Glossary

A

A&B bit signaling

Used in T1 transmission facilities and sometimes called “24th channel signaling.” Each of the 24 T1 subchannels in this procedure uses one bit of every sixth frame to send supervisory signaling information.

AAL

ATM Adaptation Layer: A service-dependent sublayer of the Data Link layer, which accepts data from other applications and brings it to the ATM layer in 48-byte ATM payload segments. CS and SAR are the two sublayers that form AALs. Currently, the four types of AAL recommended by the ITU-T are AAL1, AAL2, AAL3/4, and AAL5. AALs are differentiated by the source-destination timing they use, whether they are CBR or VBR, and whether they are used for connection-oriented or connectionless mode data transmission. See also: AAL1, AAL2, AAL3/4, AAL5, ATM, and ATM layer.

AAL1

ATM Adaptation Layer 1: One of four AALs recommended by the ITU-T, it is used for connection-oriented, time-sensitive services that need constant bit rates, such as isochronous traffic and uncompressed video. See also: AAL.

AAL2

ATM Adaptation Layer 2: One of four AALs recommended by the ITU-T, it is used for connection-oriented services that support a variable bit rate, such as voice traffic. See also: AAL.

AAL3/4

ATM Adaptation Layer 3/4: One of four AALs (a product of two initially distinct layers) recommended by the ITU-T, supporting both connectionless and connection-oriented links. Its primary use is in sending SMDS packets over ATM networks. See also: AAL.

AAL5

ATM Adaptation Layer 5: One of four AALs recommended by the ITU-T, it is used to support connection-oriented VBR services primarily to transfer classical IP over ATM and LANE traffic. This least complex of the AAL recommendations uses SEAL, offering lower bandwidth costs and simpler processing requirements but also providing reduced bandwidth and error-recovery capacities. See also: AAL.

AARP

AppleTalk Address Resolution Protocol: The protocol in an AppleTalk stack that maps data-link addresses to network addresses.

AARP probe packets

Packets sent by the AARP to determine whether a given node ID is being used by another node in a nonextended AppleTalk network. If the node ID is not in use, the sending node appropriates that node’s ID. If the node ID is in use, the sending node will select a different ID and then send out more AARP probe packets. See also: AARP.

ABM

Asynchronous Balanced Mode: When two stations can initiate a transmission, ABM is an HDLC (or one of its derived protocols) communication technology that supports peer-oriented, point-to-point communications between both stations.

ABR

Area Border Router: An OSPF router that is located on the border of one or more OSPF areas. ABRs are used to connect OSPF areas to the OSPF backbone area.

Access layer

The layer at which user workstations and servers connect in the Cisco three-layer hierarchical design model.

access list

An itemization kept by routers that determines access to and from the router for various services on the network.

access method

The way network devices approach gaining access to the network itself.

access server

Also known as a “network access server,” it is a communications process connecting asynchronous devices to a LAN or WAN through network and terminal emulation software, providing synchronous or asynchronous routing of supported protocols.

acknowledgment

Verification sent from one network device to another signifying that an event has occurred. May be abbreviated as ACK. Contrast with: NAK.

ACR

Allowed Cell Rate: A designation defined by the ATM Forum for managing ATM traffic. Dynamically controlled using congestion control measures, the ACR varies between the minimum cell rate (MCR) and the peak cell rate (PCR). See also: MCR and PCR.

active monitor

The mechanism used to manage a Token Ring. The network node with the highest MAC address on the ring becomes the active monitor and is responsible for management tasks such as preventing loops and ensuring tokens are not lost.

address mapping

By translating network addresses from one format to another, this methodology permits different protocols to operate interchangeably.

address mask

A bit combination descriptor identifying which portion of an address refers to the network or subnet and which part refers to the host. Sometimes simply called the mask. See also: subnet mask.

address resolution

The process used for resolving differences between computer addressing schemes. Address resolution typically defines a method for tracing Network layer (Layer 3) addresses to Data Link layer (Layer 2) addresses. See also: address mapping.

adjacency

The relationship made between defined neighboring routers and end nodes, using a common media segment, to exchange routing information.

administrative distance

A number between 0 and 225 that expresses the value of trustworthiness of a routing information source. The lower the number, the higher the integrity rating.

administrative weight

A value designated by a network administrator to rate the preference given to a network link. It is one of four link metrics exchanged by PTSPs to test ATM network resource availability.

ADSU

ATM Data Service Unit: The terminal adapter used to connect to an ATM network through an HSSI-compatible mechanism. See also: DSU.

advertising

The process whereby routing or service updates are transmitted at given intervals, allowing other routers on the network to maintain a record of viable routes.

