[A-H] [I-P] [Q-Z]
| A | B | C | D | E | F | G | H |
ACD (Automatic Call Distribution/Distributor) A specialized phone system, or the service it
provides, for handling many incoming calls. Typically used by airlines and hotels, it recognizes and
answers incoming calls according to instructions in a database, before sending the call to an operator
or agent. It also offers management information on the type and volume of calls and efficiency of the agents.
ACF/NCP (Advanced Communication Function/Network Control Program) In host based IBM SNA networks, it
is the control software running on a communications controller that supports the operation of the SNA backbone
ACF/VTAM (Advanced Communication Function/Virtual Terminal Access Method) In host-based IBM SNA
networks, it is the control software running or a host computer that allows the host to communicate with
Actius (Association of Computer Telephone integration Users and Suppliers) A UK forum for users
and suppliers to increase awareness of the business benefits of CTI. Act us develops education programs
and information campaigns on CTI.
Address One or a group of characters specifying the recipient or originator of transmitted data.
An address car also denote the position of data in computer memory or the data packet itself while in
transit through a network. IEEE 802.3 and 802.5 recommend having a unique address for each device worldwide.
ADPCM (Adaptive Differential Pulse Code Modulation) A ITU-TS standard technique for voice encoding
and compression. It allows an analog to be carried within a 32Kbit/s digital channel.
Adjusted Ring Length When a segment of Token Ring (in practice a dual ring) trunk cable fails, a
function known as the Wrap connects the main path to the backup path. In the worst case - the longest path
- would occur if the shortest trunk cable segment tailed, so ARL is calculated during network design to
ensure the system will always work.
Agent A software-driven process running on a communications or networking device that allows that
device to participate in a network management system. For example, an SNMP agent running on a router
provides the ability for the router to exchange information with an SNMP network management system through
the use of the SNMP protocol.
ADSP (Apple Datastream Protocol) A transport mechanism for interprocess communications between
Apple Macintosh and Dec Vax minicomputers.
AFP (Apple Filing Protocol) A standard means of presenting the filing system of a server to the
user with a consistent Apple Macintosh interface.
Aggregate bandwidth The total bandwidth of channel carrying a multiplexed bit stream.
Alerts Messages that Microsoft's LAN Manager network operating system sends under certain conditions.
The three classes of alerts are admin alerts, error alerts and printer alerts.
Algorithm A process or set of rules necessary for a computer or intelligent device to perform a
task, such as voice compression.
Alias A name, usually short and easy to remember, that is translated into another name, usually
long and difficult to remember. commonly used in the Unix realm to "abbreviate" verbose commands. Common
places for storing aliases are the shell configuration file (.cshrc or .profile) and a separate file
sources from the configuration file (usually called .alias)
Alternate buffer Two buffers are sometimes used to handle data I/0. These are a alternated
to achieve continuous throughput.
Alternate routing Safety technique enabling communication to continue in the event of node
failure or congestion. The network design allows for alternate paths through the network to arrive at
the same destination.
Analog An analog (US analog) signal is electrical and varies constantly in voltage, unlike a
digital signal which varies between two constant values, usually denoted as 0 and 1. The value of the
analog signal varies all the time during tran smission, whereas a digital signal changes on y between
two set values without intermediate variations.
Anonymous FTP Anonymous FTP allows a user to retrieve documents, files, programs and other
archived data from anywhere on the Internet without having to establish a userid and password. By using
the special userid of "anonymous" the network user will bypass local security checks and will have access
to publicly accessible files on the remote system.
Ansi (American National Standards Institute) A group that defines US standards for the information
processing industry. Ansi participates in defining network protocol standards and represents the US on
other international standards-setting bodies like ISO.
Application A program that performs a function directly for a user. FTP, mail and Telnet clients
are examples of network applications.
Applications Programming Interface (API) Software designed to make computer functions available
to an application program PC and network operating systems have them. APIs in a network must be compatible
to ensure programs are accessible to machines other than those they reside in. Some APIs, such as NetBios,
are de facto standards.
APPC (Advanced Program-to-Program Communications) A set of IBM protocols also known as LU 6.2
and Type 2.1 architectures. It functions within SNA's APPN to support peer to-peer communications between
workstations attached to SNA LANs and the applications running on those workstations. It was added to SNA
as part of the "new" SNA to support peer to-peer networking, unlike the traditional hierarchical SNA approach
in which the mainframe acts as host or master and treats the other computer as a terminal or slave.
APPC/PC A version of APPC developed by IBM to run on PC based Token Ring networks.
APPN (Advanced Peer to Peer Networking) An extension to SNA which routes information around the
IBM network without help from the host, allowing systems to adjust dynamically to the topology of the
network (dynamic routing). APPN keeps track of network topology, making it easier to connect and reconfigure.
It also creates a directory of network nodes and other resources. APPN also allows for dynamic SNA networks,
where nodes can join and leave the network as required, and session routes can be selected as needed.
AppleShare Apple system software that allows sharing of files and network services via a file
server in the Apple Macintosh environment.
AppleTalk A seven- layer protocol stack developed by Apple for communications between its Apple
Macintosh product range. Apple defines it in similar terms to the functionality of the seven-layer OSI
Application Layer The top layer in the OSI Reference Model comprising the interface between
the OSI environment and a user's application. It does not contain applications, but provides a link from
application software on one system to applications an another computer through the OSI environment. Several
applications layers support different user tasks such as e mail and file transfer and transaction processing.
Archie A system to automatically gather, index and serve information on the Internet. The initial
implementation provides an indexed directory of filenames from more than 800 anonymous FTP archives on the
Internet -- some 100 gigabytes worth of information. This information is accessible through archie client
programs, through servers reachable using the telnet command, through email servers and through forms on
the World Wide Web.
ARP (Address Resolution Protocol) The lnternet and TCP/IP protocol used to bind dynamically a
high-level IP address, such as an lnternet address, to a low-level physical hardware address. ARP operates
only across a single physical network and is limited to networks supporting hardware broadcast.
Arpanet The Advanced Research Projects Agency Network developed by the eponymous research agency in
the 1960s as the first, large scale, packet switched network. It is still in use today, connecting a large
number of universities in the U S and Europe, as well as commercial users.
ASCII The American Standard Code for Information Interchange developed by ANS-1 to encode characters
in seven bit units. These are normally padded out with an eighth bit that can represent parity to make up an
eight-bit byte. This eighth bit can also be used to make ASCII support international character sets, extending
the 128 possible seven-bit combinations to 256.
Asic (Application-Specific Integrated Circuit) Pronounced A sick, it is a Very Large Scale
Integrated circuit, custom-designed to perform one or more particular functions. Advantages include fewer
discrete components, lower power consumptio n and increased reliability.
ASN-1 (Abstract Syntax Notation-1) A formal language used for describing and implementing ISO
OSI protocols used in the automated implementation of protocol software. The protocol data units of most
Application Layer standards like ACSE, FTA M, MMS, are defined using ASN-1.
