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Online Training Demos and Learning Tutorials for Windows XP, 2000, 2003.






Basic Networking Terms and Definitions



Ethernet running on thin coax network cable at 10 Mbps.


Ethernet running on thick wire network cable at 10 Mbps.


Ethernet running on unshielded twisted pair (UTP) cable at 10 Mbps. Point-to-point network media, with one end of cable typically going to repeater/hub and other to network device.


Ethernet running on unshielded twisted pair (UTP) cable at 100 Mbps. Point-to-point network media, with one end of cable typically going to repeater/hub and other to network device.


AAL (ATM Adaptation Layer)

A collection of standard Asynchronous Transfer Mode (ATM) protocols that adapt user traffic to the cell format. AAL is subdivided into the convergence sub-layer (CS), and the Segmentation And Reassembly (SAR) sub-layer. There are several types of AALs -- AAL0, AAL1, AAL2, AAL3/4 and AAL5 -- to support the various AAL service classes.

AAL0 (AAL Type 0)

Null protocol. No cell adaptation occurs.

AAL1 (AAL Type 1)

Used for transporting time-dependent Constant Bit Rate (CBR) traffic, such as audio and video, and emulating Time Division Multiplexer (TDM)-based circuits, such as digital signal level 1 (DS1) and E1. Timing information must be exchanged between the source and the destination. AAL1 supports QoS Class A (defined under QoS in this glossary).

AAL2 (AAL Type 2)

Used for supporting time-dependent slow or Variable Bit Rate Real Time (VBR-RT) connection-oriented traffic (e.g., packetized and compressed audio and video). Timing information must be exchanged between the source and the destination. AAL2 supports QoS Class B (defined under QoS in this glossary).

AAL 3/4 (AAL Type 3 and 4)

Used for supporting both connectionless and connection-oriented Variable Bit Rate Non-Real Time (VBR-NRT) traffic. AAL3 supports quality of service (QoS) class C while AAL4 supports QoS class D. AAL3 and AAL4 are combined into one type. AAL3/4 also performs re-sequencing and cell identification operations. AAL3/4 services are suitable for supporting interworking with frame relay, SMDS and X.25.

AAL5 (AAL Type 5)

Used for supporting connection-oriented variable bit rate VBR-NRT data traffic and signaling messages. AAL5 supports quality of service (QoS) Class X. AAL5 services are suitable for supporting interworking with most data networking protocols, such as frame relay, SMDS, Ethernet and Internet Protocol (IP). AAL5 is more popular and easier to implement than AAL3/4.

ABR (Available Bit Rate)

One of five Asynchronous Transfer Mode (ATM) service categories. In this service type, the network attempts to pass the maximum number of cells but does not guarantee cell delivery. ABR supports Variable Bit Rate (VBR) data traffic with flow control, a minimum guaranteed data transmission rate, and specified performance parameters. In exchange for regulating user traffic flow, the network offers minimal cell loss of accepted traffic. Traffic parameters are Peak Cell Rate (PCR) and Maximum Cell Rate (MCR). Quality of Service (QoS) parameters are Cell Loss Ratio (CLR) and Cell Error Rate (CER).

Access Network

Portion of public switched network that connects access nodes to individual subscribers. Predominantly passive twisted pair copper wiring.

Access Nodes

Points on edge of the Access Network that concentrate individual access lines into smaller number of feeder lines. May also perform various forms of protocol conversion. Examples are Digital Loop Carrier systems concentrating individual voice lines to T1 lines, cellular antenna sites, PBXs, and Optical Network Units (ONUs).



Address Prefix

String of 0 or more bits up to maximum of 152 bits that is lead portion of one or more ATM addresses.

Address Resolution

Procedure by which client associates LAN destination with ATM address of another client or the BUS.

Administrative Domain

Collection of managed entities grouped for administrative reasons.

ADPCM (Adaptive Differential Pulse Code Modulation)

(1) Reduced bit rate variant of PCM audio encoding. (See also PCM.) This algorithm encodes difference between actual audio sample amplitude and predicted amplitude and adapts resolution based on recent differential values. (2) Coding scheme standardized by CCITT (See CCITT) that allows analog voice to be carried on 32 kbps digital channel instead of standard 64 kbps PCM channel.

ADSI (Analog Display Services Interface)

Protocol that simplifies use of advanced features by displaying text messages, generated by a remote computer or central office switch, on a user's telephone display or television set.

ADSL (Asymmetric Digital Subscriber Line)

Modems attached to twisted pair copper wiring that transmit from 1.5 to 9 Mbps downstream (to subscriber) and from 16 to 800 kbps upstream, depending on line distance.

AIN (Advanced Intelligent Network)

Bellcore's switching concept that centralizes significant amount of intelligence rather than constantly placing more information in central office switch.

AMI (Alternate Mark Inversion)

Line coding format used on T1 facilities that transmits ones by alternate positive and negative pulses.


Advanced Mobile Phone Service (US), the name applied to the original analog cellular system. Still the predominant cellular transmission scheme.

ANSI (American National Standards Institute)

U.S. body and standards-setting organization, not arm of the government. Accredits various other standards setting committees.

API (Application Programming Interface)

A set of calling conventions that define how a service is invoked through a software package.


Communications protocol developed by Apple Computer to allow networking between Macintoshes. All Macintosh computers have LocalTalk port, running AppleTalk over 230K bps serial line. Also runs over Ethernet (EtherTalk) and Token Ring (TokenTalk) network media.

Application Layer

The top layer of the network protocol stack. The application layer is concerned with the semantics of work, such as formatting electronic mail messages. (The lower layers of the network address how to represent that data and how to reach the foreign node.)

Application-Level Firewall

Firewall system providing service by processes that maintain complete TCP connection state and sequencing. Often re-addresses traffic so outgoing traffic appears to have originated from firewall, rather than internal host.

APPN (Advanced Peer to Peer Network)

IBM network architecture for building dynamic routing across arbitrary network topologies. Intended as an eventual replacement for SNA, IBM's static routed, hierarchical network architecture.

ARP (Address Resolution Protocol)

Used to dynamically discover the low-level physical network hardware address that corresponds to the high-level Internet Protocol (IP) address for a given host. ARP is limited to physical network systems that support broadcast packets that can be heard by all hosts on the network. ARP is defined in Request for Comments (RFC) 826.

ASCII (American Standard Code for Information Interchange)

A seven-level code (128 possible characters) used for data transfer.

ASP (Abstract Service Primitive)

Implementation-independent description of interaction between service-user and service-provider at particular service boundary, as defined by Open Systems Interconnection (OSI).

Asynchronous Time Division Multiplexing

Multiplexing technique in which transmission capability is organized in unassigned time slots that are assined to cells upon request of each application's instantaneous real need.

Asynchronous Transmission

A transmission method that sends units of data one character at a time. Characters are preceded by start bits and followed by stop bits, which provide synchronization at the receive terminal.

ATM (Asynchronous Transfer Mode)

A standard implementation of cell relay, a packet switching technique using packets of a fixed length, called cells. It is asynchronous because the recurrence of cells containing information from an individual user is not periodic.

ATM Address

Defined in UNI Specification as 3 formats, each having 20 bytes in length, including country, area and end-system identifiers.

ATM-ARP (ATM Address Resolution Protocol)

An address resolution protocol for mapping Asynchronous Transfer Mode (ATM) and Internet Protocol (IP) addresses. (Each host is assigned a unique IP address.) ATM-ARP can be used for discovering local area network (LAN) hosts attached to an ATM network or in classical IP over ATM.

ATM Layer Link

Section of an ATM Layer connection between two adjacent active ATM Layer entities (ATM-entities).

ATM Link

Virtual path link (VPL) or virtual channel link (VCL).

ATM Peer-to-Peer Connection

Virtual channel connection (VCC) or virtual path connection (VPC).

ATM Traffic Descriptor

Generic list of traffic parameters that can be used to capture the intrinsic traffic characteristics of requested ATM connection.

ATM User-User Connection

Association established by ATM Layer to support communication between two or more ATM service users (i.e., between two or more next higher entities or between two or more ATM-entities). Communications over an ATM Layer connection may be either bidirectional or unidirectional. Same Virtual Channel Identifier (VCI) issued for both directions of connection at interface.


ATM Forum-defined 25.6Mbit/s cell-based user interface based on IBM token ring network.

ATU-C and ATU-R (ADSL Transmission Unit, Central or Remote)

Device at end of ADSL line that stands between line and first item of equipment in subscriber premises or telephone switch. May be integrated within access node.

AUI (Attachment Unit Interface)

15-pin shielded, twisted pair Ethernet cable used (optionally) to connect between network devices and MAU.


Process of determining identity of user attempting to access system.

Authentication token

Portable device used for authenticating user. Operates by challenge/response, time-based code sequences or other techniques. May include paper-based lists of one-time passwords.


Process of determining what types of activities are permitted. Usually, authorization in context of authentication.


Automatic determination and matching of transmission speed.


Clause 28 of the IEEE 802.3u standard specifies MAC sublayer for identification of speed and duplex mode of connection being supported by device. Support optional for individual vendors.


Auto-Negotiation in Clause 28 of IEEE 802.3u standard. Ability of 10/100 Ethernet device to interpret speed or duplex mode of attached device and adjust to that rate.

AWG (American Wire Gauge)

System that specifies wire size. Gauge varies inversely with wire diameter size.



Main cable in network.

Bandwidth on Demand

Feature that allows remote access device to initiate second connection to particular site. Used to increase amount of data transferred to that site to increase desired threshold. Network manager configuring remote access server will specify number of bits or percentage of connection bandwidth threshold to trigger the secondary connection. Multilink PPP is emerging standard to allow this feature to be interoperable. Currently, the only way to ensure correct operation is to use devices on both end from same vendor.

Baseband LAN

Local Area Network that uses single carrier frequency over single channel. Ethernet, Token Ring and Arcnet LANs use baseband transmission.

