must establish a new bundle for the link. See also bundle, MP, MP+.

end system identifier

See ESI.

end-to-end delay

An Asynchronous Transfer Mode (ATM) routing metric that measures the time it takes a cell to get from one end of a connection to the other. See also admin cost, ATM, CDV.

Enhanced Throughput Cellular

See ETC.

Enigma

An important provider of network security applications. Enigma's SafeWord AS (also known as the Enigma Logic SafeWord server) is a UNIX-based software authentication server that identifies users by means of dynamic passwords (called tokens). The server identifies users at the point of connection to a TCP/IP network, and uses standard network authentication protocols and token cards. See also SafeWord authentication, SafeWord token, token, token card, token-card authentication, token-card server.

ENQ character

A control character that signifies a request for identification or status on an X.25 T3POS connection. See also ENQ handling timer, X.25 T3POS.

ENQ handling timer

A value that specifies the amount of time the packet assembler/disassembler (PAD) waits for an ENQ from the host on an X.25 T3POS connection. See also ENQ character, PAD, X.25 T3POS.

enterprise-wide network

A network that contains all or most of a company's hardware and software resources. Typically, an enterprise-wide network includes computers that run different operating systems and reside on different types of networks. Therefore, achieving interoperability is the biggest challenge facing the administrator of an enterprise-wide network.

environment variable

A system-defined or user-defined variable that provides information to the UNIX shell about the operating environment.

EOC

Embedded operations channel. In a BRI-U interface, an EOC is the out-of-band mechanism for implementing maintenance functions. Instead of using the D or B channels, EOC uses the maintenance bits of the U-interface superframe. Maintenance functions include test loopback, statistics gathering, and requests to generate errors (to verify that the block-error counters work). See also B channel, D channel.

EOP signaling

End-of-pulse signaling. A method for determining when transmission of a dial string is complete. Collection of the dialed digits is considered complete when the listening device detects an interval of silence exceeding the interval between dialed digits.

EPD

Early packet discard. EPD is an Asynchronous Transfer Mode (ATM) flow-control mechanism. The ATM flow-control processor performs EPD for unspecified bit rate (UBR), available bit rate (ABR), and variable bit rate (VBR)-nonreal time virtual channels. If a cell causes the queue for a virtual channel to exceed the discard thresholds, the virtual channel enters the EPD state. The cells in the current packet of the virtual channel are admitted to the queue. However, when the end of the current packet is detected, subsequent cells are discarded. Compare with selective discard, PPD. See also ABR, ATM, ATM Flow-Control processor, UBR, VBR-nonreal time, virtual channel.