Asynchronous communications A method of transmitting data in which each transmitted character is sent
separately. The character has integral start and finish start and stop bits so that the character can be sent at
an arbitrary time, and sep arate from any other character. It is the most rudimentary type of communication as
the originating and receiving machines do not have to be synchronized. Cheap, reliable and common among PCs and
minicomputers, its disadvantage is the large number of extr a bits needed for the data to be interpreted.
AT Modem control language for asynchronous dial-up modems designed by Hayes Micro- computer Products.
ATM (Asynchronous Transfer Mode) A cell- based data transfer technique in which channel demand
determines packet allocation. ATM offers fast packet technology, real time, demand led switching for
efficient use of network resources. It is al so the generic term adopted by ANSI and the ITU-TS to classify
cell relay technology within the realm of broadband WANs, specifically B-ISDN. In ATM, units of data are
not time related to each other and, as part of the B-ISDN standard, is specified for di gital transmission
speeds from 34Mbit/s to 622Mbit/s. IBM currently offers ATM at a non standard 25Mbit/s format. ATM will be
the high band width networking standard of the decade.
Attenuation The weakening of transmitted signals as they travel away from their point of origin.
Amplifiers can recharge the signal up to a point.
AUI (Attachment Unit Interface) The IEEE 802.3 specified cable and connector used to attach
devices to a MAU. Defined in Section 7 of the 802.3 standard.
Auto partition A feature of 10 BaseT. When 32 consecutive collisions are sensed by a port in a hub
or concentrator from its attached work station or network segment, or when a packet that far exceeds the
maximum allowable length is received , the port stops forwarding packets. The port continues to monitor
traffic and will automatically begin normal packet forwarding when the first correct packet is received.
BABT (British Approvals Board for Telecommunications) An independent organization that tests
telecommunications equipment. Its processes are known for their rigorousness and labyrinthine complexity.
Back end The server part of a client/server application. It provides services across the network
that have been requested by the client. For example, a back end may be a database server that responds to
SQL requests from a workstation runni ng a front end application.
Back-up server Software or hardware which copies files so that there are always two current
copies of each file. Also known as a shadow server.
Backbone A high-capacity network that links together other networks of lower capacity. A local
backbone network would typically be an FDDI network acting as an in building backbone to link together
multiple LANs. A wide area backbone network would typically use digital leased circuits and multiplexers
Background Task or Mode A secondary function perforated by a computer without interrupting its
current or primary task.
Back-up domain controller A server in a network domain that keeps and uses a copy of the domain's
user accounts database to validate logon requests.
Balun A transformer that levels out impedance differences so that a signal generated on to a
coaxial cable can transfer on to twisted pair. Baluns are often used so that IBM 3270 terminals can run
off twisted pair, or to allow co-axial Ethe rnet to be operated over UTP.
Bandwidth The range of frequencies a transmission line or channel can carry: the greater the
bandwidth, the greater the information - carrying capacity of a channel. For a digital channel this is
defined in bit/s. For an analog channel it is dependent on the type and method of modulation used to encode
10Base2 A form of Ethernet and IEEE 802.3 network cabling using thin coaxial. It refers to
I0Mbit/s speed Baseband transmission over 200 meters maximum length in practice 185m) and is commonly
known as Cheapernet.
10Base5 A form of Ethernet and IEEE 802.3 network cabling using thick coax. It refers to 10Mbit/s
speed Baseband transmission and 500m maximum length.
10BaseT A form of Ethernet and IEEE 802.3 network cabling using twisted pair cabling. It refers
to 10Mbit/s speed Baseband transmission twisted pair cable with a maximum segment length of 100m.
100BaseT IEEE standard from proposals by the Fast Ethernet Alliance (including 3Com and SynOptics).
It will support Category 3,4 & 5 UTP cabling.
100BaseVG-AnyLan A competing proposal to 100BaseT (promoted by Hewlett Packard, IBM and Proteon
among others) to the IEEE for a 100Mbit/s standard over voice grade UTP the cable most users already
have installed in existing 10BaseT systems. Based on Quartet Signaling and demand priority protocol, it
preserves the infrastructure and will need only a new hub and upgraded adapters in PCs/work stations.
It claims support for Category 3,4 & 5 UTP cabling for both Ethernet and Token Ring.
Baseband A term defining any network in which the information is modulated onto a single carrier
frequency. The digital input is applied directly to the transmission media without the intervention of
a modulating device, which works well if there is wide bandwidth and distances no more than several
hundred meters are involved. It is common in LANs and limited distance modems. All stations attached to
the network have to participate in every transmission. Simpler and cheaper than Broadband, it permits
only one "conversation" at a time as the whole of the bandwidth is used to transmit a single digital signal.
Ethernet is a baseband network.
Base station A fixed radio transmitter/receiver which electronically relays signals to and from
mobile voice and data terminals or handsets.
Basic Rate Access Two 64 Kbit/s "B" channels + one 16Kbit/s "D" channel (2B + D), carrying
user traffic and signaling information respectively to the user via twisted pair local loop.
Baud A unit of signaling speed, expressed in terms of the number of discrete conditions or
signal events per second. It is only the same as bit/s, when one discrete signaling condition is used
to transmit a single bit of data.
BBS See: Bulletin Board System.
BCNU Be Seeing You
Beaconing Token Ring process to recover the network when any attached station has sensed that
the ring is inoperable because of a hard error Stations can withdraw from the ring if needed. A station
detecting a ring failure upstream transmit s (beacons) a special MAC frame used to isolate the location
of the error using beacon transmit and beacon repeat modes.
Beta In the preliminary or testing stage of a product.
Binary Base 2 numeral system. The two symbols used are '0' and '1'.
Binary file Any file that is not plain, ASCII text. For example: executable files, graphic files
and compressed (ZIP) files.
Bindery A database that contains definitions for entities such as users, groups and workgroups
in the Novel NetWare LAN network operating system environment. The bindery supports the design,
organization and secure operation of the NetWare environment.
Bipolar transmission Method of sending binary data in which negative and positive states
alternate. Used in digital transmission facilities.
B-ISDN (Broadband ISDN) The proposed advanced version of ISDN, providing speeds of 155.52Mbit/s
and higher. Standards and switching technology that will work this fast are under development. It promises
universal coverage based on ATM/SDH technologies and optical fiber, supporting data, voice and video traffic.
Bit A binary unit of information that can have two values, 0 or, 1. The word comes from a
contraction of binary digit.
Bit Error Rate The percentage of received bits on a digital link that are in error relative to
the number of bits received, usually expressed to a power of 10
Bit Error Rate Tester A device for testing the reliability of a digital datacommunications link.
The BERT generates specific data patterns that are routed through a communications device for comparison
at the receiving end. The errors are counted by the BERT.
Bit Interleaving A form of Time Division Multiplexing (TDM) for synchronous protocols, including
HDLC, SDLC, BiSync and X.25 Bit inter-leaving retains the sequence and number of bits, so that correct
synchronization is achieved between both ends.
BITNet "Because It's Time" Network. An academic computer network that provides interactive
electronic mail and file transfer services, using a store-and-forward protocol. BITNet hosts are not
on the Internet per se, but are reachable by email through BITNet to Internet gateways.