Bastion host

System hardened to resist attack. Installed on network to potentially come under attack. Often component of firewalls or may be outside Web server or public access system. Generally runs some form of general purpose operating system (e.g., UNIX, VMS, WNT, etc.) rather than ROM-based or firmware operating system.


Unit of signal frequency in signals per second. Not synonymous with bits per second as signals can represent more than one bit. Baud equals bits per second only when signal represents single bit.

BBC (Broadband Bearer Capability)

Bearer class field that is part of initial address message.

BCD (Binary Coded Decimal)

Form of coding of each octet within cell, where each bit has one of two allowable states, 1 or 0.

BECN (Backward Explicit Congestion Notification)

An indicator bit in the frame relay header to notify the source of traffic that the virtual circuit is passing through a congested switch. It is set on any traffic flowing from the destination back to the source that passes through the congested switch.

BER (Bit Error Rate)

(1) Measure of transmission quality generally shown as negative exponent, (e.g., 10-7 or 1 in 107 bits in error or 1 in 10,000,000 bits in error). (2) Measure of transmission accuracy as ratio of bits received in error to bits sent (e.g., 10-9 or 1 error in 1,000,000,000 bits) is common in voice and data transmission systems.

Best Effort

A Quality of Service (QoS) class in which no specific traffic parameters and no absolute guarantees are provided. Best effort includes Undefined Bit Rate (UBR) and Available Bit Rate (ABR).

BETRS (Basic Exchange Telecommunications Radio Service)

Simplest form is "fixed cellular." Form of wireless local exchange service where handoff is not required.

BGP (Border Gateway Protocol)

An exterior gateway protocol defined in Request for Comments (RFC) 1267 and 1268.


A format for storage or transmission of binary data in which the most significant bit (or byte) comes first.


Binary, machine-readable forms of programs that are compiled or assembled, as opposed to source language forms of programs.


Characteristic of having only two states, such as current on and current off. Binary number system uses only ones and zeros.

BIP (Bit Interleaved Parity)

Method used at PHY layer to monitor error performance of link. Check bit or word is sent in link overhead covering previous block or frame. Bit errors in payload will be detected and may be reported as maintenance information.

B-ISDN (Broadband Integrated Digital Network)

Digital network with ATM switching operating at data rates in excess of 1.5 Mbps. ATM enables transport and switching of voice, data, image, and video over same infrastructure.

BISUP (Broadband ISDN User's Part)

SS7 protocol that defines signaling messages to control connections and services.

Bit (binary digit)

Smallest unit of data processing information. Assumes value of 1 or 0.


Standardized connector used with Thinnet and coaxial cable.

BOC (Bell Operating Company)

Any of 22 regulated telephone companies organized into seven Regional Bell holding companies. See RBOC and RHC.

BOOTP (Bootstrap Protocol)

Used for booting diskless nodes. Described in Request for Comments (RFC) 951 and 1084.

Border Node

Logical node in a specified peer group, with at least one link that crosses peer group boundary.

Bps (bits per second)

Units of transmission speed.

BRI (Basic Rate Interface)

ISDN scheme identified as 2B1D that permits two “bearer” channels, each operating at 64 kbps, and one “data” channel, operating at 16 kbps, to be carried over single twisted pair.


A device interconnecting Local Area Networks (LANs) at the Open Systems Interconnection (OSI) data link layer, and filtering and forwarding frames according to Media Access Control (MAC) addresses.


Wide-band technology capable of supporting voice, video and data, possibly using multiple channels.

Broadband Access

Access capable of supporting one or more broadband services.

Broadband Network

Network that uses multiple carrier frequencies to transmit multiplexed signals on single cable. Several networks may coexist on single cable without interfering with one another.


Data transmission to all addresses or functions.


Device that routes specific protocols, such as TCP/IP and IPX, and bridges other protocols, thereby combining functions of both routers and bridges.


LAN topology in which all nodes are connected to single cable, considered equal, and receive all transmissions on the medium.

BUS (Broadcast and Unknown Server)

Server that handles data sent by LE Client to broadcast MAC address (FFFFFFFFFFFF), all multicast traffic, and initial unicast frames sent by LAN Emulation Client.

BW (bandwidth)

Numerical measurement of throughput of system or network.


Data unit of eight bits.


CAC (Carrier Access Code)

Five to seven-digit number that identifies which interexchange carrier call uses. Subscribers dial these digits with each long distance call or pre-subscribe to particular carrier and let digital switch software add CAC.

CAC (Connection Admission Control)

Set of actions taken by network during call setup phase (or during call re-negotiation) to determine whether connection request should be accepted or rejected (or whether request for re-allocation can be accommodated).


Association between two or more users or between user and network entity that is established by use of network capabilities. Association may have zero or more connections.

CAT-5 (Category 5 UTP)

Unshielded twisted pair (UTP) standard cabling, commonly used with fast Ethernet and asynchronous transfer mode (ATM) interfaces for higher-speed cell transmission (more than 50 Mbps).

CBR (Constant Bit Rate)

One of the five Asynchronous Transfer Mode (ATM) classes of service. CBR supports the transmission of a continuous bit-stream of information, such as voice and video traffic, which require a constant amount of bandwidth allocated to a connection during the transmission.

CC (Continuity Cell)

A cell used periodically to check whether a connection is idle or has failed. Continuity checking is one of the Operation Administration And Maintenance (OAM) function types for fault management.

CCITT (Comité Consultatif Internationale de Telegraphique et Telephonique)

International group operating under auspices of International Telecommunications Union (ITU) and charged with establishing telecommunications standards. Name recently changed to ITU-TSS (International Telecommunications Union-Telecommunications Standards Sector).

CCR (Current Cell Rate)

The currently acceptable transmission rate for an end-system as defined by RM cells within Available Bit Rate (ABR). The field in the RM cell indicates the current complying cell rate (i.e., ACR) a user can transmit over a Virtual Channel (VC) connection.

CDMA (Code Division Multiple Access)

Digital transmission scheme claimed to be more efficient than other systems and to offer up to 20 times more call handling capacity than analog cellular systems.


Cellular Digital Packet Data

CD-ROM (Compact Disk-Read Only Memory)

Used by computer to store large amounts of data.

CDV (Cell Delay Variation)

A Quality of Service (QoS) parameter that measures the difference between the transfer delay of a single cell transfer delay and the expected transfer delay. This parameter is important for time-sensitive virtual circuits such as Constant Bit Rate (CBR) and Variable Bit Rate Real Time (VBR-RT).

CDVT (Cell Delay Variation Tolerance)

Used in Constant Bit Rate (CBR) traffic, it specifies the acceptable tolerance of the CDV (jitter).


The 53-byte basic information unit within an Asynchronous Transfer Mode (ATM) network. The user traffic is segmented into cells at the source and reassembled at the destination. An ATM cell consists of a 5-byte ATM header and a 48-byte ATM payload, which contains the user data.

CER (Cell Error Rate)

A Quality of Service (QoS) parameter that measures the number of transmitted cells that are erroneous over a specific period of time (i.e., those that contain errors when they arrive at the destination).

CES (Circuit Emulation Service)

An Asynchronous Transfer Mode (ATM) service in which Constant Bit Rate (CBR) virtual circuits use AAL1 to emulate an end-to-end physical circuit by providing a time division multiplexer (TDM)-like virtual circuit between local access circuits.


Authentication technique where server sends unpredictable challenge to user, who computes response using some form of authentication token.


Data path between two nodes.

Channelized T1/E1

T1 or E1 service that is divided into individual 64 Kbps channels, as opposed to unchannelized service, which uses the entire bandwidth of the T1 (1.544 Mbps) or E1 (2.048 Mbps). Channelized T1 or E1 lines can consist of switched lines with either in-band signaling or leased lines.

CHAP (Challenge Handshake Authentication Protocol)

Authentication scheme for PPP where password is required to begin connection and during the connection. Failure to provide correct password during login or challenge mode results in disconnect.


A computed value which is dependent upon the contents of a packet. This value is sent along with the packet when it is transmitted. The receiving system computes a new checksum based upon the received data and compares this value to the value sent with the packet. If the two values are the same, the receiver has a high degree of confidence that the data was received correctly.

CIR (Committed Information Rate)

A term used in frame relay that defines the information rate the network is committed to providing the user.


Competitive LEC

CLID (Caller ID)

Service that permits subscribers to see telephone number and/or name of calling party. Frequently, “call blocking” is offered, allowing calling parties to block display of their telephone numbers.

CLP (Cell Loss Priority)

A 1-bit field in the Asynchronous Transfer Mode (ATM) cell header specifying whether a cell is more or less likely to be discarded by an ATM network experiencing congestion.

CLR (Cell Loss Ratio)

A Quality of Service (QoS) parameter that gives the ratio of the lost cells to the total number of transmitted cells.

CMIP (Common Management Interface Protocol)

ITU-TSS standard for message formats and procedures used to exchange management information to operate, administer, maintain, and provision a network.

CO (Central Office)

A telephone company office that connects to all local loops in a given area and where circuit switching of customer lines occurs.

Coaxial Cable

Electrical cable with solid wire conductor at its center, surrounded by insulating materials and an outer metal screen conductor with an axis of curvature coinciding with inner conductor. Examples are standard Ethernet cable and Thinwire Ethernet cable.

COD (Connection Oriented Data)

Data requiring sequential delivery of its component PDUs to assure correct functioning of its supported application (e.g., voice or video).

CODEC (Coder/Decoder)

Electronic circuit converts analog voice signals into digital signals for transmission and switching, and digital signal to analog voice signals so they can be used by telephone.


Result of two network nodes transmitting on same channel at same time. Transmitted data is not usable.

Collision Detect

Signal indicating one or more stations are contending with local station's transmission. Signal is sent by the Physical layer to the Data Link layer on Ethernet/IEEE 802.3 node.

