Glossary B


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.


Authentication, Authorization, and Accounting: A Cisco description of the processes that are required to provide a remote access security solution. Each is implemented separately, but each can rely on the others for functionality. See also: authentication, authorization, and accounting.


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.


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.


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.


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.


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. See also: AAL.


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.


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 control lists

Used by Cisco routers to control packets as they pass through a router. Access lists are created and then applied to router interfaces to accomplish this.

access layer

One of the layers in Cisco's three-layer hierarchical model. The access layer provides users with access to the internetwork.

access link

A link used with switches that is only part of one Virtual LAN (VLAN). Trunk links carry information from multiple VLANs.

access list

A set of test conditions kept by routers that determines 'interesting traffic' to and from the router for various services on the network.

access method

The manner in which network devices approach gaining access to the network itself.

access rate

Defines the bandwidth rate of the circuit. For example, the access rate of a T1 circuit is 1.544Mbps. In Frame Relay and other technologies, there may be a fractional T1 connection-256Kbps, for example-however, the access rate and clock rate are still 1.544Mbps.

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.


One of the three components in AAA. Accounting provides auditing and logging functionalities to the security model.


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


Allowed Cell Rate: A designation defined by the ATM Forum for managing ATM traffic. Dynamically controlled by 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 that tokens are not lost.

address learning

Used with transparent bridges to learn the hardware addresses of all devices on an internetwork. The switch then filters the network with the known hardware (MAC) addresses.

address mapping

A methodology that translates network addresses from one format to another so that different protocols can 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.


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

administrative distance (AD)

A number between 0 and 255 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.


ATM data service unit: The terminal adapter used to connect to an ATM network through an HSSI-compatible mechanism. See also: DSU.


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


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.


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.


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 8. 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.


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


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.


An analog or digital waveform's highest value.


Analog signaling is a technique to carry voice and data over copper and wireless media. When analog signals are transmitted over wires or through the air, the transmission conveys information through a variation of some type of signal amplitude, frequency, and phase.

analog connection

Provides signaling via an infinitely variable waveform. This differs from a digital connection, in which a definite waveform is used to define values. Traditional phone service is an analog connection.

analog transmission

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


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 International Electrotechnical Commission (IEC) and the International Organization for Standardization (ISO).


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


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, enabling networks to exist in more than one 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.


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 by using routers to create a single autonomous system. See also: autonomous system.


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.


Address Resolution Protocol: Defined in RFC 826, the protocol that traces IP addresses to MAC addresses. See also: RARP.


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.


American Standard Code for Information Interchange: An 8-bit code for representing characters, consisting of 7 data bits plus 1 parity bit.


application-specific integrated circuits: Used in layer 2 switches to make filtering decisions. The ASIC looks in the filter table of MAC addresses and determines which port the destination hardware address of a received hardware address is destined for. The frame will be allowed to traverse only that one segment. If the hardware address is unknown, the frame is forwarded out all ports.


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


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 connection

Defines the start and stop of each octet. As a result, each byte in asynchronous connections requires 2 bytes of overhead. Synchronous connections use a synchronous clock to mark the start and stop of each character.

asynchronous dial-up

Asynchronous dial-up is interchangeable with analog dial-up. Both terms refer to traditional modem-based connections.

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.


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


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.


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

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.


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


The first component in the AAA model. Users are typically authenticated via a username and password, which are used to uniquely identify them.

authority zone

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


The act of permitting access to a resource based on authentication information in the AAA model.

auto duplex

A setting on layer 1 and layer 2 devices that sets the duplex of a switch port automatically.

automatic call reconnect

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


The process of two network devices communicating, trying to decide what duplex and speed will be used for data transport.

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.


A process whereby a router automatically determines if a port connection has special QoS requirement-for example, an IP phone-and allocates queue priority based on the needs without configuration.


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.


An IOS feature that allows multicast-enabled routers to detect RP and forward the summary information to other routers and hosts.

auxiliary port

The console port on the back of Cisco routers that enables you to dial the router and make console configuration settings.


Active Virtual Gateway: The term given to the router in a GLBP block that receives a virtual MAC address allocation from the active router. See also: AVG and GLBP.


Active Virtual Gateway: The term given to the router in a GLBP block that allocates virtual MAC addresses. See also: AVF and GLBP.


Architecture for Voice, Video, and Integrated Data: This is a Cisco marketing term to group their convergence efforts. Convergence is the integration of historically distinct services into a single service.

CCNP. Building Cisco Multilayer Switched Networks Study Guide (642-811)
CCNP: Building Cisco Multilayer Switched Networks Study Guide (642-811)
ISBN: 078214294X
EAN: 2147483647
Year: 2002
Pages: 174
Authors: Terry Jack © 2008-2017.
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