Bits per second The rate at which individual bits are transmitted across a communications
link or circuit; written bit/s. One thousand bit/s is 1 Kbit/s, and one million bit/s is 1 Mbit/s.
Block A group of characters or bytes treated as a unit.
BNC connector A cylindrical push-and-twist connector for connecting thin co-axial cable, such as
10Base2 "thin wire" Ethernet, and to link thin wire Ethernet to network interface cards, transceivers
and other network elements. Said to be short for Bayonet Neill-Concelman after the developers of the
connector. Also referred to as a Barrel Nut Connector.
Booting Loading a computer memory with information needed for it to operate. Remote booting
refers to loading software over the network.
Bounce The return of a piece of mail because of an error in the delivery process. Mail can be
bounced for various reasons. "Bounce" can also refer to the message indicating the error (informal usage).
Boundary Routing A 3Com proprietary name for a method of accessing remote networked locations,
such as a bank branch office. Effectively a form of bridging, the idea is to reduce the need
for technical expertise locally and the cost of equipment at the remote site and manage the
communications from head office.
BRA (Basic Rate Access) BRA provides ISDN users with access to two 64Kbit/s data channels, It is
defined in ITU-TS Recommendation I.420 which covers a 2B + D-channel where the B channel
is a 64Kbit/s channel, and the D-channel is a 16Kbit/s signaling channel.
Bridge Device connecting two separate networks at the OSI Data Link Layer (Level Two Media Access
Control Layer). Once bridging is accomplished, the bridge makes interconnected
LANs look like a single LAN, passing data between the networks and filtering
local traffic. There are two key classifications of bridge: those supporting
Spanning Tree and, for Token Ring networks, those supporting Source Routing.
Bridges connect networks using dissimilar protocols and do not interpret the
data they carry. They control network traffic and security, filtering where
necessary to boost network, performance and contain sensitive data to particular LAN areas.
BS5750 A British Standards Institute standard with certification procedures that says an
organization is in control of its quality procedures, at least in terms of consistency. Now identical to IS09000.
BSC, BiSync (Binary Synchronous Communications) Rules developed by IBM for the synchronous
transmission of binary coded data as a serial stream of binary digits. Synchronization
is achieved by using control characters recognizable as bit patterns which do not
appear within the body of the message.
BSGL (Branch Systems General License) A license which must be obtained by any organization seeking
to link its own private network to the Public Switched Telephone Network (PSTN).
A separate license must be held for each individual site.
BSI (British Standards Institute) The UK standards body responsible for input into European and
international standards setting bodies like ISO and the ITU-TS.
Broadband Also referred to as wideband. A term describing any network that multiplexes multiple,
independent network carrier frequencies on to a single cable. It allows
multiple simultaneous "conversations", since the independent networks
operate on different frequencies and do not interfere with each other.
In LAN terminology, it refers to a system in which multiple channels
access a medium, for example co-axial cable, that has a large bandwidth
using Radio Frequency (RF) modems. This may allow the co-axial cable to
carry multiple separate LANs whose transmission is being modulated at
different frequencies. In cable television (CATV), broadband describes
the ability to carry 30 or more TV channels and is synonymous with wideband.
Broadcast The simultaneous transmission of data via a network from one terminal to a set of
destinations or to all destinations.
Brouter An industry term for a device with the functionality of a bridge and router. It supports
more than two LAN connections and uses Level Two addresses for routing. The term is mostly used by bridge vendors.
BTW By The Way.
Bulletin Board System (BBS) A computer, and associated hardware, which typically privides electronic
messaging services, archives of files and any other services or
activities of interest to the bulletin board system's operator. Many
BBS's are currently operated by government, educational and research
institutions. Although BBS's have traditionally been the domain of
hobbyists, an increasing number of BBS's are connected to the Internet.
The majority, however, are still reachable only via a direct modem-to-modem
connection over a phone line.
Buffer A temporary storage place for data, designed to compensate for a difference in transmission
speeds or to hold data when there is a difference in timing of events. It can be
a software program, a storage facility or a hardware device, ensuring the data
always has somewhere to go, even if it has to be held up for while in the buffer
until it can be transmitted to the destination.
Bus topology A type of network in which all tie devices are connected in a line to a single cable.
A bus network has two distinct ends. All devices which attached to a bus
network have equal access to it and they can see all the messages th at are
put on to the network. Each device determines which messages are intended for
it alone, and selects those.
Byte Eight bits forming a unit of data. Usually each byte stores one character.
CAI (Common Air Interface) The CT2 international mobile communications standard which allows any
compliant equipment to be used on any network of the same type. CAI compliant
telepoint handsets from different vendors may therefore be used o n a telepoint
network. Vendors with CAI compliant systems include Northern Telecom and GPT.
CATV (Cable TeleVision) Cable system covering defined areas, such as the UK's franchises to
install and operate a cable system granted by the Cable Authority and Department
of Trade & Industry, offering TV channel output and, increasingly, local loop
digital telephony services The Cable Television Association is the CATV industry's
Call processing The system and process that sets up the intended connection in a switching system.
A system scans the trunk and station ports for any requests
for service. It then checks the stored instructions and look-up tables and sets t he connection up accordingly.
Carrier signal The underlying frequency or frequencies that are to carry information. They are
modulated through one or more modulation techniques to impose information on the signal.
Category 3 cable Standard for UTP voice grade cabling specified by the EIA/TIA 568 standard for use
at speeds up to I0Mbit/s including 10BaseT Ethernet.
Category 4 Cabling standard specified by EIA/TIA 568 for use at speeds up to 20Mbit/s including
16Mbit/s Token Ring.
Category 5 Cabling standard specified by ElA/TIA 568 for use at speeds up to 100Mbit/s including
FDDI (TP PMD), 100BaseT and 100BaseVG-AnyLan, and potentially ATM at 155Mbit/s.
CCITT (The International Telegraph and Telephone Consultative Committee) Former name for the
ITU-TS (International Telecommunications Union), a Specialized Agency of the United Nations.
CCITT Study Groups The ITU-TS (formerly the CCITT) operates as a series of groups considering
specialist areas. There are key study groups applicable to
networking and communications such as Study Group VII responsible
for data communicatio ns networks and the X series Recommendations
and Study Group XVIII covering digital networks including ISDN.
CCTA Central Computer & Telecommunications Agency - the Government Center for Information Systems.
Cell Relay Generic term for a protocol based on small fixed packet sizes capable of supporting
voice, video and data at very high speeds. Information is handled in fixed length cells of 53 octets.
Centronics interface A parallel interface with 36 pins that will transmit eight data bits
simultaneously. The interface originates from the Centronics Company, a printer manufacturer. It has become widely used as a
parallel interface standard.
CEPT The European Conference of Postal and Telecommunications Administrations. An association of
European Telecommunications service providers. It in turn participates
in relevant areas of the work of Cen/Cenelec. Formerly extremely powerfull
and was originally responsible for the Net standards, but these have subsequently been passed on to Etsi.
Character Interleaving A form of TDM used for asynchronous protocols. This can be used either
with extra channels, or by carrying RS232-C control signals.
