Glossary of Term and Acronyms


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1TR6 is a widely deployed, German- specific ISDN switch standard that existed prior to the ETSI NET3 standard.

IEEE 802.3 Physical layer specification for 10-Mbps Ethernet over two pairs of Category 3, 4, or 5 UTP wire.

Proposed IEEE 802.3 Physical layer specification for 100-Mbps Ethernet over two strands of fiber.

The group of proposed IEEE 802.3 Physical layer specifications for 100-Mbps over various wiring specifications.

Proposed IEEE 802.3 Physical layer specification for 100-Mbps Ethernet over four pairs of Category 3, 4, or 5 UTP wire.

Proposed IEEE Physical layer specification for 100- Mbps CSMA/CD over two pairs of Category 5 UTP or STP wire.

A 100-Mbps technology under development by Hewlett- Packard that uses a demand priority network access method.

A set of IEEE specifications for local area networks (LANs) and metropolitan area networks (MANs).
802.1: general management and internetwork operations such as bridging.
802.2: sets standards at the logical link control sublayer of the data link layer.
802.3: CSMA/CD (Ethernet) standards, which apply at the physical layer and the media access control (MAC) sublayer.
802.4: token passing bus standards. 802.5: token ring standards.
802.6: MAN standards.
IEEE 802 standards become ANSI standards and usually are accepted as international standards.

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Augmentative and Alternative Communication. Any communication which supplements or augments speech, including words, sign systems and written sysmbols. The user indicates the symbols needed from a book, chart or electronic aid. It attempts to provide those with severe expressive disorders with an efficient communication system.

ATM Adaption Layer

AAL 3/4
An AAL enables connection-oriented transfer of connectionless data, i.e., SMDS.

A low-overhead AAL tailored for data, such as Frame Relay and multiprotocol LAN packets.

ABM (Asynchronous Balanced Mode)
A communication mode used in HDLC that allows either of two workstations in a peer-oriented point-to-point configuration to initiate a data transfer.

Area Border Routers

Available bit rate

Access Method
Generally, the method by which networked stations determine when they can transmit data on a shared transmission medium. Also, the software within an SNA processor that controls the flow of information through a network.

Access Control System

Access SecuritySystem
Remote access security software that works with network-based security servers.

Active Hub
A multiport device that amplifies LAN transmission signals.

A board installed in a computer system, usually a PC, to provide network communication capabilities to and from that computer system. Also called a network interface card (NIC).

Advanced Digital Network.

Advanced Distributed Recovery Intelligence

AFP (AppleTalk* Filing Protocol)
Protocol that lets workstations access files from remote file servers. The protocol corresponds to layer 6 of the Open Systems Interconnection (OSI) reference model. See OSI.

Software that receives queries and returns replies on behalf of an application. In network management systems, agents reside in all managed devices and report the values of specified variables to management stations.

APPN Implementors Workshop

Attention Line

Adapter Management Protocol

American National Standards Institute

API (Application Program Interface)
Means of communication between programs to give one program transparent access to another. APIs serve various computing purposes. In networking, for example, an API offers software applications (such as a database manager) transparent access to OS/2* files,devices or interprocess communications.

APPC (Advanced Program-to-Program Communications)
Implementation of SNATM LU 6.2 sessions that permits personal computers in an SNA network to communicate in real time with the mainframe host and other networks.

A small Java programthat maybe used on a Web page.

An Apple* networking system that operates over STP wire at 230 Kbps.

Application layer
Layer 7 of the OSI Reference Model; implemented by various network applications, including electronic mail, file transfer and terminal emulation.

APPN (Advanced Peer-to-Peer)
NetworkinSNA facility that provides distributed processing based on Type 2.1 network nodes and LU 6.2.

Advanced Peer-to-Peer NetworkingNetwork Node

AppleTalk Remote Access Protocol

Adaptor Request Blocks

A network service that searches FTP sites for files.

ARP (Address Resolution Protocol)
Internet protocol for dynamically mapping Internet addresses to physical hardware addresses on LANs. Limited to LANs that support hardware broadcast.

Advanced Research Projects Agency Network - a system developed by ARPANET in the 1960's as the first resilient large-scale packet switched network. A precursor to the Internet, it was in use between 1971 and 1990 when it was officially dismantled.

Autonomous System Boundary Router

American Standard Code for Information Interchange

Application-Specific Integrated Circuit

Async (Asynchronous)
A form of communication in which data is sent using start and stop bits, without regard for the time needed for transmission.Compare to synchronous transmission.

Async-Sync PPP Conversion:
Method by which PPP data sent between a computers COM port and the ISDN is converted by the terminal adapter to/from asynchronous to synchronous traffic.

AT (Asynchronous Transmission)
Data transmission one character at a time, with intervals of varying lengths between transmittals. Start and stop bits at the beginning and end of each character control the transmission.

ATM (Asynchronous Transfer Mode)
A packet switching technique which uses packets, or cells, of fixed length to transmit multiple types of information (voice, video, data). Speeds vary from the 1.5 Mpbs to 622 Mbps and above. Also referred to as BISDN.

ATM Forum
An industry alliance of more than 500 companies dedicated to rapidly standardizing ATM through design and specification work.

ATM Layer
The part of the BISDN protocol stack that handles most of the ATM routing and processing.

ATM Switch
A hardware device that takes an incoming ATM cell and directs it to one or more of many potential output interfaces.

The decrease in magnitude of the power of a signal transmitted over a wire, measured in decibels. As attenuation increases, signal power decreases.

Access Unit

AUI (Attachment Unit Interface) Cable
An IEEE 802.3 cable connecting the MAU (Media Access Unit) to a networked device; AUI also may refer to the host backpanel connector to which an AUI cable attaches.

Autonomous System (AS)
In Internet (TCP/IP) terminology, a series of gateways or routers that fall under a single administrative entity and cooperate using the same Interior Gateway Protocol (IGP).

Attachment Unit Interface

A function of all repeaters, whereby a faulty segment is automatically isolated to prevent the fault affecting the entire network. The segment is automatically reconnected by the repeater when the fault condition is rectified.

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A LAN or WAN that interconnects intermediate systems (bridges and/or routers).

The main bus that carries data within a device.

Balun (balanced-unbalanced)
An impedance-matching device that connects a balanced line (such as a twisted-pair line) with an unbalanced line (such as a coaxial cable).

Measure of the information capacity of a transmission channel. Strictly speaking, bandwidth is the difference, expressed in hertz (Hz), between the highest and lowest frequencies of the channel.

(see Dynamic bandwidth allocation)

Transmission scheme in which the entire bandwidth, or data-carrying capacity, of a medium (such as coaxial cable) is used to carry a single digital pulse, or signal, between multiple users. Because digital signals are not modulated, only one kind of data can be transmitted at a time. Contrast with broadband.

Basic Rate ISDN
An a version of ISDN offering two 64 Kbps channels (B-channels) for speech or data and a 16 Kbps channel (D-channel) for signalling and control purposes. Aggregate data rate: (2x64)+16=144 Kbps.

Bastion Host
A machine placed on the perimeter network to provide publicly available services. Although secured against attack, it is assumed to be compromised because it is exposed to the Internet.

Baud Rate
The number of signal events per second occurring on a single communications channel. Often taken to mean bit rate, though not really accurate.

Bulletin Board System - a software package that interacts with one or more dial-up lines to allow users to communicate with other users by reading and writing messages aswell as enabling them to download and upload files.

The main type of channel in ISDN. It's a full duplex, 64 Kbps channel for sending data and voice. Basic Rate ISDN has two B-Channels and Primary Rate ISDN has between 6 and 30.

Token ring frame signaling that the ring is inoperative because of a serious hard error; defective cable or faulty nodes are possible causes.

Backward Explicit Congestion Notification

BER (Basic Encoding Rule)
Rule for encoding data units described in ANS.1; also, bit error rate, or the ratio of received bits that are in error.

BGP (Border Gateway Protocol)
Protocol for communications between a router in one autonomous system and routers in other ASs.

Binary synchronous communication, or bisync
Character-oriented data link protocol for half-duplex applications. Usually bisync.

BISDN (Broadband Integrated Services Digital Network)
Communications standard designed to handle high bandwidth applications such as video over broadband. See ATM.

BIinary digiT - has only two possible values 0 or 1.

Bit error rate
Percentage of bits in a transmittal received in error.

Because It's Time NETwork - a network, separate from the Internet, of educational institutions. Becoming less commonly used.

Bit rate
The number of bits travelling per second in a data stream.

BNC connector
Standard connector to link IEEE 802.3 10BASE2 coaxial cable to a transceiver.

Bell Operating Company (see RBOC).

An international standard for aggregating multiple data channels into a single logical connection. Very popular in videoconferencing applications.