Communication Server

Dedicated, standalone system that manages communications activities for other computers.


A wiring hub in a star-topology network. Sometimes refers to a device containing multiple modules of network equipment.


Phase in which LE Client discovers LE Service.


(1) ATM connection consists of concatenation of ATM Layer links to provide end-to-end information transfer capability to access points. (2) In switched virtual connection environments, LAN Emulation Management entities set up connections between each other using UNI signaling.


The data communication method in which communication proceeds through three well-defined phases: connection establishment, data transfer, and connection release. Transmission Control Protocol (TCP) is a connection-oriented protocol.


The data communication method in which communication occurs between hosts with no previous setup. Packets between two hosts may take different routes, as each is independent of the other. User Datagram Protocol (UDP) is a connectionless protocol.


Terminal used to configure network devices at boot (start-up) time.

Core Network

Combination of switching offices and transmission plant connecting switching offices together. Linked by several competing Interexchange networks in U.S. local exchange. Now extends to national boundaries in rest of world.

CPE (Customer Premise Equipment)

Telecommunications equipment provided for and/or installed by a service provider at a home or enterprise.

CRC (Cyclic Redundancy Check)

A data transmission error-detection scheme. A polynomial algorithm is performed on the data, and the resultant checksum is appended at the end of the frame. The receiving equipment performs a similar algorithm.


Noise passed between communications cables or device elements.

CRS (Cell Relay Service)

A bearer service offered by an asynchronous transfer mode (ATM) network to the end users delivers Asynchronous Transfer Mode (ATM) cells directly over the network.

Cryptographic Checksum

One-way function applied to file to produce unique “fingerprint” of file for later reference. Primary means of detecting file system tampering on UNIX.

CS (Convergence Sublayer)

(1) General procedures and functions that convert between ATM and non-ATM formats, describing functions of upper half of AAL layer. (2) Used to describe conversion functions between non-ATM protocols, such as frame relay or SMDS and ATM protocols above AAL layer.

CSA (Canadian Standards Association)

One of several bodies that develops telecommunications standards.

CSMA/CD (Carrier Sense Multiple Access with Collision Detection)

A protocol in which stations listen to the bus and only transmit when the bus is free. If a collision occurs, the packet is retransmitted after a random time-out. Ethernet uses CSMA/CD.


Circuit Switched Public Data Network


Computer Supported Telephony Application (ECMA)

CSU (Channel Service Unit)

Equipment installed on customer premises to terminate a DDS or T1 circuit. CSUs provide network protection and diagnostic capabilities.

CTD (Cell Transfer Delay)

A quality of service (QoS) parameter that measures the average time for a cell to be transferred from its source to its destination over a virtual channel (VC) connection. CTD is the sum of any coding, decoding, segmentation, reassembly, propagation, processing, and queuing delays.


Computer Telephony Integration


Cellular Telecommunications Industry Association


Technique for examining incoming packets where Ethernet switch looks only at first few bytes of packet before forwarding or filtering it. Faster than looking at whole packet but allows some bad packets to be forwarded.


DA (Destination Address)

Information sent in forward direction indicating address of called station or customer.

DA (Destination MAC Address)

Six-octet value that uniquely identifies endpoint sent in IEEE LAN frame headers to indicate frame destination.

DACS (Digital Access and Cross Connect System)

A time-slot switch that allows T1 or E1 lines to be remapped electronically at the DS-0 (64 Kbps) level. Also called DCS or DXS.

Data Connections

Data VCCs connect LECs to each other and to Broadcast and Unknown Server. Carry Ethernet/IEEE 802.3 or IEEE 802.5 data frames as well as flush messages.

Data-driven attack

Attack is encoded in innocuous-seeming data that is executed by user or other software to implement attack. Concern it may get through firewall in data form and launch attack against system behind firewall. AKA denial of service attack.

Data Encryption

Transformation of data into unreadable, meaningless data through a cryptographic transformation using key. Decryption turns unintelligible data into meaningful data using a key.


A packet or string of bytes carrying and routing data and sufficient information from source to destination.

Data Link Layer

Layer 2 of the Open System Interconnection (OSI) model. Layer 2 is concerned with transmitting units of information or frames, and associated error-checking. It establishes, maintains, and releases data-link connections between elements in a network.

dB (Decibel)

Logarithmic unit describing ratio of two powers.

dBm (Decibel Referenced to a Milliwatt)

Ratio of two power levels, in which the second is one milliwatt.

DCC (Data Country Code)

Specifies country in which address is registered. Codes are given in ISO 3166. Field length is two octets. Digits are encoded in Binary Coded Decimal (BCD) syntax. Codes left justified and padded on right with hexadecimal value "F" to fill two octets.

DCE (Data Communication Equipment)

Generic definition of computing equipment that attaches to network via DTE.

DDS (Digital Data Service)

56 or 64 kbps digital private line channel.


Digital Equipment Corporation (DEC) proprietary network architecture running on point-to-point, X.25 and Ethernet networks.

DEK (Data Encryption Key)

Used for encrypting message text and computing message integrity checks (signatures).

DES (Data Encryption Standard)

A popular, standard encryption scheme.

Dial on Demand

Automatic detection, based on network manager’s pre-defined parameters, of need to initiate dial-up connection to a remote network.


Security feature that ensures people do not log into modems to which they should not have access. When connection is requested, system checks user name for validity, then “dials back” number associated with that user name.

Digital Certificate

Package of information, digitally signed by trusted authority (usually referred to as a CA or Notary), that binds a public key to owner. Usually consists of identifier field, public key field, serial number (of certificate), activation and expiration date, and signature field. X.509 defines a standard format.

Dijkstra's Algorithm

Algorithm sometimes used to calculate routes given link and nodal state topology database.

Distributed Processing

System in which each computer or node in network performs its own processing and manages some of its data while network facilitates communications between nodes.

DLC (Digital Loop Carrier)

Digital transmission system designed for subscriber loop plant. Multiplexes many circuits onto very few wires or onto single fiber pair.

DLCI (Data Link Connection Identifier)

A unique number assigned to a Permanent Virtual Connection (PVC) endpoint in a Frame Relay network.

DLPI (Data Link Provider Interface)

UNIX International Specification, Revision 2.0.0, OSI Work Group, August 1991.

DMS (Digital Multiplex Systems)

(1) System that combines number of digital circuits. (2) Prefix for Northern Telecom family of digital central office switches (DMS-10, DMS-100/200, DMS-250, DMS-300, and DMS-500.)

DNS spoofing

Assuming DNS name of another system by corrupting name service cache of victim system or compromising domain name server for valid domain.


See Administrative Domain.

Domain Name

Text name appended to host name to form unique host name across Internets.


Transfer of a file or information from one network node to another. Generally refers to transferring a file from big node, such as server, to a small node, such as terminal or printer.

DPN (Data Packet Network)

(1) Network in which “bundles” of information are transmitted, one after another. Differs from circuit network, in which entire circuit is dedicated to particular user. (2) Prefix for Northern Telecom’s DPN data networking switches.

DS0 (Digital Subscriber Level Zero)

A 64 Kbps unit of transmission bandwidth. A worldwide standard speed for digitizing one voice conversation, and more recently, for data transmission. Twenty-four DS0s (24x64 Kbps) equal one Digital Signal Level 1 (DS1).

DS1 (Digital Signal Level 1)

Framing specification used in transmitting digital signals at 1.544 Mbps on a T1 facility or 2.048 Mbps on an E1 facility.

DS2 Channel

For a T1 line, a 6.312 Mbps channel that consists of four Digital Signal Level 1 (DS1) channels. For an E1 line, an 8.45 Mbps channel that consists of four DS1 channels.

DS3 Channel

A 44.736 Mbps line consisting of seven DS2 channels. Also called a T3 line.

DS3 PLCP (Physical Layer Convergence Protocol)

Alternate method used by older T carrier equipment to locate ATM cell boundaries. Recently been moved to informative appendix of the ATM DS3 specification and replaced by HEC method.

DSL (Digital Subscriber Line)

A modem technology for transmitting information at high speeds on existing copper telephone lines to homes and businesses. DSL requires runs of usually less than 20,000 feet to a central telephone office. Types of DSL include Asymmetric DSL (ADSL), Symmetric DSL (SDSL), and High Bit Rate DSL (HDSL).

DSLAM (Digital Subscriber Line Access Multiplexer)

Device that takes number of ADSL subscriber lines and concentrates these to single ATM line.

DSS1 (Digital Subscriber Signaling System #1)

N-ISDN UNI Signaling.

DSS2 (Digital Subscriber Signaling System #2)

B-ISDN UNI Signaling.

DSU (Data Service Unit)

(1) Equipment used to attach users' computing equipment to a public network. (2) Device located on the customer’s premises that converts a digital data signal to a digital transmission signal.

DSU (Digital Service Unit)

A user device interfacing to a digital circuit, such as DDS or T1 when com-bined with a Channel Service Unit (CSU). The DSU converts the user’s data stream to bipolar format for transmission.

DTE (Data Terminal Equipment)

(1) Generic definition of external networking interface equipment, such as modem. (2) Name applied to a piece of terminal equipment.

Dual homed gateway

System with two or more network interfaces, each of which is connected to different networks. With firewall, acts to block or filter some or all traffic trying to pass between networks.

DWS (Dialable Wideband Service)

Alternative name for Multirate ISDN, providing dialed data connectivity at desired bandwidth on per call basis (from 128 kbps through 1.536 Mbps in 64 kbps increments).



Public network addressing standard with maximum of 15 digits. ATM uses E.164 addressing for public network addressing.


The 2.048 Mbps digital carrier system common in Europe.


The European standard for high-speed digital transmission operating at 34 Mbps.


Previously European Computer Manufacturers Association

ECSA (Exchange Carrier Standard Association)

Standards body sponsored by exchange carriers and accredited by American National Standards Institute (ANSI). Recently changed to Alliance for Telecommunications Industry Solutions (ATIS).