AEP

AppleTalk Echo Protocol: A test for connectivity between two AppleTalk nodes where one node sends a packet to another and receives an echo, or copy, in response.

AFI

Authority and Format Identifier: The part of an NSAP ATM address that delineates the type and format of the IDI section of an ATM address. See also: IDI and NSAP.

AFP

AppleTalk Filing Protocol: A presentation-layer protocol, supporting AppleShare and Mac OS File Sharing, that permits users to share files and applications on a server.

AIP

ATM Interface Processor: Supporting AAL3/4 and AAL5, this interface for Cisco 7000 series routers minimizes performance bottlenecks at the UNI. See also: AAL3/4 and AAL5.

algorithm

A set of rules or process used to solve a problem. In networking, algorithms are typically used for finding the best route for traffic from a source to its destination.

alignment error

An error occurring in Ethernet networks, in which a received frame has extra bits; that is, a number not divisible by eight. Alignment errors are generally the result of frame damage caused by collisions.

all-routes explorer packet

An explorer packet that can move across an entire SRB network, tracing all possible paths to a given destination. Also known as an all-rings explorer packet. See also: explorer packet, local explorer packet, and spanning explorer packet.

AM

Amplitude Modulation: A modulation method that represents information by varying the amplitude of the carrier signal. See also: modulation.

AMI

Alternate Mark Inversion: A line-code type on T1 and E1 circuits that shows zeros as “01” during each bit cell, and ones as “11” or “00,” alternately, during each bit cell. The sending device must maintain ones density in AMI but not independently of the data stream. Also known as binary-coded, alternate mark inversion. Contrast with: B8ZS. See also: ones density.

Amplitude

An analog or digital waveform’s highest value.

Analog transmission

Signal messaging whereby information is represented by various combinations of signal amplitude, frequency, and phase.

ANSI

American National Standards Institute: The organization of corporate, government, and other volunteer members that coordinates standards-related activities, approves U.S. national standards, and develops U.S. positions in international standards organizations. ANSI assists in the creation of international and U.S. standards in disciplines such as communications, networking, and a variety of technical fields. It publishes over 13,000 standards, for engineered products and technologies ranging from screw threads to networking protocols. ANSI is a member of the IEC and ISO. See also: IEC and ISO.

anycast

An ATM address that can be shared by more than one end-system, allowing requests to be routed to a node that provides a particular service.

AppleTalk

Currently in two versions, the group of communication protocols designed by Apple Computer for use in Macintosh environments. The earlier Phase 1 protocols support one physical network with only one network number that resides in one zone. The later Phase 2 protocols support more than one logical network on a single physical network, allowing networks to exist in more than one zone. See also: zone.

Application layer

Layer 7 of the OSI reference network model, supplying services to application procedures (such as electronic mail or file transfer) that are outside the OSI model. This layer chooses and determines the availability of communicating partners along with the resources necessary to make the connection, coordinates partnering applications, and forms a consensus on procedures for controlling data integrity and error recovery.

ARA

AppleTalk Remote Access: A protocol for Macintosh users establishing their access to resources and data from a remote AppleTalk location.

area

A logical, rather than physical, set of segments (based on either CLNS, DECnet, or OSPF) along with their attached devices. Areas are commonly connected to others using routers to create a single autonomous system. See also: autonomous system.

ARM

Asynchronous Response Mode: An HDLC communication mode using one primary station and at least one additional station, in which transmission can be initiated from either the primary or one of the secondary units.

ARP

A protocol within TCP/IP (Transmission Control Protocol/Internet Protocol) and AppleTalk networks that allows a host to find the physical address of a node on the same network when it knows only the target’s logical or IP address.

Under ARP, a network interface card contains a table (known as the address resolution cache) that maps logical addresses to the hardware addresses of nodes on the network. When a node needs to send a packet, it first checks the address resolution cache to see if the physical address information is already present. If so, that address is used, and network traffic is reduced; otherwise, a normal ARP request is made to determine the address. See also: RARP.

ASBR

Autonomous System Boundary Router: An area border router placed between an OSPF autonomous system and a non-OSPF network that operates both OSPF and an additional routing protocol, such as RIP. ASBRs must be located in a non-stub OSPF area. See also: ABR, non-stub area, and OSPF.

ASCII

American Standard Code for Information Interchange: An eight- bit code for representing characters, consisting of seven data bits plus one parity bit.

application-specific integrated circuit

Abbreviated ASIC. A computer chip developed for a specific purpose, designed by incorporating standard cells from a library rather than created from scratch. Also known
as gate arrays, ASICs are found in all sorts of appliances, including modems, security systems, digital cameras, and even microwave ovens and automobiles.