A method of marking a World Wide Web address (URL) that you wish to go back to. Known as a "hotlist" in Mosaic and "Favourites" in Microsoft Internet Explorer.

Protocol that a network workstation uses on boot up to determine the IP address of its Ethernet interfaces.

Boot PROM (Programmable Read-Only Memory)
Nonvolatile memory that contains information necessary for initializing a system. Boot PROM information can be transmitted over a network.

Traffic slowdowns that result when too many network nodes try to access a single node, often a server node, at once.

To return undeliverable email to the sender. The term is sometimes used when a data packet is repeatedly sent between two routers (because of a routing problem) until it's time to live or hop count expires.

Boundary Function
Capability of SNA subarea nodes -- encountered most often in IBM 3745 high-speed communications controllers -- to offer protocol support for attached peripheral nodes.

Boundary Routing System Architecture
Software algorithms and methodology that enable a router at a central node of a wide area network to perform protocol-specific routing and bridging path table management on behalf of a router at a peripheral (leaf) node, greatly simplifying the router at the leaf node.

BPDU (Bridge Protocol Data Units)
A packet to initiate communications between devices under a spanning-tree protocol. Compare PDU.

bits per second

Bridging Application-Specific Integrated Circuit.

BRI(Basic Rate Interface)
The ISDN interface comprising two B channels and one 16 K bit/second D channel.

A device that interconnects local or remote networks no matter what higher level protocols (such as XNS* or TCP/IP) are involved. Bridges form a single logical network, centralizing network administration. They operate at the physical and link layers of the Open Systems Interconnect (OSI) reference model. See SRT (source routing transparent) bridge, and STA (Spanning Tree Algorithm). Contrast with router and gateway.

A device that can provide the functions of a bridge, router or both concurrently. A bridge/router can route one or more protocols, such as TCP/IP and/or XNS, and bridge all other traffic.

One of two methods used to transmit information around a LAN, the other being Baseband. Broadband uses modems to modulate the signal before putting it onto the LAN media. Multiple frequency channels are provided which operate independently of each carrying voice, data or video.This is usually done using Frequency Division Multiplexing (FDM).
Alternate definition - Usually taken to mean "faster than commonly occuring networks", so the real meaning depends on what the most common network speeds are at the time. At the moment anything operating at speeds faster than 34Mbps is referred to as broadband.

Broadband ISDN
A version of ISDN that works at Broadband speeds. This is different from Primary Rate ISDN which consists of a number of 64Kbps channels and is not a fully integrated service. The two main proposed Broadband ISDN rates are 150Mbps and 600Kbps.

A message forwarded to all network destinations.

Broadcast Storm
Multiple simultaneous broadcasts that typically absorb available network bandwidth and can cause network time-outs.

Area in a device for temporary storage of data in transit; can accommodate differences in processing speeds between devices by storing data blocks until they are ready to be processed by a slower device.

BUS (Broadcast and Unknown Server)
It provides the broadcast function and resolution of unknown addresses for LAN Emulation which is connection-oriented.

Bypass Mode
Operating mode on ring networks such as FDDI and token ring in which an interface has de-inserted from the ring.

Byte Order
There are two main conventions for the ordering of bytes within multi-byte integers - "big-endian" (most significant byte first) and "little-endian" (least significant byte first). This is vendor dependent, for instance SUN machines use "big-endian" where as DEC machines use "little-endian". Obviously this can cause problem when sharing sets of data between machines. Software utilities such as "dd" in Unix can be used to "byte swap" (ie convert data produced using one convention for use on a machine that uses the other).

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Speeds information processing by storing information from a transaction to use for later transactions.

CAE (Common Applications Environment)
Computer environment in which applications can be ported across various manufacturers' X/Open systems. The CAE contains standards for the operating system, networking protocols, languages and data management.

In Europe, CAPI (Common Application Interface) provides a common ISDN software platform for communication applications.

CAU (Controller Access Unit)
A managed concentrator on a token ring network -- essentially, an intelligent version of an MAU. Handles the ring in/ring out function.

Comité Consultatif International Télégraphique et Téléphonique (Consultative Committee for International Telegraph and Telephone) An international organization that develops communications standards known as "Recommendations" for all internally controlled forms of analogue and digital communication ("Recommendation X.25" for example).

A distributed system model of computing that brings computing power to the desktop, where users (clients) access resources held on servers.

Common Channel Signaling

CDDI(Copper Distributed Data Interface)
FDDI over UTP or STP copper media.

Communications Engine Controller, the main processor for the NETBuilder II.

Cell relay
Network transmission format that uses small packets of uniform size, called cells. The fixed-length cells can be processed and transmitted by hardware at very high speeds. Acts as a basis for SMDS Interface Protocol and ATM.

In SNA, a grouping of RUs (Request/Response Units) to aid error recovery.

Channel Aggregation
Channel aggregation combines multiple physical channels into one logical channel of greater bandwidth. With BRI ISDN connections, channel aggregation would combine the two 64 K bit B channels into a single, logical 128 K bit channel.

Challenge Handshake Authentication Protocol

The IEEE 802.3 10BASE2 standard (or cable used in such installations). Thinnet, another term for the standard, specifies a less expensive, thinner version of traditional Ethernet cable.

CICS (Customer Information Control System)
An IBM application subsystem that permits transactions entered at remote terminals to be processed concurrently by user applications.

CIR (Committed Information Rate)
The transport speed the frame relay network will maintain between service locations.

Circuit-Switched Network
Network that establishes a physical circuit temporarily, until it receives a disconnect signal.

CLNP (Connectionless Network Protocol)
See Connectionless Network Service.

Any of the sources of timing signals used in isochronous data transmission.

CMIP/CMIS (Common Management Information Protocol/Services)
An OSI-based protocol that provides standard ways to manage large multivendor networks.


Complementary Metal-Oxide Semiconductor

An Internet standard defining the use of CMIP (an OSI- based protocol) over TCP for managing TCP/IP networks.

CMT (Connection Management)
Process in FDDI for controlling the transition of the ring through its various operating states (off, connect, active, etc.), under the X3T9.5 specification.

Computer and Network Advisory Panel.

CO (Central Office)
A local telephone company office which connects to all local loops in a given area and where circuit switching of customer lines occurs.

Connection Oriented IPX. A native ATM protocol based on IPX under development by Novell.

Collapsed Backbone
Network architecture under which the backplane of a device such as a hub performs the function of a network backbone; the backplane routes traffic between desktop nodes and between other hubs serving multiple LANs.

Common Carrier
Licensed utility that provides communications services at government-regulated rates.

Reducing the size of a data set to lower the bandwidth or space required for transmission or storage.

Device that serves as a wiring hub in star-topology network. Sometimes refers to a device containing multiple modules of network equipment.

Conditioned analog line
Analog line to which devices have been added to imrpvoe the electrical signal.

Excessive network traffic.

Congestion Control
In a frame relay network, the mechanisms (see BECN and FECN) designed to limit excessive traffic and provide network switches with a means of alerting the access node (e.g., a router) to slow its transmission.

Connection (or Call) Spoofing:
The concept of mimicking correct responses to keep level requests alive at the local end of a temporarily broken connection is called connection (or call) spoofing. Call spoofing saves connect time charges by allowing the call to be disconnected without causing the NOS to time-out the client/host connection. It also enhances data throughput by keeping the line clear of these network administration packets.

Connectionless Communications
A form of packet-switching that relies on global addresses in each packet rather than on predefined virtual circuits.

Connectionless Network Service (CLNS)
Packet-switched network where each packet of data is independent and contains complete address and control information; can minimize the effect of individual line failures and distribute the load more efficiently across the network.

Connection-oriented Communications
A form of packet-switching that requires a predefined circuit from source to destination to be established before data can be transferred.

Connectivity system
A collection of network devices that are logically related and managed as a single entity.

Control System
Control Systems measure environmental changes and perform actions in response to those changes.

CONS (Connection-Oriented Network Service)
An OSI protocol for packet-switched networks that exchange information over a virtual circuit (a logical circuit where connection methods and protocols are pre-established); address information is exchanged only once. CONS must detect a virtual circuit between the sending and receiving systems before it can send packets.

Network access method where devices compete for the right to access the physical medium.

When all routers on a network use a consistent perspective of the network topology.

The chassis backbone in the NETBuilder II

Class Of Service

Connection-Oriented Session Protocol

Common Part Convergence Sublayer

CPE (Customer Premises Equipment)
Terminating equipment, such as terminals, phones and modems, supplied by the phone company, installed at customer sites and connected to the phone company network.