Cheapernet Thin wire Ethernet.
Circuit switching The transmission technique in which a physical circuit is established between
sender and receiver before transmission takes place. When the transmission is complete, the circuit is freed.
CLI (Calling Line Identification) A service available on digital phone networks that tells the
person being called which number is calling them. The central office equipment identifies the phone number of the caller,
enabling information about the caller to be sent along with
the call itself. Synonymous with ANI (Automatic Number Identification).
Client/server computing The division of an application into two parts; a front end client and
a back end server. It allows multiple front ends running on a PC or Unix workstation (client) to access the same SQL based
server database at the same time over the LAN. The aim is to off-load
as much processing as possible to the intelligent desktop leaving
only the shared information and the software for managing it at the
central server. An application that is running in such a fashion with
client and server linked by a LAN is termed a bifurcated application.
Cluster controller An IBM device that allows multiple 3270 terminals to be linked directly to
a host computer, or into a SNA network through the use of a communications
controller. A cluster controller is a Control Unit in IBM speak.
CMIP/CMIS (Common Management Information Protocol/Common Management Information Services) ISO
OSI connection oriented network management protocol and set of services. Well accepted in the WAN and telecommunications
world, they have not yet been widely adopted for LAN management.
CMOT (CMIP/CMIS over TCP) The use of SO CMIP/CMIS network management protocols to manage
gateways in a TCP/IP Internet. CMOT is a co-recommended standard with SNMP.
CNL Connect No Login (prompt). NETCOM shorthand for times when a user establishes a connection
to a POP but does not receive a login prompt.
Co-axial cable A cable comprising a central wire surrounded by a second tubular screening of
fine wire. Associated with IBM for linking terminals and other devices needing high-speed links, coax is used in Ethernet.
It is difficult to add or remove devices from a coaxial LAN as the cable is unwieldy and thick so is being superseded by UTP.
Collision The result of two devices on a shared transmission medium, like Ethernet, transmitting
simultaneously. Data is corrupted and both devices must retry their transmissions. A delay mechanism used by both senders
drastically reduces the chances of another collision.
Collision detection Devices at each end of a link are designed to detect collisions instantly
and attempt to resend. This is the principle on which CSMA/CD is based and the access control method for Ethernet. An
alternative is to resend if there is no acknowledgment of receipt from the remote device.
Communications Controller A switching unit central to the implementation of host-based IBM SNA
networks. Typically the network is built around a backbone of
interlinked communications controllers to which host computers
and Control Units ( CUs) are attached.
Communications Manager An individual often underpaid and invariably overworked, dedicated to
providing cost effective, ultimately flexible networking to users.
Communications Networks The UK's leading monthly magazine for networking professionals and
Communications Server A specialized network server that provides access to external networks,
communications facilities and hosts that cannot be directly connected
to the LAN. Typically it will enable workstation users running
appropriate workstation software, such as terminal emulation software,
to access asynchronous communications links and typically modems attached to the communications server.
Communications Toolbox An extension of the Apple Macintosh operating system that provides protocol
conversion and the drivers needed for communications tasks.
Concentration A technique used to get the most out of a composite multiplexer link. Usually a
statistical multiplexer, or concentrator, is used to focus channel
inputs on to the composite ink by removing the portions of the transmission carrying no data.
Concentrator A central chassis into which various modules, such as bridging, supervisory,
10BaseT and other peripheral cards are plugged.
Connection-oriented service The transport of packets of information from one network node to
a destination node following an established network connection.
Connectionless Service The transport of a single datagram or packet of information from one
network node to a destination node or multiple nodes without establishing a network connection.
Contention The process whereby multiple users make requests for transmission bandwidth across a
transmission link but the pool of bandwidth is less than the
aggregate amount of bandwidth the users could request between
them. Contention is used to resolve which users gain access to
the bandwidth. When this is applied to multiplexers, it is concerned
with the multiplexer's ability to allow a number of channels to
contend for transmission bandwidth that is less than the sum of all the channel rates.
Control point A program that manages an APPN network node and its resources, enabling
communications to other control points in the network.
Converter A repeater that also converts from one media type to another, such as from fiber to
copper. Often called a media adapter.
CPE (Customer Premises Equipment) Telecommunications- communications equipment, including
PBXs and wiring, located in a user's premises.
Cracker A cracker is an individual who attempts to access computer systems without authorization.
These people are often malicious, as opposed to hackers, and have many means at their disposal for breaking into a system.
CRC (Cyclic Redundancy Check) A method of detecting errors in the serial transmission of data. A
CRC for a block of data is calculated before it is sent, and is then sent along with the
data. A new CRC is calculated on the received data. If the new CRC does not match the
one that has been sent along with the data then an error has occurred.
Cross-Connect An ATM switch usually comprising three functional areas. System control The central
control unit, which also provides the management interface of the system; the ATM "fabric block" providing the system
switching capacity; term ination groups to provide the external
interfaces and the functions of the ATM layer of the network node. Each
of these functional system areas is configured according to the specified
needs of the respective network node. Each functional area usually has its
own monitoring and control units for safeguarding the high availability of
the complete system.
Crosstalk Unwanted interference from another adjacent communications channel . The signal from the
adjacent channel is inserted into the original communications channel.
CSMA/CD (Carrier Sense Multiple Access with Collision Detection) The access method used in Ethernet.
All nodes are attached to a single cable and contend equally
for access to the transmission medium. if two nodes attempt to
send data at the same time, they "sense" each other's signal and
immediately stop sending. They will both try to send again after
Waiting a random number of microseconds.
CSMA/CA (Carrier Sense Multiple Access with Collision Avoidance) A method of network access not
covered by OSI standards and used in AppleTalk networks.
CSU (Channel Service Unit) (1) In the US, data transmission equipment to repeat the signal from the
carrier and link to CPE. Vendors add value to CSUs by adding performance
monitoring and management. (2) In Europe, CSUs are sold for their value
features like diagnostics and performance monitoring. The basic repeating
function is prd in the NTU (networking terminating unit). CSUs monitor
quality on El, E2 or E3 circuits in terms of transmission and line loading.
CT1 First generation analog domestic cordless telephone (non-cellular).
CT2 Two-way digital cordless telephony technology, particularly relevant to cordless PBXs. In its
public guise, it becomes a one way telepoint service now no longer available in the UK but prevalent in the Far East.
CT3 Ericsson's proprietary cordless telecommunications system.
CTI (Computer Telephone Integration) A generic name for the technology automatically relating
computers and PABXs via applications such as ACD, power dialing, IVR
and other customer facing or agent facing services. Also known by older,
prop rietary names CIT (Computer Integrated Telephony) and CSTA (Computer
Supported Telephony Applications).
Cyberspace A term coined by William Gibson in his SF novel Neuromancer (1984) to describe the
interconnected "world" of computers and the society that gathers around them.
Darpa (Defense Advanced Research Projects Agency) Formerly called Arpa, this US government agency
that funded research and experimentation with the Arpanet and later, the connected Internet- The group
within Darpa responsible for the Arpanet is ISTO (information Systems Techniques Office), formerly IPTO
(Information Processing Techniques Office).