Enterprise Computer Telephony Forum

EGP (Exterior Gateway Protocol)

A protocol that distributes routing information to the routers connecting autonomous systems. Today, the term “router” is commonly used in place of the term “gateway.” There is also a routing protocol called EGP, defined in STD 18, Request for Comments (RFC) 904.


Electronics Industry Association

ELA (Emulated LAN)

The Asynchronous Transfer Mode (ATM) segment of a virtual local area network (VLAN) based on the ATM forum Local Area Network Emulation (LANE) standard. A VLAN consists of an ELAN segment and traditional LAN segment.

EMI (Electromagnetic Interference)

Electromagnetic waves emitted by some electrical devices that distort or overwhelm other communications signals.


Encapsulating data is a technique used by layered protocols in which a low level protocol accepts a message from a higher-level protocol, then places it 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.

Encrypting router

See tunneling router and virtual private network.

Enterprise Network

An information infrastructure that often combines private and public facilities to cover all of the locations operated by a single company or corporate enterprise with a single communications fabric.

ESF (Extended Superframe Format)

A T1 framing format that uses the framing bit to provide mainte-nance and diagnostic functions.


Most popular LAN technology in use today, with configuration rules defined by IEEE standard 802.3. 10 Mbps, CSMA/CD baseband network that runs over thin coax, thick coax, twisted pair or fiber optic cable.


Apple Computer's protocol for Ethernet transmissions.

ETSI (European Telecommunications Standards Institute)

Primary telecommunications standards organization.



Federal Communications Commission

FCS (Frame Check Sequence)

Any mathematical formula that derives numeric value based on bit pattern of transmitted block of information and uses that value at receiving end to determine existence of any transmission errors.

FDDI (Fiber Distributed Data Interface)

An American National Standards Institute (ANSI) standard for fiber-optic links with data rates up to 100 Mbps.

FEC (Forward Error Correction)

Technique for detection and correction of errors in digital data stream. Frequently used in data transmission systems. Redundant bits are transmitted along with payload. Location and value of these bits in message allows receiving station to detect and correct errors.

FECN (Forward Explicit Congestion Notification)

A bit set by a frame relay network to notify an interface (DTE) that congestion avoidance procedures should be initiated by the receiving device.

Fiber Optics

A transmission medium consisting of thin glass filaments. Light beams travel through the fiber-optic line, carrying large amounts of data over long distances.

File Server

Computer that stores data for network users and provides network access to that data.


Process where an Ethernet switch or bridge reads contents of packet, finds that packet does not need to be forwarded, and drops it. Filtering rate is rate at which device can receive packets and drop them without any loss of incoming packets or delay in processing.


System or combination of systems that enforces boundary between two or more networks, controlling access from one to the other.


Alterable programs in semipermanent storage, such as some type of read-only or flash reprogrammable memory.

Flash ROM

See ROM.

Flow Control

A congestion control mechanism in which an Asynchronous Transfer Mode (ATM) system implements flow control.

FOTS (Fiber Optic Transmission System)

Generic term applied to any fiber optic transmission system.

Fractional T1

A service provided by carriers in which a full T1 link is leased to the customer, but the service charge is calculated based only on the number of timeslots used.

FRAD (Frame Relay Access Device)

A device responsible for framing data with header and trailer infor-mation (control information) before presenting the frame to the frame relay switch.


A piece of a packet. When a router is forwarding an Internet Protocol (IP) packet to a network that has a maximum packet size smaller than the forwarded packet size, it is forced to break up that packet into multiple fragments. These fragments will be reassembled by the IP layer at the destination host.


A data link layer “packet” that contains the header and trailer information required by the physical medium. Network layer packets are encapsulated to become frames. The terms packet, datagram, segment, and message are also used to describe logical information groupings.

Frame Relay

A network interface providing high-speed frame or packet transmission with minimum delay and an efficient use of bandwidth.

Frame Relay Frame

A variable-length unit of data in frame relay format that is transmitted as pure data through a frame relay network.


At the physical and data link layers of the Open System Interconnection (OSI) model, bits are fit into units called frames. Frames contain source and destination information, flags to designate the start and end of the frame, plus information about the integrity of the frame. All other information, such as network protocols and the actual payload of data, is encapsulated in a packet, which is encapsulated in the frame.

FRS (Frame-Relay Service)

Connection oriented service capable of carrying up to 4096 bytes per frame.

FTP (File Transfer Protocol)

A protocol that allows a user on one host to access and transfer files to and from another host over a network. FTP is usually the name of the program the user invokes to execute the protocol. The Internet Protocol (IP) version is defined in STD 9, Request for Comments (RFC) 959.

FTTC (Fiber to the Curb)

Fiber placed in long distance network, feeder plant, and distribution plant. Fiber then proceeds to curb, with copper going from curb to home.

FTTH (Fibre to the Home)

Network where optical fibre runs from telephone switch to subscriber's location or home.

FTTK (Fiber to the Kerb)

See Fiber to the Curb.

Full Duplex

A circuit or device permitting transmission in two directions at the same time.

FUNI (Frame User Network Interface)

A frame-based interface which supports signaling and Quality of Service (QoS) to an Asynchronous Transfer Mode (ATM). To inter-operate with a frame relay end system, the ATM switch should support FRF.8, which is the frame relay/ATM service Internetworking specification.



ITU-T Recommendation, “Physical/Electrical Characteristics of Hierarchical Digital Interfaces.”


ITU-T Recommendation, “Synchronous Frame Structures Used at Primary and Secondary Hierarchy Levels.”


Voice compression algorithm used in H.324 .


ITU-T Recommendation, “ATM Cell Mapping into Plesiochronous Digital Hierarchy (PDH).”


One of the versions of DSL (Please see DSL).


Generic Address Resolution Protocol


Today, the term “router” is used in place of the original term “gateway,” a communications device/program that passes data between networks with similar functions but dissimilar implementations. A router or gateway should not be confused with a protocol converter, in which a router is a Layer 3 (network layer) gateway, and a mail gateway is a Layer 7 (application layer) gateway.

Gbps (Giga Bits per Second)

Giga is the prefix representing 109, or one billion. For example, 8 gbps is 8 billion data bits per second.

GFC (Generic Flow Control)

A 4-bit field within the Asynchronous Transfer Mode (ATM) cell header that may be used to identify whether or not an ATM system implements congestion control.


Generic Multicast Resolution Protocol

GSM (Global System for Mobile Communications)

Comprehensive network specification that includes transmission scheme, network architecture, and network services. Current standard in Europe and many countries in Asia and proposed standard for personal communication services in North America.


Generic VLAN Resolution Protocol



Multiplexing and control protocol for H.324.


Video compression algorithm used in H.324.


A set of International Telecommunication Union (ITU) standards that define a framework for the transmission of real-time voice communications through Internet protocol (IP)-based packet-switched networks. The H.323 standards define a gateway and a gatekeeper for customers who need their existing IP networks to support voice communications.


New communications standard for sharing video, voice, and data over single analog telephone line.

H0 Channel

384 kbps channel that consists of six contiguous DS0s (64 kbps) of T1 line.

H10 Channel

North American 1472 kbps channel from T1 or primary rate carrier. Equivalent to 23 64 kbps channels.

H11 Channel

North American primary rate used as single 1536 kbps channel. Uses 24 contiguous DS0s or entire T1 line, except for 8 kbps framing pattern.


European primary rate used as single 1920 kbps channel (30 64 kbps channels or entire E1 line, except for 64 kbps framing and maintenance channel.

Half Duplex

A circuit or device capable of transmitting in two directions, but not at the same time.

Hardware Address

See Network Address.


ISDN bearer services with pre-defined speeds and starting and stopping locations on PRI that are contiguously transported from one PRI site through networks to another PRI site.

HDLC (High-level Data Link Control)

A synchronous, bit-oriented link layer protocol for data transmission. Frame relay is an example of an HDLC-based packet protocol.

HDSL (High Bit-Rate Digital Subscriber Line)

A high-performance twisted pair transmission technology, best known as an enhanced transport mechanism for T1 or E1 service. It is designed for the local loop between a customer’s premises and an area exchange central office.


The portion of a packet that precedes the actual data and contains source and destination addresses, error checking, and other fields.


Ethernet-defined SQE signal quality test function.

HEC (Header Error Control)

Using fifth octet in ATM cell header, ATM equipment may check for error and correct contents of header. Check character is calculated using CRC algorithm allowing single bit error in header to be corrected or multiple errors to be detected.

Hello Packet

Type of PNNI Routing packet exchanged between neighboring logical nodes.

Heterogeneous network

A network running multiple network layer protocols.


A term used in routing. A path to a destination on a network is a series of hops, through routers, away from the origin.

Host Table

List of TCP/IP hosts on network along with their IP addresses.



ITU-T Specifications for Traffic Measurement.


B-ISDN ATM Layer Specification.


B-ISDN ATM Layer (AAL) Functional Description.


B-ISDN ATM Layer (AAL) Specification.


ITU-T Recommendation for B-ISDN User-network Interface.

IASG (Internetwork Address Sub-Group)

Range of internetwork layer addresses summarized in internetwork layer routing protocol.

ICMP (Internet Control Message Protocol)

An extension to the Internet Protocol (IP). ICMP enables the generation of error messages, test packets and informational messages related to Internet protocol. It is defined in STD 5, Request for Comments (RFC) 792.

IEC (Inter-exchange Carrier)

Long distance telephone company. See IXC.

IEEE (Institute of Electrical and Electronic Engineers)

An international professional society issuing its own standards. The IEEE is a member of the American National Standards Institute (ANSI) and International Standards Organization (ISOs).