Customer Premises Network

CSMA/CD (Carrier-Sense Multiple Access with Collision Detection)
Channel access method used by Ethernet and IEEE 802.3 in which devices transmit only after finding the data channel clear for some period of time. When two devices transmit simultaneously, a collision occurs and the colliding devices delay their retransmissions for a random length of time.

CSU/DSU (Channel Service Unit/Data Service Unit)
A digital interface unit that connects end user equipment to the local digital telephone loop.

Custom Signaling
ISDN signaling protocols used in AT&T and Northern Telecom switches prior to the advent of the National ISDN 1 standard.

Campus-Wide Information System.

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Destination MAC Address

DAC (Dual Attached Concentrator; also digital-to-analog converter)
A device that is attached to and allows access to both rings in an FDDI network.

Delivery of Advanced Network Technology in Europe - an organisation based in Cambridge, England. For more information see the DANTE web site.

DAS (Dual-Attached Station)
A station with two connections to an FDDI network, one to each logical ring. If one of the rings should fail, the network automatically reconfigures to continue normal operation.

Direct Access Storage Device

Data flow control layer
Layer 5 of the SNA architectural model.

Data link control layer
Layer 2 in the SNA architectural model.

DCE (Data Communications -- or Data Circuit- terminating --Equipment)
A communications device that can establish, maintain and terminate a connection (for example, a modem). A DCE may also provide signal conversion between the data terminal equipment (DTE) and the common carrier's channel. Contrast DTE.

D-Channels are the 16 Kbps (basic rate) or 64 Kbps (primary rate) full duplex ISDN signaling channels which carries messages between the customer equipment and the public switch. Messages can communicate call request information (phone numbers) and incoming call information, for example.

Digital Data Service

Digital Equipment Corporation's proprietary network architecture.

Dedicated line
A transmission circuit installed between two sites of a private network and "open," or available, at all times.

Default route
Entry in a routing table that can re-direct any frames for which the table has no definitive listing for the next hop.

Amount of time a call spends waiting to be processed.

Opposite of modulation; the process of retrieving data from a modulated carrier wave.

Data Encryption Standard - an encryptian scheme approved for use within the US by the National Security Agency (NSA).

Designated router
In OSPF, each multiaccess network with at least two attached routers has a designated router. The designated router has special duties in the running of the protocol, such as generating a link state advertisement for the multiaccess network. This concept helps reduce the number of adjacencies required on a multiaccess network, which cuts routing protocol traffic and the size of the topological database.

Dial up
A type of communication that is established by a switched-circuit connection using the telephone network.

Digital modem:
A digital modem accepts an analog call over a 64 K bit ISDN channel and interprets it via software as a call originated by an analog modem. This process generally requires the use of a DSP to break the analog signal into an equivalent digital bit stream.

DLC (Data Link Control)
The SNA layer responsible for transmission of data between two nodes over a physical link.

DLCI (Data Link Connection Identifier)
A value in frame relay that identifies a logical connection.

DLSw Data Link Switching
It is a method of encapsulating, or tunneling, Logical Link Control Type 2 (LLC2) packets from LAN-based SNA and NetBIOS applications, enabling them to traverse a non-SNA backbone. Specified in FRC 1434.

Data Link Control Exchange

Dependent LU Requester/Dependent LU Server

Direct Memory Access

Distributed Management Architecture

Domain (Domain Name)
Part of the DNS naming hierarchy which identifies a particular network or sub-network. The unique address that identifies a network or Internet site consist of two or more domains, separated by dots, starting with the most specific and ending with the most general. Any given network may have more than one Domain Name, but any one Domain Name can only apply to one network.

Domain Name System, maps Internet Protocol addresses to named computers via a set of distributed databases which are automatically updated.

Demand Protocol Architecture

Demand Priority Access Method

DQDB (Distributed Queue Dual Bus)
Communication protocol proposed by IEEE 802.6 committee for use in MANs.

Distributed Repeater Architecture

DRI (Distributed Recovery Intelligence)
The ability to track down a network problem and automatically isolate the malfunctioning node.

Drop cable
A cable that connects a network device such as a computer to a physical medium such as an Ethernet network. Drop cable is also called transceiver cable because it runs from the network node to a transceiver (a transmit/receiver) attached to the trunk cable. Compare AUI cable.

DS (Digital Signal)
Standard specifying the electrical characteristics for data transmission over four-wire telco circuits. DS1 is 1.544 Mbps and DS3 is 44.736 Mbps. Also referred to as T1 and T3.

Digital (transmission) System 1, or Digital Signal level 1; refers to the 1.44 Mbps (U.S.) or 2.108 Mbps (Europe) digital signal carried on a T1 circuit.

Digital (transmission) System 3, or Digital Signal level 3; refers to the 44 Mbps digital signal carried on a T3 circuit.

A digital signal processor (DSP) is a CPU that is tailored to handle complex mathematical functions.

Downstream Physical Unit

DSU/CSU (Data service unit/channel service unit)
A data service unit/channel service unit connects an external digital circuit to a digital circuit on the customers premises. The DSU converts data into the correct format, and the CSU terminates the line, conditions the signal, and participates in remote testing of the connection.

DTE (Data Terminal Equipment)
End-user equipment, typically a terminal or computer, that can function as the source or destination point of communication on the network. Contrast DCE.

Data Terminal Ready (modems)

Dedicated Token Ring

Dual-attached servers
Servers attached to both paths of an FDDI ring for load balancing and redundancy.

Dual homing
Method used to connect a DAS or DAC to a pair of concentrators on an FDDI ring; used when server or station availability is critical in a network.

DXI Data Exchange Interface
Allows a DTE (such as a router) and a DCE (such as an ATM DSU) to provide an ATM UNI for networks.

Dynamic Bandwidth Allocation:
The ability to add and drop B channels based on the sending of threshold data levels. Specifically, the ability to raise a call over a 2nd B channel when the first B channel becomes saturated and to drop the call when data rates decline.

Dynamic routing
Routing that adjusts automatically to changes in network topology or traffic.

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European designation for T-1.

European designation for T-3.

EBCDIC (Extended Binary Coded Decimal Interchange Code)
An 8-bit data-exchange code used in IBM* mainframes, other computer systems, and associated communications equipment. EBCDIC and ASCII are the two most widely used data codes.

EDI (Electronic Data Interchange)
Method for passing database information and other transactions in standard form electronically between locations or organizations.

Ethernet Data Link

Electronically Erasable Programmable Read-Only Memory

EGP (Exterior Gateway Protocol)(TCP/IP)
The service by which gateways exchange information about what systems they can reach; generally, an exterior gateway protocol is any internetworking prototcol for passing routing information between autonomous systems.

EISA (Extended Industry Standard Architecture)
PC bus systems that is an alternative to IBM's Micro Channel Architecture (MCA). The EISA architecture, backed by an industry consortium headed by Compaq*, is compatible with the IBM AT bus; MCA is not.

E-mail Attachment
A binary file attached to an e-mail message carrying more complex information, for example, a document with formatting codes, images or sound.

Wrapping a data set in a protocol header. For example, Ethernet data is wrapped in a specific Ethernet header before network transit. Also, a method of bridging dissimilar networks where the entire frame from one network is simply enclosed in the header used by the link-layer protocol of the other network.

Applying a specific algorithm to data so as to alter the data's appearance and prevent other devices from reading the information. Decryption applies the algorithm in reverse to restore the data to its original form.

End system
End-user device on a network. Also, a nonrouting host or node in an OSI network.

Enterprise network
Larger network connecting most major points in a company. Differs from a WAN in that it is typically private and contained within a single organization.

Individual, manageable device in a network. Also, OSI terminology for a layer protocol machine. An entity within a layer performs the functions of the layer within a single computer system, accessing the layer entity below and providing services to the layer entity above at local service access points.

Erasable Programmable Read-Only Memory

End System To End System Protocol

Extended Super Frame

ES-IS (end system to intermediate system protocol)
The OSI protocol by which end systems such as networks personal computers announce themselves to intermediate systems such as hubs.

Extended Simple Mail Transfer Protocol - proposed extensions to SMTP for things like maximum message length and allowing content wider than 7 bits. Related to the MIME specifications. The core of the specification is in RFC 1425 and more is in RFC1426 and RFC1427.

IEEE-standard data link protocol that specifies how data is placed on and retrieved from a common transmission medium. Forms the underlying transport vehicle used by several upper-level protocols, including TCP/IP and XNS. See CSMA/CD for a description of Ethernet's media-access method. Contrast TR (token ring).

Explorer super frame
Frame sent out by a networked device in a source route bridging environment to determine the optimal route to another networked device.

Specifies the ISDN numbers, including telephone numbers up to 15 digits long.