DassiII A message based signaling system following the ISO based model developed by BT to provide
multi-line IDA interconnection to the BT network.
Data compression A way of reducing the amount of data to be transmitted by applying one of severs
techniques that reduce the number of bits needed to represent the information. When the data is received It
is decompressed into its original form.
Database server A database installed as a back-end or server component of a client-server system,
which can be accessed over a LAN by one or more client, or front-end applications through the use of query
language, typically SQL. The server part of the program is responsible for updating the records, ensuring
that multiple access is available to authorized users, protecting the data and communicating with other
servers holding relevant data. The client end of the program requests records and then modifies them,
while the server tracks records down for the client and adds new ones.
Datagram A method of sending data in which parts of the message are sent in random order. The
recipient machine has the task of reassembling the parts in the correct sequence. The datagram is a
connectionless, single packet message or item of data that can traverse a network at OSI Level Three,
the Network Layer. It typically does not involve end-to-end session establishment or delivery-confirmation
acknowledgment. As well as the information within the datagram, there is a destination network address and
usually a source network address.
Data link A direct serial data communications path between two devices without intermediate
Data Link Layer Layer Two of the ISO OSI model is responsible for the transmission of information
over a physical medium. After establishing the link it ensures the error-free delivery of the information
through the use of error detection, error recovery and flow control. The contention access methods such
as CSMA/CD and Token passing are Layer Two activities.
Data PBX A switching system for data traffic that allows terminals and workstations connected
by individual cables to the Data PBX selectively to link to one or more host computers over asynchronous
circuits through the use of contention.
DCA (Defense Communication Agency) The US government agency responsible for the installation of
Defense Data Networks, like Arpanet and Milnet, and PSNs. The DCA writes contracts for operation of the
DDN and pays for network services.
DCA (Document Content Architecture) The IBM approach to storing documents as two types of
document group: draft documents and final form documents. For presentation, the draft document is
transformed into a final document through an office s ystem.
DCE (Data Circuit Terminating Equipment) Communications equipment installed in a user's
premises responsible for establishing, maintaining and terminating a connection. A modem is an example.
DCE (Distributed Computing Environment) A suite of software utilities and operating system
extensions that will, in theory, create applications on networks of heterogeneous hardware - PCs, Unix
workstations, minicomputers and mainframes. The DCE is the product of the OSF. The DCE is designed to
simplify the building of heterogeneous client/server applications and provides seven general services:
Remote Procedure Call, Security, Naming (directory), Distributed File System, Threads, Time and PC
Integration. DDE (Dynamic Data Exchange) - A Microsoft messaging specification. When DDE-compliant
applications are combined, dynamic documents can be created which update each other as data changes.
DDM (Distributed Data Management Architecture) An IBM SNA LU 6.2 transaction providing
users with facilities to locate and access data in the network. It involves two structures: DDM Source,
and DDM Target. The Source works with a transactio n application to retrieve distributed data and
transmits commands to the Target program on another system where the data that has been requested is
stored. The Target interprets the DDM commands, retrieves the data and sends it back to the Source that
originated the request.
DDCMP (Digital Data Communication Message Protocol) The DecNet- specific Link Level protocol
that operates at Layer Two of the Digital Network Architecture.
DDN (Defense Data Network) Used generally to refer to Milnet, Arpanet and the TCP/IP protocols
those networks use. More specifically refers to Milnet and associated parts of the connected Internet
that connect military installations.
DecNet Proprietary peer-to-peer network technology originally developed for use in wide area
networking by the Digital Equipment Corporation (Dec) and evolved to include significant Ethernet-based
LAN capabilities. It is the implementation of the Digital Network Architecture (DNA).
Dect (Digital European Cordless Telecommunications) A standard governing pan-European digital
mobile telephony. Based on advanced TDMA technology, Dect covers cordless PBXs, telepoint and residential
Demand Priority Access method providing support for time-sensitive applications such as video
and multimedia as part of the proposed 100BaseVG standard offering l00Mbit/s over voice-grade UTP cable.
By managing and allocating access to the network centrally, at a hub rather than from individual
workstations, sufficient bandwidth for the particular application is guaranteed on demand. Users, say
its proponents, can be assured of reliable, continuous transmission of information.
Demodulation Technique for retrieving information from a modulated signal. Demonstrated by the
eponymous modem (modulator/demodulator).
Des (Data Encryption Standard) An algorithm designed by the US National Bureau of Standards for
the encryption and de-encryption of data using a 64-bit key.
Device driver In the context of computer networking a device driver is a software module forming
part of a computer operating system, or software that interacts with the operating system. It aims to
control communications equipment, such as a LAN network adapter card and facilitate the transfer of
information to and from the network. Other examples of device driver programs include software to support
the activities of printers, disks and mice.
DIA (Document Interchange Architecture) An IBM term defining the sets of functions needed for
document handling in an IBM environment, including storage and distribution.
Dialer A program which establishes and maintains your connection to the Internet, as well as
provides Winsock support. Popular dialers include Trumpet Winsock and the Windows '95 Dial Up Networking.
Dialup A temporary connection between machines established with modems over a standard phone line.
Digital signal A signal with only two values, normally 0 and 1, during transmission, unlike an
analog signal whose values constantly vary.
Direct attachment The IBM term for linking a device or LAN directly to a host computer through an
appropriate Control Unit, like a cluster controller.
Disk server A device equipped with disks and a program permitting users to create and store files
on those disks. Each user has access to their own section of disk on the disk server. The aim is to give users
access to disk space that they would not normally have on their PC. The disk server is linked to the PCs via
a LAN. The next level of sophistication would be a file server.
Diskless workstation A PC or workstation attached to a LAN that has neither floppy nor hard disks,
but relies on disk storage provided by a file server attached to the same LAN. When the diskless workstation
is first initialized it uses a re mote boot program stored in a remote boot prom/eprom on its network adapter
card to initialize a session with the file server. The workstation then loads its operating system, such as
MS-Dos, from the server and executes the normal server login procedure.
Distributed database A database stored on more than one networked computer. The database is split
up across these machines, and not replicated.
Distributed name service A technique for storing network node names so that the information is
stored throughout the network, and can be requested from, and supplied by, any node.
Disoss (Distributed Office Support Systems) IBM software typically forming part of an IBM Office
Distributed computing The trend away from having big, centralized computers such as mini-computers
and mainframes to bring processing power to the desk top. Often confused with distributed processing.
Distributed processing An approach that allows one application program to execute on multiple
computers linked together by a network. The networked computers share the work between them.
DLS (Data Link Switching) An enhancement to source routing which transports source route packets
over a resilient IP/OSPF network and provides local termination of LLC2 sessions to avoid LLC timeouts in
large or busy networks. It is the idea l mechanism for mixed LAN-to-LAN and interactive SNA traffic since
it can recover from network problems quickly using OSPF. It is rapidly becoming accepted as a major standard.