IEEE 802.3

Local Area Network protocol suite with 10 Mbps or 100 Mbps throughput. Uses Carrier Sense Multiple Access bus with Collision Detection CSMA/CD media-access method and physical and data link layer specifications of local area network. Includes 10BASE2, 10BASE5, 10BASE-FL, and 10BASE-T Ethernet implementations. Allows users to share network cable, but only one station can use the cable at a time. Variety of physical medium-dependent protocols are supported.

IEEE 802.5

Local Area Network protocol suite commonly known as Token Ring. Standard originated by IBM for token-passing ring network that can be configured in star topology. Supports versions 4 Mbps and 16 Mbps.

IETF (Internet Engineering Task Force)

A group that was initially responsible for developing specifications required for the interoperable implementation of Internet Protocol (IP).

IGP (Interior Gateway Protocol)

A protocol that distributes routing information to the routers within an autonomous system. (Today the term “router” is used in place of “gateway.”

IISP (Interim Inter-switch Signaling Protocol)

A protocol that uses user network interface (UNI)-based signaling (i.e., UNI 3.0/3.1) and pre-fix routing for switch-to-switch communication. IISP is formally known as Private Network-Network Interface (PNNI) Phase 0.

ILMI (Interim Local Management Interface)

A Simple Network Management Protocol (SNMP)-based network management interface between an end-system and an asynchronous transfer mode (ATM) switch for status and configuration reporting and registering/deregistering ATM addresses.

IMA (Inverse Multiplexing over ATM)

A method to pass Asynchronous Transfer Mode (ATM) traffic over multiple E1/T1 links while maintaining the ATM quality of service and optimizing bandwidth usage.

Insider Attack

Attack originating from inside protected network.

Instance ID

Subset of an object's attributes that serve to uniquely identify MIB instance.


Series of interconnected local, regional, national, and international networks, linked using TCP/IP. Links many government, university, and research sites. Provides e-mail, remote login, and file transfer services.

Internet Address

Also known as an Internet Protocol (IP) address. This is a 32-bit hardware-independent address assigned to hosts using the transmission control protocol/Internet protocol (TCP/IP) suite.

Internet Datagram

Unit of data exchanged between pair of internet modules that includes internet header.

Internet Protocol Suite

Official name of TCP/IP, as used in Internet standards documents. See TCP/IP.


Process of connecting two networks together. Result is referred to as an internet.


General term used to describe the industry composed of products and technologies used to link networks together.


Closed network of computers that uses similar technology to the Internet, such as Web servers and browsers, to make information available to controlled group of users. May have connection to Internet or may exist on Internet, achieving controlled access through passwords or other means.

Intrusion detection

Detection of break-ins or break-in attempts, manually or via software expert systems that operate on logs or other information available on network.

IOP (Interoperability)

Ability of equipment from different manufacturers (or different implementations) to operate together.

IP (Internet Protocol)

The network layer protocol of the transmission control protocol/Internet protocol (TCP/IP) suite. Defined in STD 5, Request for Comments (RFC) 791. It is a connectionless, best-effort packet switching protocol.

IP Address

See Network Address.


IP Control Protocol.

I-PNNI (Integrated Private Network-Network Interface)

PNNI-compatible protocol used to exchange information between routers that augment or replace protocols such as Open Shortest-Path First (OSPF) and Internetwork Packet Exchange (IPX). This enables the integration of existing router-based connectionless networks and Asynchronous Transfer Mode (ATM) networks.

IP splicing /hijacking

Attack where active, established, session is intercepted and co-opted by attacker. May occur after authentication has been made, permitting attacker to assume role of authorized user. Primary protections rely on encryption at session or network layer.

IP spoofing

Attack where system attempts to illicitly impersonate another system by using the IP network address.

IP Telephony

The transmission of voice over an Internet Protocol (IP) network. Also called Voiceover IP (VoIP), IP telephony enables users to make telephone calls over the Internet, intranets, or private Local Area Networks (LANs) and Wide Area Networks (WANs) that use the Transmission Control Protocol/Internet Protocol (TCP/IP).

IPX (Internetwork Packet Exchange)

Netware network layer (Layer 3) protocol for transferring data from servers to workstations.


Uses ISDN transmission technology to deliver data at 128kbps into IDSL modem bank connected to router.

ISDN (Integrated Services Digital Network)

A carrier-provided service that enables a variety of switched digital data and voice transmission to be accommodated simultaneously.

ISO (International Standards Organization)

An international organization involved in writing communi-cations standards.

ISO Layered Model

References that specify how dissimilar computing devices, such as Network Interface Cards (NICs), bridges and routers, exchange data over a network. Model has seven layers.

ISP (Internet Service Provider)

Organization offering and providing Internet services to public, with own computer servers to provide services offered.

ITU (International Telecommunication Union)

A European-based, international advisory committee recommending worldwide standards for transmission.

ITU H.222

ITU-T Study Group 15 standard that addresses multiplexing of multimedia data on ATM network.

ITU Q.2100

B-ISDN Signaling ATM Adaptation Layer Overview.

ITU Q.2110

B-ISDN Adaptation Layer, Service Specific Connection Oriented Protocol.

ITU Q.2130

B-ISDN Adaptation Layer, Service Specific Connection Oriented Function for Support of Signaling at UNI.

ITU Q.2931

Signaling standard for ATM to support Switched Virtual Connections. Based on signaling standard for ISDN.

ITU Q.931

Signaling standard for ISDN to support SVCs. Basis for signaling standard developed for Frame Relay and ATM.

ITU Q.933

Signaling standard for Frame Relay to support SVCs. Based on the signaling standard for ISDN.

ITU-T (International Telecommunications Union Telecommunications)

International body of member countries that defines recommendations and standards relating to the international telecommunications industry. Defined and published fundamental standards for ATM. (Previously CCITT.)



The deviation of a transmission signal in time or phase. It can introduce errors and loss of synchro-nization in high-speed synchronous communications.


kbps (kilobits per second)

Represents 103 or 1000. For example, 64 kbps is 64,000 data bits per second.


Popular file transfer and terminal emulation program.


LAN (Local Area Network)

A data transmission facility connecting a number of communicating devices (computers, terminals and printers) within a single room, building, campus or other limited geographical area.

LANE (LAN Emulation)

An Asynchronous Transfer Mode (ATM) forum standard that provides support for native Local Area Network (LAN) protocols across an ATM network by emulating the media access control (MAC) protocol. LANE defines a single Virtual Local Area Network (VLAN) consisting of traditional LAN segments and an Emulated Local Area Network (ELAN) segment across the ATM network. Routers will connect multiple VLANs.

Leaky Bucket

A flow control algorithm in which cells are monitored to check whether they comply with the established connection parameters. Non-conforming cells are either tagged or dropped from the network. The analogy is taken from a bucket with a hole in its bottom that allows the fluid to flow out at a certain rate.

Line Speed

Maximum rate, expressed in bps, at which data can reliably be transmitted over line using given hardware.


A format for storage or transmission of binary data in which the least significant byte (bit) comes first.

LLC (Logical Link Control)

The upper portion of the data link layer, as defined in Institute of Electrical and Electronic Engineers (IEEE) 802.2. The LLC sub-layer presents a uniform interface to the user of the data link service, which is usually the network layer. Beneath the LLC sub-layer is the media access control (MAC) sub-layer.

LMI (Local Management Interface)

A frame relay network management mechanism that uses Data Link Connection Identifiers (DLCIs) 0 and 1023 to pass management messages over the User-To-Network Interface (UNI).

Load Balancing

A technique that distributes network traffic along parallel paths to maximize the available network bandwidth while providing redundancy.


Apple Computer's proprietary 230 Kbps baseband network protocol. Uses CSMA/CD access method over unshielded twisted pair wire.

LSB (Least Significant Bit)

Lowest order bit in binary representation of numerical value.

LSSGR (LATA Switching System Generic Requirements)

Multi-volume set of Bellcore technical references dealing with basic switching requirements used by switch manufacturers, procurement staffs, planners, and switch technicians.

LTE (SONET Lite Terminating Equipment)

ATM equipment terminating communications facility using SONET Lite Transmission Convergence (TC) layer. Usually reserved for end user or LAN equipment. Does not implement some maintenance functions used in long haul networks, such as termination of path, line, and section overhead.


MAC (Media Access Control)

A protocol that defines the way workstations gain access to transmission media. MAC is most widely used in reference to Local Area Networks (LANs). For Institute of Electrical and Electronic Engineers (IEEE) LANs, the MAC layer is the lower sub-layer of the data link layer protocol.

MAN (Metropolitan Area Network)

A network that provides regional connectivity within a metropolitan area (such as city). MANs are categorized between Local Area Networks (LANs) and Wide Area Networks (WANs).

MAU (Medium Attachment Unit)

Device used to convert signals from one Ethernet medium to another.

Mbps (Megabits per second or millions of bits per second)

Measure of digital transmission speed used in computer and telephone networks.

MBS (Maximum Burst Size)

A traffic parameter that specifies the maximum number of Asynchronous Transfer Mode (ATM) cells in a burst that can be transmitted at the Peak Cell Rate (PCR).

MCR (Maximum Cell Rate)

An Available Bit Rate (ABR) traffic parameter (in cells per second) that gives the slowest rate at which the network controls the flow of the source on an ABR Virtual Channel (VC) connection.

MIB (Management Information Base)

A collection of objects that can be accessed via a network management protocol, such as Simple Network Management Protocol (SNMP). The objects represent values that can be read or changed.

MIB Attribute

Single piece of configuration, management, or statistical information that pertains to specific part of PNNI protocol operation.

MIB Instance

Incarnation of MIB object that applies to specific part, piece, or aspect of PNNI protocol's operation.

MII (Media Independent Interface)

New standard developed for Fast Ethernet in IEEE 802.3u specification. Fast Ethernet equivalent to AUI in 10 Mbps Ethernet, allowing different types of Fast Ethernet media to be connected to Fast Ethernet device via common interface.