Edge Device
A device, such as a router or Ethernet-to- ATM switch, that is directly connected to an ATM network. The UNI defines the connection between the edge device and the ATM network switch. It is the first device a user sees when sending traffic to the ATM network. Also called an end device.

Extended Generalized Programming Environment

Electronic Industries Assn/Telecommunications Industries Assn

Extended Industry Standard Architecture

Ethernet LAN module

Ethernet switching module

Extended Simple Mail Transfer Protocol. Proposed extensions to SMTP (see SMTP) to achieve such things as increased maximum message length and allowing content wider than 7-bits. Related to the MIME spec. Defined in RFCs 1425, 1426 and 1427.

ETSI is the European Telecommunications Standards Institute.

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Frequently Asked Questions - a help document that contains answers to frequently asked questions.

File Allocation Table - the DOS filesystem.

Fast Ethernet
A 100-Mbps technology based on the 10BASE-T Ethernet CSMA/CD network access method.

Fault tolerance
Generally, the ability to prevent a problem on a device from affecting other devices on the same port.

FDDI concentrator module

FCS (Frame Check Sequence; First Customer Shipment)
Extra characters added to a frame for error control purposes; HDLC term adopted by subsequent link layer protocols.

First Customer Ship date. The released final product shipped to the 3Com sales channel.

FDDI (Fiber-Optic Distributed Data Interface; Fiber Distributed Data Interface)
LAN technology that permits 100-megabit-per-second (Mbps) data transfer. ANSI has proposed FDDI as standard X3T9.5.

Fast Ethernet Alliance

FDDI enterprise access module

Forward Explicit Congestion Notification

FEP (Front-End Processor)
Device or board that provides a network interface for networked devices. In SNA, typically an IBM 3745 device.

Fiber-optic cable
A transmission medium that uses glass or plastic fibers, rather than copper wire, to transport data or voice signals. The signal is imposed on the fibers via pulses (modulation) of light from a laser or a light-emitting diode (LED). Because of its high bandwidth and lack of susceptibility to interference, fiber-optic cable is used in long-haul or noisy applications.

File Server (See Server)
A machine on a network whose sole purpose is to centrally store application software and user data.

A service that responds to queries and retrieves user information remotely.

A router or workstation with multiple network interfaces that controls and limits specific protocols, types of traffic within each protocol, types of services, and direction of the flow of information.

PROM (Programmable Read-Only Memory) technology providing nonvolatile storage that can be electrically erased in the circuit and reprogrammed; developed by Intel and licensed to other semiconductor companies.

Technique where routing information received by a routing device is sent out through every interface on that device except the one on which the information was received.

Flow control
Method for ensuring that a transmitting entity does not overwhelm a receiving entity with data.

Fast link pulse

Flexible Media Stack products from 3Com

FOIRL (Fiber Optic Inter-Repeater Link)
Fiber-optic signalling methodology based on the IEEE 802.3 fiber-optic specification.

Fully Qualified Domain Name - the full DNS address of a machine including all the the domains it resides in (including the "root" domain which is signified by a final dot ".").

Fractional T-1
A WAN communications service that provides the user with some portion of a T1 circuit which has been divided into 24 separate 64 Kb channels; Fractional E-1 in Europe.

A piece of a larger packet that has been broken down into smaller units.

Breaking a packet into smaller units when transmitting over a network medium that cannot support the original size of the packet.

Set of bits that form an elementary block of data to be sent over a communications channel. Usually, a frame contains its own control information, including the transmission address and data for error detection.

Frame Relay
A packet-switching wide-area technology for interconnecting LANs at high speeds. Frame relay defines the interface between user equipment and a WAN; it does not define internal operation of the network or the interfaces or protocols used within the WAN itself. For this reason, the term "frame- relay cloud" is often used to describe the internal operation of a WAN that has a frame-relay interface.

File Service Protocol

FTAM (File Transfer, Access and Management)
The OSI remote file service and protocol. See FTP.

FTP (File Transfer Protocol)
(TCP/IP) The Internet application and protocol used to send complete files over TCP/IP services.

Full duplex
The ability of a device or line to transmit data simultaneously in both directions.

Full-Duplex Token Ring
Part of the 802.5 standard that defines dedicated and full-duplex communication for Token Ring networks at speeds of 32 Mbps.

Functional grouping
Placing all users performing the same function, and the servers they require, on the same ring.

Frame-based UNI

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Device that can interconnect networks with different, incompatible communications protocols. The gateway performs a layer-7 protocol-conversion to translate one set of protocols to another (for example, from TCP/IP to SNA or from TCP/IP to X.25). A gateway operates at Open Systems Interconnection (OSI) layers up through the Session Layer. Contrast bridge and router.

GGP (Gateway-to-Gateway Protocol)
MILNET protocol that uses distributed shortest path algorithm to control how core gateways (or routers) should exchange access and routing information.

Graphics Interchange Format - an image file format.

A network based versatile menu-driven information service.

GOSIP (Government Open Systems Interconnection Profile)
U.S. government version of the OSI protocols. GOSIP compatibility is a requirement in government networking purchases.

Giga Bits per Second (equivalent to 1,000 Mbps)

Graphic User Interface

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Half duplex
Data transmission that can occur in two directions over a single line, but only one direction at a time. Contrast full duplex.

HDLC (High-Speed Data Link Control)
A protocol defined by the International Standards Organization and used in X.25 communications; specifies an encapsulation method for data on synchronous serial data links. Various manufacturers have proprietary versions of HDLC, including IBM's SDLC.

HEPCCC or HEP triple-C
High Energy Physics Computing Consultative Committee.

Hierarchical routing
Routing based on a hierarchical addressing system. IP routing algorithms use IP addresses, for example, which contain network numbers, host numbers and, frequently, subnet numbers.

HLM (Heterogenous LAN Management)
Management of LANs that contain dissimilar devices running different protocols and different applications.

A unit that equates to the passage of a packet through one router.

Hop Count
A routing metric used to measure the distance between a source and a destination.

Any computer on the network that is a repository for services available to other computers on the network.

High-Performance Routing

High-Performance Scalable Networking (3Com)

High-Speed Serial

HSSI (High-Speed Serial Interface)
A de facto standard for high-speed serial communications at up to 52 Mbps over WAN links.Used for the physical connection between a router and a DSU.

Hyper Text Markup Language - the format of WWW documents.

HEPCCC Technical Sub Committee.

Hyper Text Transfer Protocol.

The centre of a star topology network or cabling system.

A distributed hypertext system mostly popular in Europe.

Connections between hypermedia or hypertext documents.

Hypertext documents that includes or links to others forms of media (eg sound or video).

Text that, when selected, has the ability to present connected documents.

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IAB (Internet Activities Board)
The technical body that oversees the development of the Internet suite of protocols.

ICMP (Internet Control Message Protocol)
(TCP/IP) The collection of messages exchanged by IP modules in both hosts and gateways to report errors, problems and operating information.

IDRP (Interdomain Routing Protocol)
OSI protocol that controls how routers in different domains communicate with each other.

IEEE (Institute of Electrical and Electronics Engineers)
Committees that develop and propose computer standards, such as the 802 protocols, which define the physical and data link protocols of communication networks. Members represent an international cross section of users, vendors and engineering professionals.

Internet Engineering Task Force

IGP (Interior Gateway Protocol)
The protocol used to exchange routing information between collaborating routers in the Internet. RIP and OSPF are examples of IGPs.

Interim Local Management Interface. An interim requirements definition in ATM Forum UNI 3.1. It supports bidirectional exchange of management information between UNI Management Entities related to the ATM layer and physical layer parameters.

Interactive Mail Access Protocol. A more sophisticated version of POP which adds more mailbox capabilities. Defined in RFC1203.

Interprocessor Messaging System in 3Coms Multi- Processor Architecture

Input packet filtering
Filtering applied to packets immediately upon reaching the router, before they reach the router's internal forwarding processing. Since the packets never enter the router, the router itself is protected against an external attack.

Integrated IS-IS
Routing protocol based on the OSI routing protocol IS- IS, but with support for IP or other networks. Integrated IS-IS implementations send only one set of routing updates and are more efficient than two separate implementations.

Interactive Video (IV)
A computer linked by software to a video system which allows the user control of a video disc and view the information on it.

Interexchange carrier (IXC)
A long-distance telephone company offering circuit- switched, leased-line or packet-switched service or some combination.

InterLata are connections between local access companies, i.e., long distance connections.

internet (lower case i)
An internet is any collection of two or more connected networks.

Internet (upper case I)
The vast collection of connected networks (ie an "internet"), comprising large backbone nets (MILNET, NSFNET and CREN for example) and an array of regional and local campus networks worldwide. All of which uses the TCP/IP protocol suite.