ASN.1

Abstract Syntax Notation One: An OSI language used to describe types of data that is independent of computer structures and depicting methods. Described by ISO International Standard 8824.

ASP

AppleTalk Session Protocol: A protocol employing ATP to establish, maintain, and tear down sessions, as well as sequence requests. See also: ATP.

AST

Automatic Spanning Tree: A function that supplies one path for spanning explorer frames traveling from one node in the network to another, supporting the automatic resolution of spanning trees in SRB networks. AST is based on the IEEE 802.1 standard. See also: IEEE 802.1 and SRB.

asynchronous transmission

Digital signals sent without precise timing, usually with different frequencies and phase relationships. Asynchronous transmissions generally enclose individual characters in control bits (called start and stop bits) that show the beginning and end of each character. Contrast with: isochronous transmission and synchronous transmission.

ATCP

AppleTalk Control Protocol: The protocol for establishing and configuring AppleTalk over PPP, defined in RFC 1378. See also: PPP.

ATDM

Asynchronous Time-Division Multiplexing: A technique for sending information, it differs from normal TDM in that the time slots are assigned when necessary rather than preassigned to certain transmitters. Contrast with: FDM, statistical multiplexing, and TDM.

ATG

Address Translation Gateway: The mechanism within Cisco DECnet routing software that enables routers to route multiple, independent DECnet networks and to establish a user-designated address translation for chosen nodes between networks.

ATM

Asynchronous Transfer Mode: The international standard, identified by fixed-length 53-byte cells, for transmitting cells in multiple service systems, such as voice, video, or data. Transit delays are reduced because the fixed-length cells permit processing to occur in the hardware. ATM is designed to maximize the benefits of high-speed transmission media, such as SONET, E3, and T3.

ATM ARP server

A device that supplies logical subnets running classical IP over ATM with address-resolution services.

ATM endpoint

The initiating or terminating connection in an ATM network. ATM endpoints include servers, workstations, ATM-to-LAN switches, and ATM routers.

ATM Forum

The international organization founded jointly by Northern Telecom, Sprint, Cisco Systems, and NET/ADAPTIVE in 1991 to develop and promote standards-based implementation agreements for ATM technology. The ATM Forum broadens official standards developed by ANSI and ITU-T and creates implementation agreements before official standards are published.

ATM layer

A sublayer of the Data Link layer in an ATM network that is service independent. To create standard 53-byte ATM cells, the ATM layer receives 48-byte segments from the AAL and attaches a 5-byte header to each. These cells are then sent to the Physical layer for transmission across the physical medium. See also: AAL.

ATMM

ATM Management: A procedure that runs on ATM switches, managing rate enforcement and VCI translation. See also: ATM and VCI.

ATM user-user connection

A connection made by the ATM layer to supply communication between at least two ATM service users, such as ATMM processes. These communications can be uni- or bidirectional, using one or two VCCs, respectively. See also: ATM layer and ATMM.

ATP

AppleTalk Transaction Protocol: A transport-level protocol that enables reliable transactions between two sockets, where one requests the other to perform a given task and to report the results. ATP fastens the request and response together, assuring a loss-free exchange of request-response pairs.

attenuation

In communication, weakening or loss of signal energy, typically caused by distance.

AURP

AppleTalk Update-based Routing Protocol: A technique for encapsulating AppleTalk traffic in the header of a foreign protocol that allows the connection of at least two noncontiguous AppleTalk internetworks through a foreign network (such as TCP/IP) to create an AppleTalk WAN. The connection made is called an AURP tunnel. By exchanging routing information between exterior routers, the AURP maintains routing tables for the complete AppleTalk WAN. See also: AURP tunnel and exterior router.

AURP tunnel

A connection made in an AURP WAN that acts as a single, virtual link between AppleTalk internetworks separated physically by a foreign network such as a TCP/IP network. See also: AURP.

authority zone

A portion of the domain-name tree associated with DNS for which one name server is the authority. See also: DNS.

automatic call reconnect

A function that enables automatic call rerouting away from a failed trunk line.

autonomous confederation

A collection of self-governed systems that depend more on their own network accessibility and routing information than on information received from other systems or groups.

autonomous switching

The ability of Cisco routers to process packets more quickly by using the ciscoBus to switch packets independently of the system processor.

autonomous system (AS)

A group of networks under mutual administration that share the same routing methodology. Autonomous systems are subdivided by areas and must be assigned an individual 16-bit number by the IANA. See also: area and IANA.

autoreconfiguration

A procedure executed by nodes within the failure domain of a Token Ring, wherein nodes automatically perform diagnostics, trying to reconfigure the network around failed areas.