MMF (Multimode Fiberoptic Cable)

Fiberoptic cable in which signal or light propagates in multiple modes or paths. Since these paths may have varying lengths, transmitted pulse of light may be received at different times and smeared so pulses may interfere with surrounding pulses. May cause signal to be difficult or impossible to receive, sometimes limiting distance over which MMF link can operate.


Modulator-demodulator device for changing transmission signals from digital to analog for transmission over phone lines. Used in pairs, one at each end of line.

MPOA (Multi-Protocol Over ATM)

Internet Engineering Task Force (IETF)-defined specifications and procedures that enable network layer protocols to operate directly on top of an Asynchronous Transfer Mode (ATM) and provide end-to-end internetworking between hosts in an ATM and non-ATM environment.

MPOA Client

Edge Device Functional Group (EDFG) or Host Behavior Functional Group (HBFG). device that implements client side of one or more of MPOA protocols, (i.e., SCP client and/or RDP client).

MPOA Server

Any one of ICFG or RSFG.

MPOA Service Area

Collection of server functions and their clients. Collection of physical devices consisting of MPOA server plus its set of clients.

MPOA Target

Set of protocol address and path attributes (e.g., internetwork layer QoS, other information derivable from received packet) describing intended destination, which MPOA devices may use as lookup keys.

MSB (Most Significant Bit)

Highest order bit in binary representation of numerical value.

MTBF (Mean Time Between Failures)

Average period of time equipment or component remains working before failure.

MTP (Message Transfer Part)

Level 1 through 3 protocols of SS7 protocol stack. MTP 3 (Level 3) used to support BISUP.


Device that allows several users to share single circuit. Funnels different data streams into single stream. At other end of communications link, another multiplexer reverses process by splitting data stream back into original streams.

Multiport Repeater

Repeater, either standalone or connected to standard Ethernet cable, for interconnecting up to eight Thinwire Ethernet segments.


Name Server

Software that runs on network hosts charged with translating (or resolving) text-style names into numeric IP addresses.

Native Address

Address that matches one of given node's summary addresses.


Narrow Band PCS.

NCP (Network Control Program)

Program that runs on VMS machines to configure local network hardware and remote network devices.


Network Control Protocol.

NDIS (Network Driver Interface Specification)

See 3COM/Microsoft, LAN Manager: Network Driver Interface Specification, October 8, 1990.

NE (Network Element)

System that supports at least Network Element Functions and may also support Operation System Functions/Mediation Functions. May be realized as either stand alone device or geographically distributed system. Cannot be further decomposed into managed elements in context of given management function.


Microsoft's networking protocols for its LAN Manager and Windows NT products.


Novell-developed Network Operating System (NOS). Provides file and printer sharing among networks of Personal Computers (PCs). Each network must have at least one file server, and access to other resources is dependent on connecting to and logging into file server. File server controls user logins and access to other network clients, such as user PCs, print servers, modem/fax servers, and disk/file servers.


Interconnected system of computers that can communicate with each other and share files, data, and resources.

Network Address

Every node on network has one or more associated addresses, including at least one fixed hardware address, such as “ae-34-2c-1d-69-f1” assigned by device's manufacturer. Most nodes also have protocol specific addresses assigned by network manager.

Network Layer

A layer in the Open System Interconnection (OSI) reference model. The network layer provides address resolution and routing protocols. Address resolution enables the network layer to determine a unique network address for a node. Routing protocols allow data to flow between networks and reach their proper destination. Examples of network layer protocols include Internet Protocol (IP) and Address Resolution Protocol (ARP).

Network Management

Administrative services for managing a network, including configuring and tuning, maintaining network operation, monitoring network performance, and diagnosing network problems.

Network-Level Firewall

Firewall in which traffic is examined at network protocol packet level.

NFS (Network File System)

The recognized standard protocol for accessing files over a network as if they were on local disks. Defined in Request for Comments (RFC) 1094.

NIC (Network Interface Card)

Adapter card inserted into computer that contains necessary software and electronics to enable station to communicate over network.

NII (National Information Infrastructure)

"Seamless web of communications networks, computers, databases, and consumer electronics that puts vast amounts of information at users' fingertips." Al Gore

N-ISDN (Narrowband Integrated Services Digital Network)

Services include basic rate interface (2B+D or BRI) and primary rate interface (30B+D, Europe and 23B+D, North America or PRI). Supports narrowband speeds at/or below 1.5 Mbps. Same as ISDN.

NNI (Network Node Interface or Network-to-Network Interface)

International Telecommunication Union (ITU) standard interface between nodes within the same network. The Asynchronous Transfer Mode (ATM) forum distinguishes between two standards; one for private networks called Private Network-Network Interface (PNNI), and one for public networks known as Public NNI.

NOS (Network Operating System)

Software for network that runs in file server and controls access to files and other resources from multiple users. Provides security and administrative tools. Novell's NetWare, Banyan's VINES, and IBM's LAN Server are NOS examples.


Non-Return to Zero bit encoding.


Non-Return to Zero Inverted bit encoding.


Circuit bandwidth or speed provided by aggregation of nx64 kbps channels (where n= integer > 1). 64K or DS0 channel is basic rate provided by T Carrier systems.


OAM (Operation Administration and Maintenance)

A management framework defined by the International Telecommunication Union (ITU). OAM cells are special-purpose Asynchronous Transfer Mode (ATM) cells exchanged between two ATM entities for network fault and performance management, analysis and fault isolation.

OC (Optical Carrier)

A hierarchy of optical signals used to classify speeds or capacities of fiber lines, especially as related to the Synchronous Optical Network (SONET) standard. The basic speed is OC-1 (~52 Mbps).


Term for eight (8) bits that is sometimes used interchangeably with byte.

ODI (Open Data Link Interface)

A standard interface specification developed by Novell to enable PC adapter cards to run multiple protocol stacks.

OSI (Open Systems Interconnection)

A seven-layer model of network communications developed by the International Standards Organization (ISO).

OSPF (Open Shortest-Path First)

A link-state routing protocol defined in Request for Comments (RFC) 1247. OSPF supports packet authentication and up to 255 hops.



An ordered group of data and control signals transmitted through a network as a subset of a larger message.

PAD (Packet Assembler and Disassembler)

Device performing the interface between X.25 data network and asynchronous device such as personal computer. Assembles user data into packets with identifying information used to control routing.

PAP (Password Authentication Protocol)

Authentication scheme for PPP links. Password can be specified for both devices on remote link. Failure to authenticate results in dropped connection prior to start of data transmission.

Parity Bit

An additional non-information bit added to a group of bits to ensure that the total number of bits in the character is even or odd.

PBX (Private Branch Exchange)

A private telephone exchange.

PC (Protocol Control)

Mechanism that given application protocol may employ to determine or control performance and health of application. For example, protocol liveness may require protocol control information be sent at some minimum rate. Some applications may become intolerable to users if they are unable to send at minimum rate.

PCI (Protocol Control Information)

High-speed PC data bus.

PCR (Peak Cell Rate)

An Asynchronous Transfer Mode (ATM) traffic parameter (in cells per second) that characterizes the source and gives the maximum rate at which cells can be transmitted. It is calculated as the reciprocal of the minimum inter-cell interval (the time between two cells) over a given Virtual Channel (VC) connection.

PDU (Protocol Data Unit)

Message of a given protocol comprising payload and protocol-specific control information, typically contained in header. Passes over protocol interfaces that exist between layers of protocols (per OSI model).

PHY (Physical layer)

Layer 1 of the Open System Interconnection (OSI) model. Layer 1 is concerned with electrical, mechanical and handshaking procedures over the interface connecting a device to the transmission medium. In Asynchronous Transfer Mode (ATM), Layer 1 is the bottom layer of the ATM protocol reference model and is subdivided into two sub-layers: Transmission Convergence (TC) and Physical Medium (PM). It provides ATM cell transmission over the physical interfaces that interconnect the ATM devices.

Physical Address

Address identifying a single node.

Physical Layer

See PHY.

Physical Layer (PHY) Connection

Association established by PHY between two or more ATM entities. Consists of concatenation of PHY links to provide end-to-end transfer capability to PHY SAPs.

PLCP (Physical Layer Convergence Protocol)

Defined by the IEEE 802.6 and used for DS3 transmission of ATM. ATM cells are encapsulated in 125microsecond frame defined by PLCP, which is defined inside DS3 M-frame.

PLL (Phase Lock Loop)

Mechanism that transfers timing information within data stream and receiver derives signal element timing by locking its local clock source to received timing information.

PM (Physical Medium)

Actual physical interfaces, including STS-1, STS-3c, STS-12c, STM-1, STM-4, DS1, E1, DS2, E3, DS3, E4, FDDI-based, Fiber Channel-based, and STP. Range in speeds from 1.544 through 622.08 Mbps.

PMD (Physical Media Dependent)

Sublayer that defines parameters at lowest level, such as speed of bits on media.

PNNI (Private Network-Network Interface)

The inter-switch interface within a private Asynchronous Transfer Mode (ATM) domain. The PNNI trunking protocol provides hierarchical ATM-layer routing and Quality of Service (QoS) support.

PNNI Protocol Entity

Body of software in switching system that executes PNNI protocol and provides routing service.

PNNI Routing Control Channel

VCCs used for exchange of PNNI routing protocol messages.

PNNI Routing Domain

Group of topologically contiguous systems running one instance of PNNI routing.

PNNI Routing Hierarchy

Hierarchy of peer groups used for PNNI routing.

PNNI Topology State Element

Collection of PNNI information flooded among all logical nodes within peer group.

PNNI Topology State Packet

Type of PNNI Routing packet used for flooding PTSEs among logical nodes within peer group.

POP (Point Of Presence)

A site where a collection of telecommunications equipment (usually digital leased lines and multi-protocol routers) exists.