Internet Address
(See IP Address)

Series of networks interconnected by routers or other devices that functions as a single network. Sometimes called an internet, which is not synonymous with the Internet.

A private network that is solely for the use of it's owners.

Inverse multiplexing
The logical aggregation of multiple switched circuits to achieve a higher effective transmission speed.

Input/Output (operations or requests) per second

IP (Internet Protocol)
(TCP/IP) The standard for sending the basic unit of data, an IP datagram, through an internet.

IP Address
The unique set of four numbers, in the range 1 - 254 (eg., by which every host machine on the Internet is identified.

Internet Protocol/Routing Internet Protocol.

IP spoofing
The use of a forged IP source address to circumvent a firewall. The packet appears to have come from inside the protected network and to be eligible for forwarding into the network.

IPX (Internetwork Packet Exchange, Network Protocol)
LAN protocol developed by Novell for NetWare*.

Internet Relay Chat - allows multiple users to send messages to all other participating users in real time.

IS (Intermediate System)
A bridge, router, gateway or hub that interconnects network segments.

ISDN (Integrated Services Digital Network)
The recommendation published by CCITT for private or public digital telephone networks where binary data, such as graphics and digitized voice and data transmission, pass over the same digital network that carries most telephone transmissions today.

Information Systems department in corporations

Industry Standard Architecture

Internet Switching Engine

IS-IS (Intermediate System to Intermediate System (OSI protocol)
The OSI protocol by which intermediate systems exchange routing information.

International Standards Organisation.

Internet Service Provider.

International Telecommunications Union Standardization Sector

International Telephone Union

(See Interexchange Carrier)

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The uncontrolled transmission of oversized frames to the network by a faulty device.

Joint Academic NETwork. a datacommunicatinos network linking universities and other academic institions around the UK to each other and the Internet. Maintained by UKERNA. For more information see JANET web page.

Java is a new programming language invented by Sun Microsystems which is specifically designed for writing programs that can be safely downloaded to your computer through the Internet without fear of viruses. Using small Java programs (Applets), Web pages can become more sophisticated and include such things as animation, calculators and text and graphics editors.

Joint Information Services Committee. For more information see JISC web page.

Degradation of the signal as it traverses the cable or the Network Interface Cards (NICs). Common in token ring environments and causes errors in accuracy of signal.

A file format defined by the Joint Photographics Experts Group for still picture compression.

Joint Product Specification

Another searching tools for finding information on specific gophers.

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Kilo Bits Per Second (one thousand bits per second)

Kilo Packets Per Second (one thousand packets per second)

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LAM (Lobe Access Module)
Terminals connect to a LAM, which is connected to a CAU.

LAN (local Area Network)
An assembly of computing resources such as PCs, printers, minicomputers and mainframes linked by a common transmission medium such as coaxial cable.

LANE (LAN Emulation)
As specified by the ATM Forum, it is a standard implementation for making edge devices appear as though they were attached to a LAN. An emulated LAN has all of the benefits (and weaknesses) of the traditional LANs they are emulating. Currently only Ethernet and Token Ring LAN Emulation are specified. 3Com, and most other vendors, are implementing Ethernet LAN Emulation first.

LAN Segmentation
Dividing LAN bandwidth into multiple independent LANs to improve performance.

LAT (Local Area Transport)
A protocol developed by Digital Equipment Corporation.

LATA (Local Access Transport Area)
A telephone company term that defines a geographic area; sometimes corresponds to an area code.

The time delay between when the first bit of a packet is received and the last bit is forwarded.

A level of the OSI Reference Model. Each layer performs certain tasks to move the information from sender to receiver. Protocols within the layers define the tasks for networks, but not how the software accomplishes the tasks. Interfaces pass information between the layers they connect.

Line Control Protocol

Leased line
A transmission line reserved by a communications carrier for the private use of a customer.

LAN Emulation

LEC (Local Exchange Carrier)
Local phone company.

LEC LAN Emulation Client
An edge device, directly attached to the ATM Network operating in a ATM Emulated LAN. This can be, for example, a PC, a router, or a Ethernet-to-ATM switch directlyconnected to the ATM switch running LAN Emulation Client software.

LAN Emulation Configuration Server

low-entry networking

LAN Emulation Server

Physical connection between two nodes in a network. It can consist of a data communication circuit or a direct channel (cable) connection.


A program that watches for incoming mail in a given mailbox and forwards all messages to a list of addresses associated with that mailbox.

LLC (Logical Link Control; Link Layer Control)
Upper part of ISO layer two.

Logical Link Control 2

Local Management Interface

Load balancing
In routing, the ability of the router to distribute traffic over all its network ports that are the same distance from the destination address. It increases the use of network segments, which increases effective network bandwidth.

Local loop
The line from a telephone customer's premises to the telephone company CO (central office).

Logical ring
Network which is treated logically as a ring even though it may be cabled as a physical star topology.

LAN Security Architecture

Link Support Layer

LU (Logical Unit)
One end of a communication session in an SNA network. LU 6.2 provides peer-to-peer communications over an SNA network.

LAN emulation User Network Interface

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MAC (Media Access Control)
A method of controlling access to a transmission medium. For example, token ring, Ethernet, FDDI, etc.

The location (often a flat file) where email messages are stored. On mail servers sometimes referred to as the spool file or maildrop.

MAN (Metropolitan Area Network)
A data communication network covering the geographic area of a city (generally, larger than a LAN but smaller than a WAN). FDDI can provide a private MAN, while IEEE 802.6 can provide a public MAN.

MAU (Multistation Access Unit)
A hub in a token ring network; each MAU supports up to eight nodes and servers and can be connected to other units to create large networks. Also Medium Attachment Unit (IEEE 802.3).

Medium Attachment Unit

Mega Bytes

Mega Bits Per Second (one million bits per second)

MCA (Micro Channel Architecture)
The basis for the IBM Micro Channel bus used in high- end models of IBM's PS/2 series of personal computers.

Multicast Server

MIB (Management Information Base)
A collection of objects that can be accessed via a network management protocol.

The process of dividing the network into many small Ethernet segments, each with a small client population.

MIDI Interface
A device which allows the interchange of signals between a computer and a music synthesizer.

Media-Independent Interface

In 1984, NSF funding for the Computer Science Network led to ARPANET being split into two - MILNET for unclassified military traffic and ARPANET for research and non-military use.

Multipurpose Internet Mail Extensions. A standard format to allow multimedia in mail messages.

Multiple Link Interface Driver

Multi-Level (three-level) Encoding

MODulator-dEMdulator. A device which translates digital signals from a computer into analogue signals which can be transmitted over and ordinary telephone line and vice versa.

Process by which signal characteristics are transformed to represent information. Types of modulation include frequency modulation (FM), where signals of different frequencies represent different data values.

The original graphical web browser. Developed by the NCSA.

Multi-Processor Architecture for the NETBuilder II used in the 6-port Ethernet and the ATM UNI modules

A format for motion video compression defined by the Moving Photographics Experts Group.

MultiProtocol Over ATM

Multiprotocol Router Plus (Novell NetWare)

Multi-Services Hub (3Com)

(Multiple SNA Network Facility)

Mail Transport Agent (eg sendmail). The software which handles the delivery of messages.

Mean Time Between Failures

Mean Time To Repair

MTU (Maximum Transmission Unit)
The largest possible unit of data that can be sent on a given physical medium. For example, the MTU of Ethernet is 1,500 bytes.

Mail User Agent. The software the user uses to retrieve and send email to or from a server.

A special form of broadcast where copies of the packet are delivered only to a subset of all possible destinations.

Multi-link PPP
Multi-link PPP is a variant of PPP that addresses the additional features of compression and channel aggregation. PPP-ML, as it is known, is outlined in IETF RFC 1717.

A technique that enables several data streams to be sent over a single physical link; also, (ISO) a function by which one connection from a layer is used to support more than one connection to the next higher layer.

MUX (Multiplexer)
A telecommunications device that funnels multiple signals onto a single channel.

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National ISDN1
NI-1 is the ISDN standard in the US. It is the first successful attempt to standardize at a level allowing the same end user equipment to connect transparently to differentswitch vendor's equipment. Prior to this standard, all end user equipment needed to understand the particulars of the switch to which it was connected.

NETBuilder II router

NBP (NetBIOS protocol)(3Com)
Developed for LAN Manager; small, high-performance transport protocol with a NetBIOS interface.

Network Control Protocol

Network Control Server

Network Control Server/AT

National Center for Supercomputing Applications. A federally funded organization ine the USA whose mission is to research and develop high-technology resources for the scientific community.

NDIS (Network Driver Interface Specification)
Produced by Microsoft, a specification for a generic, hardware-and protocol-independent device driver for NICs.