DMA (Direct Memory Access) A technique for high-speed data transfer between a device such as LAN
network adapter card and the computer memory. DMA bypasses the Central Processing Unit of the computer, PC
or workstation, allowing the device to transfer a block of information directly across the bus into system
DMI (Desktop Management Interface) A set of APIs outlined by the DMTF, comprising three components:
service layer, component interface and management interface.
DNA (Digital Network Architecture) The network architecture of Digital Equipment Corporation with
eight layers. The DNA is similar in structure to OSI at lower levels, except that the top three layers of
the DNA correspond to the top two lay ers in the OSI model.
DNS (Domain Name System) The online distributed database system used by Internet to map names into
IP addresses. DNS servers throughout the connected Internet implement a hierarchical namespace that allows
sites freedom in assigning machine names and addresses. DNA also supports separate mappings between mail
destinations and IP addresses.
Domain A group of nodes on a network that form an administrative entity. It could also be a number
of servers grouped together and named to simplify network administration and security. Every computer on the
LAN belongs to at least one domain. Being logged in on one domain, however, does not limit resources in other
domains to which the user has access permissions.
Dos - Disk Operating System comprising one or a suite of programs managing a disk-based computer
system. Dos schedules and supervises work, allocating computer resources and the operation of peripherals.
Versions of Dos from different vendor s exist: Microsoft's MS-Dos is the most common. Dos 3.1 was the first
version of MS and PC Dos able to support LAN functions separate, of course, from the network's own operating
system - notably including record and file locking which is now standard on multi-user systems.
Dos LAN Manager A Dos version of Microsoft's network operating system LAN Manager. It gives Named
Pipes (an applications interface) support to Dos machines, enabling them to use the client/server environment.
Downlink Transmission from a satellite to an Earth Station.
DPA (Demand Protocol Architecture) A technique for loading protocol stacks dynamically as they
are required. It is associated with adapter cards in workstations and servers. Only the protocol stacks
that are needed for a particular communica tions sessions are loaded. Examples of stacks that could be
loaded include TCP/IP, XNS, SPX/IPX and NetBios.
DPNSS (Digital Private Networks Signaling System) Signaling standard for digital private networks
within the UK formulated jointly by BT and PABX manufacturers.
DQDB (Distributed Queue Dual Bus) The standard for future Mans which operates as a dual bus, each
carrying data in both directions. A queuing system maintains transmission order. Some similarity with ATM
encourages evolution between the tech nologies.
Drop cable A cable that links a network adapter to an external transceiver attached to a co-axial
LAN such as Ethernet. Also called an Attachment Unit Interface cable or transceiver cable.
DS1 (Digital Signal 1) Transmission standard at T1 speeds, or 1.544Mbit/s
DS3 (Digital Signal 3) Transmission standard at T3 speeds, or 44.736Mbit/s. DS3 allows the
combination of 28 DSls or a single DS3 facility - also known as a T3 circuit.
DSE (Digital Switching Exchange) A node in a telecommunications network.
DSU (Data Service Unit) Data transmission equipment used to interface to a digital circuit
at customer site. It converts the customer's datastream, such as X.21 to E1 or T1 for transmission through
the CSU, which is often contained, function ally within the DSU device. DSUs can convert data to or from a
native port on a router to an E1, E2 or E3 leased line, primary rate ISDN or SMDS, DSU functionality can be
built into devices such as some routers or multiplexers. In Europe a DSU can convert E1 bandwidth into
RS.449, X.21, V.35 or other serial interface via a router. A DSU with an HSSI interface will deliver E2
or E3 bandwidth from the WAN to an HSSI router on a LAN.
DTE (Data Terminal Equipment) A piece of equipment where a communications path ends. The user's
equipment is collectively termed DTE and can include PCs and display terminals.
DTMF (Dual Tone Multi-Frequency) A term for push button or Touchtone (an AT&T trademark)
dialing. The pushed button makes a tone, actually the combination of two tones - of high and low frequency.
They are necessary to access advanced network features such as call barring and call forwarding. DTMF
penetration in the mass-market the UK is small but growing, but high in the business community.
Duplex Simultaneous, two-way independent transmission of data.
Dynamic node address An Apple-patented feature of AppleTalk under which each node assigns itself
a unique address code each time it is initialized. Conventionally, nodes are assigned fixed addresses that
do not change.
Dynamic routing A process for selecting the most appropriate path or route for a packet or
datagram to travel around a network. At the end of each leg of the journey of the packet across the network
the router decides on the most appropriate path for the packet or datagram to follow if there are multiple
routes available. This is done using network status information gathered from around the Internet and passed
from router to router through the use of routing information protocols.
E1 The European standard for high-speed data transmission at 2.048Mbit/s - 32 64Kbit/s channels are
Earth station Transmitting or receiving point for satellite transmission.
EBCDIC (Extended Binary Coded Decimal Interchange Code) An IBM-developed eight-bit binary code that
can represent 256 characters. It allows the representation of control codes and graphics in a logical format.
It was created to represent data in particular types of data processing and communications terminal devices.
Echo cancellation A technique used in high-speed modems to isolate and remove unwanted signal energy
created by echoes of the transmitted signal. (An echo is the return of a transmitted signal). It would be
noticeable by its absence on trans atlantic or other time-differentiated phone conversations.
ECMA (European Computer Manufacturers Association) An organization dedicated to the co-operative
development of standards applicable to computer technology. It works closely with certain ITU-TS Study
Groups and ISO Sub-committees.
EDI (Electronic Data Interchange) The interchange of trading information through the use of an
electronic messaging system. A form of paperless trading that is rapidly expanding through organizations
such as Geis.
EGP (Exterior Gateway Protocol) The protocol used by a gateway in one autonomous system to
advertise the IP addresses of networks in that system to a gateway in another autonomous system.
EIA/TIA The US Electronics Industries Association and Telecommunications Industries Association
which have merged. The EIA is a US trade organization for the electronics industry that concentrates on
hardware interface standards. Best known for the RS232C and RS422 standards that specify the electrical
characteristics of interconnections between terminals and computers or between two computers.
EIA/TIA 568 The EIA/TIA's most famous standard specifying a maximum horizontal cable run of 90
meters, allowing 10 meters for hub and device attachment resulting a total cable length of 1000 meters.
Its related Service Bulletins TSB 36 and 40 define three categories of cabling system for data communications:
Categories 3, 4 & 5.
EISA (Extended Industry Standard Architecture) A 32-bit adaptation of the 8/16-bit buses originally
developed by IBM and now standard in almost all PCs that use Intel's 8086 and 80X86 chips. The
EISA bus is a joint development from Compaq an d other PC manufacturers. Contrast with Microchannel
Electronic mail, e-mail The electronic transmission and reception of messages and text-based
information without the need for the recipient to be present at the time of the transmission.
Available either as a value-added service from providers such as GEIS, IBM and MCI or on LAN
systems such as ccMail. De jure standards are evolving, but gateway software in the interim
is now sufficiently mature to cope with most interactivity.