POTS (Plain Old Telephone Service)

Only name recognized around world for basic analog telephone service. Takes lowest 4kHz of bandwidth on twisted pair wiring. Any service sharing line with POTS must use frequencies above POTS or convert POTS to digital and interleave with other data signals.

PPP (Point-to-Point Protocol)

Provides a standard means of encapsulating data packets sent over a single-channel Wide Area Network (WAN) link. PPP is the standard WAN encapsulation protocol for the interoperability of bridges and routers over synchronous or asynchronous circuits.

Presentation Layer

The Open System Interconnection (OSI) layer that determines how application information is represented or encoded while in transit between two end systems.

PRI (Primary Rate Interface)

An ISDN interface providing 23 “B” channels, each operating at 64 kbps, and single “D” channel also operating at 64 kbps to customer’s premises.


A formal set of conventions governing the formatting and relative timing of message exchange between two communicating systems.

Protocol Control Information

Information exchanged between corresponding entities, using lower layer connection, to coordinate their joint operation.


Software agent that acts on behalf of user. Generally accepts connection from user, makes decision whether or not user or client IP address is permitted to use proxy, may perform additional authentication, and completes connection on behalf of user to remote destination.

Proxy ARP

The technique in which one machine, usually a router, answers Address Resolution Protocol (ARP) requests intended for another machine. By “faking” its identity, the router accepts responsibility for routing packets to the “real” destination. Proxy ARP allows a site to use a single Internet Protocol (IP) address with two physical networks.

PSTN (Public Switched Telephone Network)

The telecommunications network commonly accessed by standard telephones, key systems, Private Branch Exchange (PBX) trunks and data equipment.

PTI (Payload Type Identifier)

A 3-bit field within the Asynchronous Transfer Mode (ATM) cell header indicating: the ATM Adaptation Layer (AAL) used; whether a congestion has been experienced (EFCI); and whether or not the cell contains Operation Administration And Maintenance (OAM) information. When an AAL5 frame passes through Segmentation And Reassembly (SAR), the PTI within the last cell identifies the end of this AAL5 frame.

PVC (Permanent Virtual Connection)

A permanent, virtual connection established by the network management between an origin and a destination.

PVP (Permanent Virtual Path)

A set of Permanent Virtual Channels (PVCs) that exist between two cross points.


QoS (Quality of Service)

A group of service classes that define the performance of a given circuit. For an Asynchronous Transfer Mode (ATM), there are a number of different QoS parameters.

QoS Class 0

Refers to the best-effort service Undefined Bit Rate (UBR).

QoS Class 1

Specifies the parameters for circuit emulation and the transport of Constant Bit Rate (CBR) uncompressed video. ATM adaptation layer type 1 (AAL1) supports this kind of delay-sensitive connection-oriented service.

QoS Class 2

Specifies the parameters for the transport of Variable Bit Rate (VBR) audio and video. ATM adaptation layer type 2 (AAL2) supports this kind of delay-sensitive connection-oriented service.

QoS Class 3

Specifies the parameters for connection oriented data transfer. ATM adaptation layer type 3 and 4 (AAL3/4) and AAL5 support this delay-tolerant class that is intended to provide interoperability with SMDS and Internet Protocol (IP).

QoS Class 4

Specifies the parameters for connectionless data transfer. ATM adaptation layer type 3 and 4 (AAL3/4) or AAL5 can be used to support this delay-tolerant class that is also intended to provide inter-operability with SMDS and Internet Protocol (IP).

QoS Class X

Refers to the connection-oriented transport service in which the traffic type (Constant Bit Rate or Variable Bit Rate and timing requirements (delay sensitive or non-sensitive) are defined by the user. QoS Class X is known as an unrestricted service class that is supported by ATM adaptation layer type 5 (AAL5).


RADSL (Rate Adaptive ADSL)

Version of ADSL where modems test line at start up and adapt their operating speed to maximum line can handle.

RAM (Random Access Memory)

Computer’s direct access memory that can be accessed very quickly and overwritten with new information. Loses its content when power is turned off.

RD (Routing Domain)

Group of topologically contiguous systems running one instance of routing.

Remote Access

Access to network resources not located on same physical Ethernet (entire site network topology).

Remote Control

Form of remote access where device dialing in assumes control of another network node. All remote keystrokes translated into keystrokes on network node. Used primarily with IPX protocol.

Remote Node

Form of remote access where device dialing in acts as peer on target network. Used with IP and IPX protocols.


A device that automatically amplifies, restores, or reshapes signals before retransmission to compensate for distortion and/or attenuation.

RFC (Request for Comments)

Documents written by the research and development community to describe Internet protocols and standards submitted to the Internet Engineering Task Force (IETF) for review. All Internet standards are written as RFCs.

RIP2 (Routing Information Protocol)

Used to discover agents and the routes that Internet Protocol (IP) packets must traverse. This is done automatically using periodic broadcasts. RIP2 also supports IP subnets.

RISC (Reduced Instruction Set Computing)

Computer processing technology in which microprocessor understands few simple instructions, providing fast, predictable instruction flow.


Application that provides terminal interface between UNIX hosts using TCP/IP network protocol. Unlike Telnet, assumes remote host is (or behaves like) UNIX machine.

RM-Remote Monitoring RMON (Remote Monitoring)

A management information base that enables a network monitoring device to be configured and read from remote locations.

ROM (Read-only Memory)

Memory device that retains its information even when power is off. ROM version of network device does not need to download as contains entire executable code, so never needs to reload it. Frequently provided as "flash ROM", which can be reprogrammed by downloading if user chooses.

Route Server

Physical device that runs one or more network layer routing protocols and uses route query protocol to provide network layer routing forwarding descriptions to clients.


An interconnection device that connects individual Local Area Networks (LANs). Unlike bridges, which logically connect at Open System Interconnection (OSI) Layer 2, routers provide logical paths at OSI Layer 3. Like bridges, remote sites can be connected using routers over dedicated or switched lines to create Wide Area Networks (WANs).

Routing Computation

Process of applying mathematical algorithm to topology database to compute routes. Many types may be used. For example, Djikstra algorithm.

Routing Constraint

Generic term that refers to topology constraint or path constraint.

Routing Protocol

General term indicating protocol run between routers and/or route servers to exchange information used to allow computation of routes. Result is one or more forwarding descriptions.

RSVP (Resource reSerVation Protocol)

A protocol developed for supporting different quality of service (QoS) classes for Internet Protocol (IP) applications.



Freely available authentication system, developed at Bellcore (based on paper by Leslie Lamport of DEC) that avoids many types of password attacks.


Emerging standard for secure firewall-to-firewall communication.

SA (Source Address)

Address from which the message or data originated.

SAAL (Signaling ATM Adaption Layer)

Resides between the ATM layer and the Q.2931 function and provides reliable transport of Q.2931 messages between Q.2931 entities (e.g., ATM switch and host) over ATM layer and two sublayers: common part and service-specific part.

SAP (Service Access Point)

Remote SAP used when application initiates an outgoing call to remote ATM device to specify ATM address of remote device and provide further addressing that identifies target software entity within remote device. Local SAP used when application prepares to respond to incoming calls from remote ATM devices to specify ATM address of device housing application and provide further addressing that identifies the application within local device. Several groups of SAPs are specified as valid for Native ATM Services.


Service Advertisement Protocol

SAR (Segmentation and Reassembly)

Segments the information frames into cells at the source and reassembles these cells back into information frames at the destination. These activities occur at the lower part of the ATM Adaptation Layer (AAL). Each AAL type has its own SAR format.

SCAI (Switch-to-Computer Applications Interface)

Protocol that allows subscriber’s computer to interact with digital switch, making it possible to coordinate information in database with incoming and outgoing phone calls to allow, for example, company representative to receive customer call and simultaneously receive customer's file for viewing on desktop workstation.

SCCP (Signaling Connection and Control Part)

SS7 protocol that provides additional functions to Message Transfer Part (MTP). Typically supports Transaction Capabilities Application Part (TCAP).

SCR (Sustainable Cell Rate)

A traffic parameter that characterizes a bursty source and specifies the max-imum average rate at which cells can be sent over a given Virtual Channel (VC) connection. It can be defined as the ratio of the Maximum Burst Size (MBS) to the minimum burst inter-arrival time.

Screened Host

Host on a network behind screening router. Degree of access to screened host depends on screening rules in router.

Screened Subnet

Subnet behind screening router. Degree of access to subnet depends on screening rules in router.

Screening Router

Router configured to permit or deny traffic based on set of permission rules installed by administrator.

SDH (Synchronous Data Hierarchy)

The European standard for using optical media as the physical transport for high-speed, long-haul networks.

SDH (Synchronous Digital Hierarchy)

ITU-TSS International standard, similar to SONET, for transmitting information over optical fiber.

SDSL (Symmetric Digital Subscriber Line)

HDSL plus POTS over single telephone line. Name has not been adopted by standards group, but is being discussed by ETSI. Operates over POTS and would be suitable for symmetric services to premises of individual customers.

SE (Switching Element)

Device or network node that performs ATM switching functions based on VPI or VPI/VCI pair.

Secure virtual private network (secure VPN)

Secure private network using unsecured public networks as carriers. Users may use their network as though it were perfectly secure, isolated LAN, even though it is connected to unsecured public networks.


Single ATM link or group of interconnected ATM links of ATM connection.


Computer that provides resources to be shared on network, such as files (file server) or terminals (terminal server).

Session stealing

See IP splicing.

Shared Ethernet

Ethernet configuration in which number of segments are bound together in single collision domain. Hubs produce this type of configuration where only one node can transmit at a time.

SLIP (Serial Link Internet Protocol)

An Internet protocol for host dial-up connection. SLIP frames are encapsulated Internet protocol (IP) datagrams in which SLIP adds just a few bytes of control data.

SMDS (Switched Multimegabit Data Service)

Public, wide-area packet-switched data service developed by Bellcore that provides DS-1 to DS-3 switched access.