Net3 (Norme Europenne de Telecommunications) is the European-wide standard for ISDN. The trade name for the standard is EuroISDN.

NetBIOS (Network BIOS [basic input/output system)
Standard interface to networks on IBM PC and compatible systems.


Another graphical world wide web browser.
Network Identifier

Network Layer
OSI layer that is responsible for routing, switching and subnetwork access across the entire OSI environment.

Near End Crosstalk

Network File System

The backbone of the Internet since 1990 when ARPANET was dimantled.

A Usenet discussion group or bulletin board.

Network Interface Card. See Adapter.

Internet Network Information Center. Responsible for setting up and administering domain names.

National Information Infrastructure (US).

Network Information Service

Narrowband ISDN

NetWare loadable module

Network Layer Protocol IDentifier

NetWare Link Services Protocol (Novell)

Network-to-Network Interface

News Network Transfer Protocol. Defined in RFC1036, this is a common method by which articles are transferred on Usenet.

Any computer that is connected to a network.

Node emulators
A Personal Routing concept that creates logical proxies for remote users so that they appear as local clients on the corporate LAN.

Network Operating System

National Research and Education Network - the proposed broadband successor to the Internet in the US.

Network Service Access Point,an OSI layer 3 (or layer 3 style) network addresses

A NT1 is the equipment required to convert from the two wire U interface to the four wire S/T interface. This equipment is NOT REQUIRED outside of North America.

Network Termination, type 2

NT File System. The file system designed for Microsoft Windows NT.

Network Technical Support Alliance

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OC Optical Carrier
The optical specification over SONET. An OC level is the optical equivalent of an STS signal. Transmission rates are based on 51.84 Mbps (OC-1). A c following a OC level identifies concatenation of payload (i.e., OC-3c).

ODI (Open Data-link Interface)
Novell specification providing standardized access to networks.

Overnight Hardware Replacement (3Com)

OSI (Open Systems Interconnection)
The goal for internetworking computers from different vendors, to be achieved by full adherence to international standards. See OSI Reference Model.

OSI Reference Model
Seven-layer network architecture model of data communication protocols developed by ISO and CCITT. Each layer specifies particular network functions such as addressing, flow control, error control, encapsulation and reliable message transfer.

Open Software Foundation

OSPF (Open Shortest Path First)
Routing protocol for TCP/IP networks.

Output packet filtering
Filtering applied to packets after they have been through the router's internal forward processing.

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When data is transmitted through networks it is often split up into small packets rather than being sent as a continuous byte stream.

Packet-switched network
A network in which data is transmitted in units called packets. The packets can be routed individually over the best available network connection and reassembled to form a complete message at the destination.

Packet Switching
Type of data transfer that occupies a communication link only during the time of actual data transmission. Messages are split into packets and reassembled at the receiving end of the communication link.

PAD (Packet Assembler-Dissembler)
The mechanism for disassembling packets at the sending end and assembling them to form the complete message at the receiving end; traditionally used in X.25 networks.

Programmable Array Logic

Password Authentication Protocol

Positive Acknowledgment with Retransmission

Parallel TaskingTM
3Com technology which allows adapters to transmit data to the network before an entire frame has been loaded from the computer into the adapter's buffer, and to transmit data to the computer's main memory before an entire frame has bee received from the network. In effect, a frame can reside on the network, the adapter and in computer memory simultaneously, boosting throughput.

Path Control Layer
Layer 3 in the SNA architecture model. This is the SNA layer that routes packets through an internetwork.

Private Branch Exchange

Peripheral Component Interconnect (Bus)

Physical Connection Management (FDDI)

PCMCIA (Portable Computer Memory Card Industry Association)
An industry group that has developed a standard for credit-card-size peripherals for portable computers.

Plesi-synchronous Data Hierarchy.A transmission method used on some high-speed data lines.

Public Data Network

Packet-Driver Specification

PDU (Protocol Data Unit)
OSI terminology for "packet." A PDU is a data object exchanged by protocol machines within a given layer of the OSI Reference Model containing both Protocol Control Information and user data.

Peer-to-Peer Communications
A type of communications and data exchange between peer entities on two or more networks.

Privacy-Enhanced Mail. Relatively new standard for email security using crytographic techniques for authentication and privacy of messages. Defined in RFCs 1421, 1422, 1423 and 1424. RIPEM is an attempt to implement this.

Perimeter Network
A small, single-segment network between a firewall and the Internet for services that the organization wants to make publicly accessible to the Internet without exposing the network as a whole.

Personal office internetworking
Providing individual remote users access to corporate LAN resources.

Personal Routing system architecture
A combination of hardware and software that makes a remote user, connected over a WAN link, appear as a logical local client of the enterprise network.

Packet Exchange Protocol

The physical layer of FDDI; also, a term for FDDI fiber optic cable. In the layer structure, PHY is positioned between the MAC and the PMD.

Physical Control Layer
Layer 1 in the SNA architecture model.

Physical Layer
First layer of the OSI Reference Model; governs hardware connections and byte-stream encoding for transmission. It is the only layer that involves a physical transfer of information between network nodes.

Physical Media
Any physical means for transferring signals between OSI systems. Considered outside the OSI Model, and sometimes referred to as "Layer 0," or the bottom of the OSI Reference Model.

Ping (Packet internet groper)
A program used to test reachability of destinations by sending them an ICMP echo request and waiting for a reply. Ping is used as a verb: "Ping the host to see if it is available."

PLL (Phase Locked Loop)
Function that ensures accurate signal timing in a token ring network automatically, as opposed to Tank circuits which must be adjusted manually.

PMD (Physical layer Medium-Dependent single mode)
The sublayer of the ATM Physical Layer that defines connectors, fiber optic parameters, etc.

PNNI Private Network-Node Interface.
Allows multi-vendor switch interoperability for SVC setup. It will eventually allow dynamic ATM networks to be constructed with heterogeneous (multi-vendor) components.

A method of controlling the sequence of transmission by devices on a multipoint line by requiring each device to wait until the controlling processor requests it to transmit.

Point of Presence

Post Office Protocol - POP2 and POP3 and standards used on some email servers.

3 meanings:
1 - the physical connection that allows data flow into or out of a computer (eg the serial port of a pc).
2 - part of an address (eg a URL) on a networked computer that specifies a particular service running on that computer. Most services run on standard port numbers in which case the port need not be specified.
3 - the translation of an application from one computer platform to another.

Port Density
The number oof ports, physical or logical, per network device.

A page description language, accepted by many printers.

Most sites that can receive email have someone responsible for ensuring the delivery of email, known as the postmaster. If you have a problem with email or can't find a user's address by other means, you can send email to postmaster@host (where host is the site you're trying to contact).

POTS stands for Plain Old Telephone Service. POTS is the existing analog telephone lines.

Particle Physics Network Coordinating Group. For more information see PPNCG web page.

PPP (Point-To-Point Protocol)
Successor to SLIP; provides router-to-router and host-to-network connections over both synchronous and asynchronous circuits.

Packets Per Second

Presentation Layer
OSI layer that determines how application information is represented (encoded) while in transit between two end systems.

Presentation Services Layer
Layer 6 of the SNA architecture model.

Primary Rate ISDN (PRI)
The Primary Rate Interface consists of 6-30 (23 in the US) B channels and two (one in the US) 64 K bit/second D channel, delivered over a single E1 link (2.08 M bit/second). In the US PRI is delivered over the same physical link as a T1 (1.55 M bit/second) link.

Programmable read-only memory

Protocol converter
Device for translating the data transmission code and/or protocol of one network or device to the corresponding code or protocol of another network or device, enabling equipment with different conventions to communicate with one another.

Protocol Islands
Network topologies confined to a single leaf network that have no interconnection needs with other leaf networks or the central node.

Protocol Stack
Related layers of protocol software that function together to implement a particular communications architecture.

Protocol Translator
Network device or software that converts one protocol into another, similar, protocol.

Packet Switching Data Network

Public Switched Telephone Network. The traditional analogue telephone network as opposed to ISDN.

Public Telephone Operator. See PTT.

PTT, or Public Telephone and Telegraph, is a generic term for European telephone companies. Most are (currently) state owned and operated. The Deutsche Bundespost is one example of a PTT.

Physical Unit

Physical Unit type 2

PVC (Permanent Virtual Circuit)
Generally, a virtual circuit that is permanently established. PVCs save bandwidth associated with circuit establishment and tear down in situations where certain virtual circuits exist all the time.

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This is the link layer protocol for ISDN.

This is the network layer protocol for ISDN. Q.931 was developed for out of band call control.

QoS (Quality of Service)
Term to describe delay, throughput, bandwidth, etc. of a Virtual Connection.