Emoticon An ASCII glyph used to indicate an emotional state, typically used in email or
Usenet messages. Although originally intended mostly as jokes, emoticons or some
other explicit humor indication are virtually required under certain circumstances
in high-volume text-only communication forums such as Usenet. The lack of verbal
and visual cues can otherwise cause what were intended to be humorous, sarcastic,
ironic or otherwise non-serious comments to be badly misinterpreted, resulting
in arguments and flame wars. An example of an emoticon is :-) (a smiley face)
To see some examples of emoticon's click
EMS (Element Management System) The level of a network management system concerned
with collecting network management information from, and setting parameters on
the network elements. Network elements consist of data communications and tele
Emulation Hardware or software, or a combination of the two, that behaves like another
device or program, like PCs emulating dumb terminals.
Encapsulation The process of sending data encoded in one protocol format across a network
operating a different protocol, where it is not possible or desirable to
convert between the two protocols. For example, where Ethernet LANs attach
to an FDDI backbone, it is not possible to convert between the different
packet formats, so the Ethernet packet is encapsulated in its entirety
inside an FDDI packet as it crosses the bridge on to the FDDI network.
When the encapsulated Ethernet packet reaches the bridge connecting the
destination Ethernet LAN to the FDDI network, the Ethernet packet is
stripped out of the FDDI packet and put, unchanged, on to the destination
Ethernet LAN. Also known as protocol tunneling.
Encryption Encryption is the manipulation of data in order to prevent any but the intended
recipient from reading that data. There are many types of data encryption
and they are the basis of network security.
Entry point An IBM network management term. An entry point provides management functions
for itself and the devices attached to it. It has to be an SNA-addressable
unit, allowing it to participate in network management by monitoring its
own environment and exchanging information and messages with a Focal point.
Error control A means of ensuring that information received across a transmission link is
correct. The techniques involved typically use error detection to detect if the transmitted data has
been corrupted. The error control technique involves asking for data to be retransmitted until a correct
version is received.
Error correction A technique to restore data integrity in received data that has been corrupted
during transmission. Error correction techniques involve sending extra data along with the original data
being sent. It allows the correct form of the data to be reconstructed from the extra information if the
original has been corrupted. This extra information is calculated using particular error correction
algorithms such as Hamming Code. It allow errors to be detected and the original data reco nstructed.
This is sometimes termed forward error correction.
Error detection A set of techniques that can be used to detect errors in received data. Techniques
that are applicable include parity checks involving parity bits, checksums or a Cyclic Redundancy Check.
Ethernet The most widely LAN transmission network. Based on a bus network topology, it runs at a
maximum 10Mbit/s - in practice far less - and adopts CSMA/CD techniques operating over convention co-axial
cable, thin wire co-axial cable and unshielded twisted pair cabling. A fiber-optic implementation has also
been defined. Originally developed by Xerox, Intel and Dec, Ethernet has moved through V1 and V2
proprietary definitions, and has now been standardized by the IEEE as the IEEE 802.3 sta ndard. This has
several implementations - 10Base5 for use over conventional co-axial cable, 10BaseF for use over optic fiber,
and 10BaseT for use over Unshielded Twisted Pair (UTP) cabling.
Ethernet switching A technique inspired by Kalpana bringing the advantages of a parallel networking
architecture to current contention- based Ethernet LANs. Each LAN can be segmented, each with its own
10Mbit/s path. When users on different segments exchange data, an Ethernet switch dynamically connects
the two separate Ethernet channels without interfering with other network segments. The switch can create
multiple independent connections between separate segments, allowing multiple parallel data exchanges. This
multiplies network bandwidth without modification to Ethernet end station hardware or software. Once
disparaged by other vendors, it now forms part of the portfolio of the major hub and router manufacturers.
Etsi (European Telecommunications Standards Institute) A European standards body established in
1988 by a decision of the CEPT. It has taken over the work of the CEPT the area of developing the Net-Normes
Europeene de Telecommunication, Net standards.
Eutelsat Inter-governmental organization founded in 1983 with the launch of Eutelsat 1, Eutelsat aims to
provide and operate a space segment for public intra-European international telecommunications services. The
segment is also used to meet domestic needs by offering leased capacity, primarily for television. UK and
France are the main shareholders, with about 25 members in total.
Ewos (European Workshop for Open Systems) A forum aimed at promoting OSI standards and undertaking
the development of functional profiles. Its work includes OSI Layers One to Four, FTAM, MHS, ODA, Directory
Services and the VT protocol.
FAQ Frequently Asked Question. In its usual context, FAQ refers to collected answers to often-asked
questions on Usenet newsgroups. These are periodically posted to the newsgroups in question, to the .answers
newsgroups (news.answers, rec.answers, comp.answers, etc.) and are stored on the FAQ FTP archive on
Fast Ethernet Proposed 100Mbit/s technology for workstation LANs from the eponymous Fast Ethernet
Alliance, which includes 3Com and SynOptics. It has been adopted by the IEEE as the basis for the 100BaseT
Fast packet switching A WAN technology capable of transmitting data, digitized voice and digitized
image information. It makes use of short, fixed length packets (or cells) that are all the same size. The
underlying switching technology is based on the statistical multiplexing of data and voice in fixed length
cells. Any of these packets could carry digital voice, data or digital image information. All the packets
travel at Level Two of the OSI Model, and routing is performed on the basis of the Level Two addressing.
Fast packet is an effective way of making best use of available bandwidth. It offers the benefits of
conventional multiplexing techniques and circuit switching techniques. It is one of the transmission
technologies being developed for use with B-ISDN. The switch used to route packets in a fast packet network
is termed a fast packet switch. Also, fast packet technology can carry data transmissions that enter the
network using a frame relay access method. For particularly high speed networking, an implementation of
fast packet switching known as ATM is being commercially developed.
Fault tolerance A method of making a computer system or network resilient to faults or breakdowns
to avoid lost data and downtime. For servers this involves such techniques as disk mirroring, disk duplexing
or mirrored servers. For LANs and WANs it may involve the use of multiple redundant transmission links.
Fax server A specialized IVR system which sends facsimile messages to a fax machine designated by
DTMF tones. What amounts to a database of fax text resides in the server that is accessed via a user's DTMF
phonepad. Requests result in the fax pages being delivered to the chosen fax and the subscriber charged a fee.
FCC (Federal Communications Commission) US regulatory and approvals agency.
FDDI (Fiber Distributed Data Interface) An optical fiber-based token-passing ring LAN technology
with dual counter-rotating rings. Each ring carries data at a rate of 100 Mbit/s using a 125MHz transmission
frequency. It has been standardized by the American National Standards Institute (ANSI). An FDDI network has
two modes of attachment: a device may be a Single Attach Station - attached to one ring - or a Dual Attach
Station - attached to both rings. Typical applications of FDDI are in the area of high speed LAN backbones.
File server A computer, attached to a LAN, that runs a Network Operating System (nos). This lets
the file server regulate communications among the workstations connected to it across the LAN, and to
manage shared resources available on the file server, such as hard disk storage and printers. A file
server may be dedicated: the computer is used only as a file server; or non- dedicated: the underlying
computer that the LAN nos runs on is used for another task simultaneously, for example as a workstation.
File Transfer The copying of a file from one computer to another over a computer network.