SMTP (Simple Mail Transfer Protocol)

A protocol, defined in STD 10, Request for Comments (RFC) 821, used to transfer electronic mail between computers. It is a server-to-server protocol, so other protocols are used to access the messages.

SNMP (Simple Network Management Protocol)

The network management protocol of the Transmission Control Protocol/Internet Protocol (TCP/IP) suite.

SONET (Synchronous Optical Network)

A standard for using optical media as the physical transport for high-speed, long-haul networks. SONET basic speeds start at 51.84 Mbps and go up to 2.5 Gbps.

Source MAC Address

Six-octet value uniquely identifying end point sent in IEEE LAN frame header to indicate source of frame.

SS7 (Signaling System 7)

A signaling method separate from the voice or data channel that lets intelligent network elements exchange information among themselves.

Store and Forward

Technique for examining incoming packets on Ethernet switch or bridge where whole packet is read before forwarding or filtering. Slightly slower process than cut-through, but ensures that all bad or misaligned packets are eliminated from network by switching device.

STP (Shielded Twisted Pairs)

General term for shielded cabling systems that are designed specifically for data transmission. Also means Spanning Tree Protocol, a routing protocol that eliminates routing loops in a network.

STS-1 (Synchronous Transport Signal 1)

SONET standard for transmission over OC-1 optical fiber at 51.84 Mbps.

STS-n (Synchronous Transport Signal “n”)

SONET standards for transmission over OC-n optical fiber by multiplexing “n” (integer) STS-1 frames (e.g., STS-3 at 155.52 Mbps, STS-12 at 622.08 Mbps, and STS-48 at 2.488 Gbps).

STS-nc (Synchronous Transport Signal “n” concatenated)

SONET standards for transmission over OC-n optical fiber by multiplexing “n” (integer) STS-1 frames (e.g., STS-3 at 155.52 Mbps, STS-12 at 622.08 Mbps, and STS-48 at 2.488 Gbps but treating information fields as single concatenated payload).


A portion of a network, which may be a physically independent network segment, which shares a network address with other portions of the network. It is distinguished by a subnet number.

SVC (Switched Virtual Circuit)

A logical connection between two points that is established dynamically and exists during transmission only. In Asynchronous Transfer Mode (ATM) networks, the Switched Virtual Circuit (SVC) connection is established via signaling. End systems transmit their User Network Interface (UNI) 3.1 or 4.0 signaling request via the Q.2931 signaling protocol.


Multiport Ethernet device designed to increase network performance by allowing only essential traffic on attached individual Ethernet segments. Packets are filtered or forwarded based upon their source and destination addresses.

Synchronous Transmission

Transmission in which data bits are sent at a fixed rate, with the transmitter and receiver synchronized.



A digital transmission link with a capacity of 1.544 Mbps used in North America. Typically channeled into 24 digital subscriber level zeros (DS0s), each capable of carrying a single voice conversation or data stream. T1 uses two pairs of twisted pair wires.


ANSI standards sub-committee dealing with Network Interfaces.


ANSI standards sub-committee dealing with Inter-Network Operations, Administration and Maintenance.


ANSI standards sub-committee dealing with performance.


ANSI standards sub-committee dealing with services, architecture, and signaling.


ANSI standards sub-committee dealing with digital hierarchy and synchronization.


A digital transmission link with a capacity of 45 Mbps, or 28 T1 lines.

TCP (Transmission Control Protocol)

An Internet standard transport layer protocol defined in STD 7, Request for Comments (RFC) 793. It is connection-oriented and stream-oriented, as opposed to User Datagram Protocol (UDP).

TCP (Test Coordination Procedure)

Set of rules to coordinate test process between lower tester and upper tester. Enables lower tester to control operation of upper tester. May, or may not, be specified in abstract test suite.

TCP/IP (Transmission Control Protocol/Internet Protocol)

Also known as the Internet protocol suite. This suite of protocols is used on the Internet, often for heterogeneous internetworking.

TDM (Time Division Multiplexer)

A device that divides the time available on its composite link among its channels, usually interleaving bits (Bit TDM) or characters (Character TDM) of data from each terminal.

TE (Terminal Equipment)

Endpoint of ATM connection(s) and termination of various protocols within connection(s).


The virtual terminal protocol in the Internet suite of protocols. It lets users on one host use the Internet to access and work as terminal users of a remote host.

TFTP (Trivial File Transfer Protocol)

A simplified version of the file transfer protocol that transfers files but does not provide password protection or user directory capability.


Half-inch diameter coax cable.


Thin coaxial cable similar to that used for television/video hookups.


Rate of data transmitted between two points in given amount of time (e.g., 10 Mbps).


Telecommunication Industry Association

Token Ring

Developed by IBM, this 4 or 16 Mbps network uses ring topology and token-passing access method.

TP-MIC (Twisted-Pair Media Interface Connector)

Connector jack at end user or network equipment that receives twisted pair plug.

Traffic Policing

Mechanism whereby any traffic that violates the traffic contract agreed to at connection setup is detected and discarded.

Traffic Shaping

A method of smoothing bursty traffic in order to present a more uniform traffic rate to the network.

Transmission Control Protocol

Protocol that provides reliable transmission of packets over IP.

TTL (Time to Live)

A field in the Internet Protocol (IP) header that indicates how long this packet should be allowed to survive before being discarded. It is primarily used as a hop count.


Refers to the encapsulation of protocol A within protocol B, such that A treats B as though it were a data link layer.

Tunneling Router

Router or system capable of routing traffic by encrypting it and encapsulating it for transmission across untrusted network, for eventual de-encapsulation and decryption.

Twisted-Pair Cable

Inexpensive, multiple-conductor cable comprised of one or more pairs of 18 to 24-gauge copper strands twisted to improve protection against electromagnetic and radio frequency interference. May be either shielded or unshielded. Because of narrow bandwidth, used in low-speed communications, as telephone cable only.


UBR (Undefined Bit Rate)

An economical class of Quality of Service (QoS) on a moment-to-moment basis, without guaranteeing service levels.

UDP (User Datagram Protocol)

A connectionless transport protocol without any guarantee of packet sequence or delivery. UDP functions directly on top of Internet Protocol (IP).

UNI (User Network Interface)

The interface, defined as a set of protocols and traffic characteristics, between the CPE and the Asynchronous Transfer Mode (ATM) network.

UNI 4.0

This UNI specification refers to signaling issues in Available Bit Rate (ABR), Virtual Path (VP) and Quality of Service (QoS) negotiations.


Connectivity from border node to upnode.


Border node’s outside neighbor in common peer group. Must be neighboring peer of one of border node’s ancestors.

UT (Upper Tester)

Representation in ISO/IEC 9646 of means of providing, during test execution, control and observation of upper service boundary of IUT, as defined by chosen Abstract Test Method.

UTOPIA (Universal Test and Operation Physical Interface for ATM)

An interface to provide connectivity at the Physical Layer (PHY) level among Asynchronous Transfer Mode (ATM) entities.

UTP (Unshielded Twisted Pair)

General term for all cabling systems that are not shielded and are used for transmission of data.


V.34 Modem

International standard for 28.8 kbps modem connections.


International standard for “video phone ready” V.34 modems that defines how video phone software runs on V.80 modems.


International standard for 56 kbps modem connections.

VADSL (Very high speed ADSL)

Same as VDSL or subset of VDSL, if VDSL includes symmetric mode transmission.

VBR (Variable Bit Rate)

Traffic containing bursts but centered around an average bandwidth.

VBR-NRT (Variable Bit Rate Non Real Time)

One of the VBR service types for transmitting traffic where timing information is not critical. Since this service type is delay-tolerant, it is well suited for bursty traffic such as data communications.

VBR-RT (Variable Bit Rate Real Time)

One of the two VBR service types for transmitting traffic that depends on timing and control information. It is suitable for carrying delay-sensitive traffic such as packeted video and audio.

VC (Virtual Channel)

A connection established between end users, in which packets are forwarded along the same path and bandwidth is not permanently allocated until it is used.

VCI (Virtual Channel Identifier)

A 16-bit value in the asynchronous transfer mode (ATM) cell header that provides a unique identifier for the Virtual Channel (VC) within a Virtual Path (VP) that carries that particular cell.

VDSL (Very high data rate Digital Subscriber Line)

Modem for twisted-pair access operating at data rates from 12.9 to 52.8 Mbps, with corresponding maximum reach ranging from 4500 to 1000 feet of 24-gauge twisted pair.

Virtual Circuit

A network service that provides connection-oriented service regardless of the underlying network structure.


Self-replicating code segment. Viruses may or may not contain payloads, attack programs, or trapdoors.

VLAN (Virtual Local Area Network)

A network architecture that allows geographically distributed users to communicate as if they were on a single physical Local Area Network (LAN) by sharing a single broadcast and multicast domain. Asynchronous Transfer Mode (ATM) forum LAN emulation supports virtual Local Area Networks (VLANs).

VP (Virtual Path)

A term to describe a set of Virtual Channels (VCs) between cross points, grouped together.

VPI (Virtual Path Identifier)

An 8-bit value in the cell header that identifies the VP and accordingly the virtual channel to which the cell belongs.

VPN (Virtual Private Network)

Public network service in which a customer is provided a network that appears as though it were a private network. The advantage of VPNs over the dedicated private networks is that the VPNs allow a dynamic use of the network resources and offer very reliable, high-speed and less expensive communications.


WAN (Wide Area Network)

A network that typically spans nationwide distances and usually utilizes public telephone networks.

Workgroup Switching

Configuration in which number of users are connected to Ethernet network via switch that allows each user to get greater throughput than would be available through hub.


X.25 Gateway Access Protocol

Allows node not directly connected to public data network to access facilities of that network through intermediary gateway node. Protocol standard governing packet-switched networks.

Source: Intel


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