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Rate Adaption
The conversion of non-ISDN data traffic to a format compatible with ISDN 64-Kbps transmission.

Remote Boot and Configuration Service

RBOC stands for Regional Bell Operating Company. The companies (the Baby Bells) are the seven regional telephone companies that were spun off as part of the AT&T divestiture in 1984. Examples of a RBOC are Bell Atlantic, Bell Northern, Pacific Bell, US West, Nynex.

Software that intercepts requests for resources within a computer and analyzes them for remote access requirements.

Remote bridge
Bridge that connects physically dissimilar network segments across WAN links.

Remote Networking
Extending the logical boundaries of a corporate LAN over wide-area links to give remote offices, telecommuters, and mobile users access to critical information and resources.

Remote Office Internetworking
Connecting LAN-based remote offices to the corporate LAN.

Remote User
A telecommuter, individual contractor, business traveler, or nomadic user who needs client access to a corporate enterprise LAN over dial-up WAN links.

Device that connects 802.3 network cable segments. Regeneration and retiming ensure that the signal is clearly transmitted through all segments. (The functionality is defined in detail in the IEEE 802.3 specification.)

Reverse ARP
Reverse Address Resolution Protocol

Request For Comments from the IETF

RFC 822
The document containing the original specification for the format of Internet email messages.

Resilient Home Architecture (3Com)

Routing Information Field

Ring latency
Time required for a signal to propagate once around a ring in a Token Ring or IEEE 802.5 network.

Ring Topology
Network topology in which a series of repeaters are connected to one another by unidirectional ransmission links to form a single closed loop. Each station on the network connects to the network at a repeater.

RIP (Routing Information Protocol)
A routing protocol for TCP/IP networks.

Reduced Instruction Set Computing

RMON (Remote Monitoring)
Subset of SNMP MIB II allows flexible and comprehensive monitoring and management capabilities by addressing up to 10 different groups of information.

Root bridge
Appointed by the spanning tree and used to determine which managed bridges to block in the spanning tree topology.

Protocol-dependent device that connects subnetworks together. It is useful in breaking down a very large network into smaller subnetworks. Routers introduce longer delays and typically have much lower throughput rates than bridges.

The selection of a communicatinos path for the transmission of information from source to destination.

Routing bridge
MAC-layer bridge that uses network layer methods to determine a network's topology.

Routing Protocol
Protocol that accomplishes routing through the implementation of a specific routing algorithm.

Routing Table
Table stored in a router or some other internetworking device that keeps track of routes (and, in some cased, metrics associated with those routes) to particular network destinations.

Routing Update
Message sent from a router to indicate network reachability and associated cost information. Routing updates are typically sent at regular intervals and after a change in network topology.

Remote Procedure Call

Remote Program Load

Redundant Power System (3Com)

Read The Fine Manual. Though often the "F" is taken to mean something more obscene. RTFM.MIT.EDU is an anonymous ftp site that maintains a copy of almost every FAQ produced.

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SAA (Systems Application Architecture)
An IBM architecture designed to allow all of IBM products to communicate and function together.

Signaling AAL

SAC (Single-Attached Concentrator; also Security Access Control)
See DAC.

serial asynchronous module

Service Advertisement Protocol

Service Access Point

Segmentation And Reassembly. One of two sublayers in an AAL.

SAS (Single-Attached Station)
See DAS.

Synchronous Digital Hierachy, the ITU-T version of SONET. The basic SDH rate (STM-1) is 155.52 Mbps (mostly equivalent to SONETs STS-3/OC-3).

Shielded Data Distributed Interface (FDDI over STP)

SDLC (Synchronous Data Link Control (IBM version of HDLC))
IBM communications line discipline or protocol associated with SNA, SDLC provides for control of a single communications line or link, accommodates a number of network arrangements and operates in half- or full-duplex over private or switched facilities.

Service Data Unit

Secure logging
A method whereby an audit trail of system activity is received from a bastion host and placed in a secure location.

Splitting an overloaded ring into two or more separate rings, linked by a bridge/router or multipurpose hub.

Serial interface
Interface which requires serial transmission, or the transfer of information in which the bits composing a character are sent sequentially. Implies only a single transmission channel.

In the context of the client-server model is the software (and sometimes the physical machine) that supplies the requested resources to the client.

Server clustering
Placing all the servers on one or more rings in a central location.

Server Farm
A cluster of servers in a centralized location serving a wide user population.

Session Layer
OSI layer that provides means for dialogue control between end systems.

Standard Generalized Markup Language. A generic language for representing documents.

Shared Ethernet
Conventional CSMA/CD Ethernet configuration, where all stations are attached to a hub sharing 10 Mbps of bandwidth; only one station can transmit at a time.

Simplex transmission
Data transmission that can occur in only one direction on a given line. Compare half duplex and full duplex.

Single mode fiber
Fiber with a relatively narrow diameter, through which only one mode will propagate. Carries higher bandwidth than multimode fiber, but requires a light source with a narrow spectral width.

Software Investment Protection Service

SLIP (Serial Line Interface Protocol)
Internet protocol used to run IP over serial lines such as telephone circuits or RS-232 cables interconnecting two systems. SLIP is now being replaced by PPP.

Intelligent management agents in devices and logical connectivity systems that reduce the computational load on the network management station and reduce management-oriented traffic on the network.

Switched Multimegabit Data Service. A standard for very high-speed data transfer.

Structure of Management Information

system management module

Specific Multicast Server

SMT (Station Management)
That part of the FDDI specification that manages stations on the ring, as defined by the X3T9.5 specification.

SMTP (Simple Mail Transfer Protocol)
Protocol governing mail transmissions; defined in RFC 821, with associated message format descriptions in RFC 822.

SNA (Systems Network Architecture)
IBM's proprietary network architecture.

SNAP (Sub Network Access Protocol)
Internet protocol that operates between a network entity in the subnetwork and a network entity in the end system and specifies a standard method of incapsulating IP datagrams and ARP messages on IEEE networks.

SNMP (Simple Network Management Protocol)
Preferred network management system for TCP/IP-based internets.

SNP (Subnetwork Protocol)
(TCP/IP) Protocol residing in the subnetwork layer below IP that provides data transfer through the local subnet. In some systems, an adapter module must be inserted between IP and the Subnetwork Protocol to reconcile their dissimilar interfaces.

SONET (Synchronous Optical Network)
Standard for digital transmission at rates from 51.84 Mbps to beyond 2.4 Gbps (gigabits per second).

Spam or Spamming
A term used to refer to the inappropriate use of a mailing list or Usenet. Usually meaning the message has been sent to a large number of newsgroups many of which the message will not be relevent to. It's been suggested that the term comes from a Monty Python sketch where the word Spam is repeated over and over. (Spam is a Registered Trademark of Hormel Corporation, for it's processed meat product).

Spanning tree
A technique that detects loops in a network and logically blocks the redundant paths, ensuring that only one route exists between any two LANs; used in an IEEE 802.1d bridged network.

Service Profile ID, uniquely identifies a B channel on the ISDN network. The SPID must be stored in any device accessing the ISDN.

Split horizon
Routing technique where information about routes is prevented from exiting router interfaces through which that information was received. Useful in preventing routing loops.

The use of a forged IP source address to circumvent a firewall. The packet appears to have come from inside the protected network, and to be eligible for forwarding into the network.

SQE (Signal Quality Error)
Transmission sent by a transceiver back to the controller to let the controller know whether the collision circuitry is functional.

Source Routing

SRT (source routing transparent) Bridge
Proposed IEEE 802.1 bridge to combine source routing (in which the source end system provides routing information) with transparent bridging (in which the bridge makes independent message handling choices and therefore is transparent to the message source and destination).

Service-Specific Connection-Oriented Protocol.

Service Specific Convergence Sublayer

The signaling system number 7 (SS7) protocol is used in the public networks to establish connections between switches. ISDN connections to switches that support SS7 have access to true 64 K bit connections between public switches. ISDN connections to switches that do not support SS7 (such as those in the Pacific Bell region) are limited to 56 kbps on each B channel as the switch signaling must be accommodated in-band.

S/T Interface
An S/T interface is a four wire BRI interface presented to the customer by the PTTs in non-US markets.

STA (Spanning Tree Algorithm)
Function of managed bridges which allows redundant bridges to be used for network resilience, without the broadcast storms associated with looping. If a bridge fails, a new path to a redundant bridge is opened.

A group of network devices that are logically integrated into a single system.

Star topology
Network configuration where all the nodes are connected to a central point via individual cables.

Static routing
System in which routing information is manually entered into the routing table.

Spanning Tree Explorer

Synchronous Transport Module. Specifies the electrical and optical transmission over SDH. Transmission rates are based on 155.52 Mbps (STM-1, equivalent to SONETs OC-3).

STP (Shielded Twisted Pair)
Common transmission medium which consists of a Receive (RX) and a Transmit (TX) wire twisted together to reduce crosstalk. The twisted pair is shielded by a braided outer sheath.

Synchronous Transport Signal. The logical signal specification for SONET frame structure. This specifies electrical transmission rates based on 51.84 Mbps (STS-1). OC-3 is 155.52 Mbps.

Collection of OSI end systems and intermediate systems under the control of one administrative domain and using a single network access protocol. For example, private X.25 networks, a series of bridged LANs. Compare Autonomous System.

The broadband upgrade to JANET.

SVC (Switched Virtual Circuit)
Virtual circuit that can be dynamically established on demand.

Switched 56
Switched data transmission service at 56 Kbps (as opposed to service on dedicated leased lines).

Switched Ethernet
An Ethernet hub with integrated MAC layer bridging or switching capability to provide each port with 10 Mbps of bandwidth; separate transmissions can occur on each port of the switching hub, and the switch filters traffic based on destination MAC address.

Switched Virtual LAN
A logical network consisting of several different LAN Emulation domains controlled through and intelligent network management application.

Switching hubs
Hubs that use intelligent Ethernet switching technology to interconnect multiple Ethernet LANs and higher-speed LANs such as FDDI.

Synchronous transmission
Form of usually high-speed data communication that uses synchronization bytes instead of start or stop bits to tell the receiving device about the coming transmission. More complex that asynchronous.

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Communications circuit provided by long-distance carriers for voice and data transmission (1.544 Mbps in U.S. or 2.054 Mbps in Europe); may be divided in 24 separate 64 Kb channels; E-1 in Europe.

Digital communications circuit standard created by AT&T that operates at 44 Mbps.

Terminal Adapter

Transparent Bridging

Tank Circuit
Ensures accurate signal tracking in token ring networks and prevents degradation of the signal; must be adjusted manually.

TCP/IP (Transmission Control Protocol/Internet Protocol)
Set of protocols developed by the U.S. Defense Department's Advanced Research Projects Agency (ARPA) during the early 1970s. Its intent was to develop ways to connect different kinds of networks and computers. TCP/IP does not have the functionality that OSI provides.

TDM (Time Division Multiplexing)
Technique where information from multiple channels may be allocated bandwidth on a single wire based on time slot assignment.

topology database update

Terminal Equipment

A common campus-wide information system developed at MIT.

Text Encoding Initiative. An effort to create a large flexible SGML DTD for coding various types of written texts.

Telco is a popular abbreviation for "telephone company." The RBOCs are a subset of all telcos.

A program which allows users to remotely use computers TCP/IP networks.

Trans-European Network at 34 Mbits/second. For more information see TEN-34 web page.

Terminal server
Communications processor that connects asynchronous devices to a LAN or WAN through network and terminal emulation software.

Trivial File Transfer Protocol

See Cheapernet.

A dumb terminal for IBM* mainframes; generally, a terminal emulation standard for connecting to mainframe resources.

Token Ring interface coupler

Terminal emulation software that allows a terminal to appear to an IBM host as a 3270 model 2 terminal.

Control information frame, possession of which grants a network device the right to transmit.

TP (Twisted Pair)
Cable consisting of two 18 to 24 AWG (American Wire Gauge) solid copper strands twisted around each other. The twisting provides a measure of protection from electromagnetic and radio-frequency interference.

twisted-pair/physical medium dependent

TR (Token Ring)
As defined in IEEE 802.5, a communications method that uses a token to control access to the LAN. The difference between a token bus and a token ring is that a token ring LAN does not use a master controller to control the token. Instead, each computer knows the address of the computer that should receive the token next. When a computer with the token has nothing to transmit, it passes the token to the next computer in line.

Transaction Services Layer
Layer 7 in the SNA architecture model.

Translation Bridging
Bridging between networks with dissimilar MAC sublayer protocols.

An integrated management solution from 3Com that is based on groups of logically related devices and includes a powerful set of management tools.

Transparent bridging
Bridging scheme preferred by Ethernet and IEEE 802.3 networks in which bridges pass frames along one hop at a time based on tables associating end nodes with bridge ports.

Transport Layer
OSI layer that is responsible for reliable end-to-end data transfer between end systems.

Technical Support Alliance (3Com and Novell)


Cable contains two coaxial cable runs. Typically used in a IBM AS/400 environment and connects IBM 5250 to host.

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Universal Asynchronous Receiver/Transmit chip, used as communications (COM) port in personal computers. Maximum data rates vary with model; the National Semiconductor 16550A

Universal Data Link Control

UDP (User Datagram Protocol)
Internet standard protocol that allows an application on one machine to send a datagram to an application program on another machine.

U Interface
A U interface is a two wire interface presented to the customer by the telco in the US market. The customer is responsible for converting this signal to the four wire S/T interface in order to make a connection.

United Kingdon Education and Research Networking Association.

User-to-Network Interface

Uniform Resource Locator. Standardized format for specifying a network service or document within an HTML document.

The global news-reading network.

Unshielded Twisted Pair See STP.

Unix-Unix Copy Protocol. A method of exchanging data between to unix machines.

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CCITT analog facsimile analog modem signaling standard providing up to 14400 kbps data rates and backwards compatible to the V.29 standard, which supports speeds up to 9600 kbps.

CCITT standard for data transfer at 2400 bits/second via a modem.

CCITT analog modem signaling standard providing up to 14400 kbps data rates.

CCITT standard for modem data transfer at 28800 bits/second.

CCITT analog modem data compression standard. Provides a theoretical maximum of 4:1 compression, although 2:1 is more commonly experienced.

The CCITT recommendation governing data transmission at 48 Kbps using 60 to 108 KHz band circuits.

VC (Virtual Channel)
A point-to-point or point-to- multipoint connection between ATM end-stations. Can either be switched or permanent. It is identified by the combination of the VCI and VPI.

VCC (Virtual Channel Connection)
They are defined by a series of VCs logically assigned to make an end-to-end link.

VCI Virtual Connection Identifier
A 16-bit identifier having only local significance on the link between ATM nodes. See VC.

A network service that allows users to search Gopher systems for documents.

VFS (Virtual File Storage)
Intermediate format used for data in transmit from one system to another system. It is used as the transit format for File Transfer, Access and Management (FTAM) and provides a set of common file operations that all FTAM systems understand (such as copy or delete).

Virtual Channel Identifier
A unique numerical tag for every virtual channel across an ATM interface.

Virtual Circuit
Circuit-like service provided by the software protocols of a network, enabling two end points to communicate as though connected by a physical circuit. Network nodes provide the addressing information needed in the packets that carry the source data to the destination.

Virtual Route
SNA terminology for virtual circuit. A logical connection between subarea nodes that is physically realized as a particular explicit route.

Very Large-Scale Integration

VN3 and VN4 are the French ISDN standards.

VPC Virtual Path Connection
VPI Virtual Path Identifier. An 8-bit identifier identifying semi-permanent connections between ATM endpoints. See VC.

Very Small Aperture Terminal. A satellite system (for voice, data or video) where the satellite dish is less 3 metres in diameter. VSATs are a type of downlink but may have uplink capability.

VT series
A series of text-based terminals introduced by Digital Equipment Corporation (DEC). Designed for use with large mainframe computers and standard enough to work with machgines from a variety of vendors. The VT100 and VT102 were the most common though they were superceded by VT220 and VT320. Programs that allow users to connect to remote machines (like Telnet) often do so by emulating VT series terminals.

Virtual Telecommunications Aaccess Method (IBM)

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Wide-Area Information Service. A service which allows users to intelligently search for information among databases distributed around the Internet.

WAN (Wide Area Network)
Public or private computer network serving a wide geographic area.

World Wide Web. The Internet-based hypertext system that you are currently using to view this definition.

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See X Windows.

A protocol suite for X Windows applications.

Recommendations developed by CCITT that define a protocol for communication between user devices and a circuit-switched network.

Recommendations developed by CCITT that define a protocol for communication between packet-switched Public Data Networks and user devices in the packet-switched mode.

International standard for a store-and-forward message handling system in a multivendor environment.

A standard which defines electronic mail directory services.

An X-formated balck and white bitmap image.

An X-formatted colour bitmap image.

eXchange IDentifier

XNS (Xerox Network Systems)
Peer-to-peer protocol involving layered data communications protocols developed by Xerox and incorporated into Ethernet local area networks.

X Windows
A program that provides Unix machines with a graphical user interface.

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There are no terms in this glossary beginning with the letter "Y".

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ZDL (Zero Delay Lockout)
Technology designed to prevent beaconing stations on a token ring from inserting into the ring and causing faults.