Finger A program that displays information about a particular user, or all users, logged on the
local system or on a remote system. It typically shows full name, last login time, idle time, terminal
line and terminal location (where applicable). It may also display plan and project information files
created by the user.
Firewall Any of several ways to protect a network from an untrusted host or network. Consists
of mechanisms to block network traffic and mechanisms to permit network traffic.
Flame A strong opinion or criticism of something, usually as a frank inflammatory statement,
in an electronic mail message.
Flow control The procedures for controlling the rate of transfer of data between two points
in a data network, such as between a protocol converter and a printer. This avoids data loss when a
recipient device's buffer is full. Buffers play an essential role in overall flow control in a network.
FNC (Federal Networking Council) A US group of representatives from those federal agencies
involved in the development and use of federal networking, especially those networks using TCP/IP, and
the connected Internet. The FNC coordinates research and engineering. Members include representatives
from the DoD, DOE, Darpa, NSF, Nasa and HHS.
Focal point An IBM Network management term; it consolidates the functions needed to manage
centrally all parts of a network. It provides an end-to-end network view and receives information from
entry points and service points. NetView is IBM 's key implementation of the focal point.
Foirl (Fiber Optic Inter-Repeater Link) Defined in IEEE 802.3 and implemented over two fiber
links, transmit and receive, this medium may be up to 500m and 1 kilometer long depending on the number
of repeaters in the network.
For Your Information (FYI) FYIs convey general information about topics related to TCP/IP, the
Internet and others. It is used colloquially as an acronym in Usenet and IRC.
Fractional services Bandwidth available from carriers in increments of 64Kbit/s, such as Mercury's
Frame A group of bits sent over a link. A frame may contain control and addressing information,
as well as error detection - for example CRC information - and forward error correction information. The
size and composition of the frame varies according to the protocol. Often used synonymously with packet.
Frame relay A data communications interface originating from ISDN designed to provide high speed
frame or packet transmission with minimum delay and efficient use of bandwidth. It is a variation on the
X.25 interface and form of fast packet switching. It derives its name from using the Data Link or "frame"
OSI layer Two to route or "relay" a packet directly to its destination instead of terminating the packet
at each switching node. This eliminates processing overheads and increases throughput speed. Based on
the ITU-TS Lap-D standard, it uses variable-length packets and applicable only to sub-broadband, T3/E3
or lower, data transmission. Like Ethernet, or token ring, frame relay assumes that connections are
reliable. It does not have error detection and error control within the network, which helps to speed up
the protocol. When errors occur frame relay relies on higher level protocols for error control. Frame
relay is often viewed as a replacement for X.25, primarily for LAN-to-LAN bursty traffic. Voice over frame
relay is available, but the subject of debate. It will also become an access method for ATM-based WANs.
FDM (Frequency Division Multiplexing) A multiplexing scheme in which the available transmission
frequency range is divided into narrower bands. Each of these bands is used to carry a separate channel.
FSK (Frequency Shift Keying) A technique for modulating data that use two frequencies. Frequency
shifts between the two frequencies are generated when the binary digital level changes. So one particular
frequency is used to represent a binary one, and a second frequency is used to indicate a binary zero. FSK
is used in low speed modems when, in full-duplex transmission, two different frequencies are used in each
direction, resulting in four different frequencies being used.
Front end The client part of a client/server application that requests services across a network
from a server, or back end. It typically provides an interactive interface to the user, for example, a data
entry front end, allowing data to be entered into a server through the use of SQL.
FTAM (File Transfer Access and Management) ISO 8671 standard which plays a key role in integrated
message handling as the vehicle for interchanges of EDI information between applications. FTAM controls the
transfer of whole files or parts of files between end systems.
FTP (File Transfer Protocol) The TCP/IP standard, high-level protocol for transferring files from
one machine to another. Usually implemented as applications level programs, FTP uses the Telnet and TCP
protocols. Full duplex - A channel capable of transmitting in both directions at the same time.
Functional profile A defined stack of ISO OSI Layer elements, such as Gosip, Map or Top. Functional
profiles were developed in order to ensure that, when defined, ISO OSI stacks could interoperate. Due to
the different protocol elements at each OSI layer, it was possible to define stacks that were syntactically
correct, but would not be able to exchange in-formation due to differences at particular layers. A functional
profile that has been defined as a standard is a standardized profile. Likewise, an International Standard
Profile is an OSI functional profile.
FWIW For What It's Worth
G recommendations A series of standards defined by the ITU-TS covering transmission facilities.
They are: G.703 2.048Mbit/s - transmission facilities running at 2.048Mbit/s that use the ITU-TS recommended
physical and electrical interface specified in G.703; G.703 641K - likewise for transmission facilities
running at 64Kbits/s; G.703 - the ITU-TS standard 1984 current version for the physical and logical traits
of transmissions over digital circuits. G.703 now includes specifications for th e US 1.544Mbit/s as well
as the European 2.048Mbit/s, and circuits with larger bandwidths on both continents. G.703 is still generally
used to refer to the standard for 2.048Mbit/s; G.821 - ITU-TS Recommendation that specifies performance
criteria for dig ital circuits for ISDN.
Gateway Network interconnection device and software that operate at OSI Layer Seven. A gateway
supports a full stack of the relevant protocol, such as SNA, DecNet, ISO, TCP/IP, and can covert to a
non-seven layer protocol, such as async or BSC. It is typically used to provide access to wide area
networks over asynchronous or X.25 links from a LAN environment. Examples include pads and protocol converters.
GGP (Gateway-to-Gateway Protocol) The protocol that core gateways use to exchange routing
information, GGP implements a distributed shortest path routing computation.
GIF Graphic Interchange Format. An image compression algorithm that facilitates the transfer
of high quality images over a network. A GIF can be of any resolution but only has 8 bit (256) color.
Gigabyte A billion bytes, which is large enough to hold 1,250 copies of Moby Dick.
Gopher A distributed information service that makes available hierarchical collections of
information across the Internet. Gopher uses a simple protocol that allows a single gopher client to
access information from any accessible gopher server, providing the user with a single "gopher space"
of information. The clients are generally text menu-based. Public domain versions of the client and
server are available.
Gosip (Government Open Systems Interconnect Profile) Country specific ISO OSI functional
profiles that have been defined as part of national procurement policies. The US has US Gosip which
is defined as a Federal Information Processing Standard. The UK has the UK Gosip which is defined by
the Central Computer and Telecommunications Agency. The UK Government will not buy equipment unless
it supports OSI as specified in Gosip. Unfortunately, the various Gosips as published by the UK, US
and Japanese governments are all slightly different.
GSM (Global System for Mobile communications) A two-way, pan-European digital cellular system.
Its specification is in line with ISDN and ITU-TS System 7 signaling and approved by almost all European
countries. GSM operates at 900MHz and is a forerunner to the mass-market Personal Communications Networks,
based on the same set of standards. GSM services include current digital subscriber services and the unique
Short Message Service - a superior form of paging offering up to 160 alphanumeric characters with guaranteed
GUI (Graphical User Interface) Often pronounced "goo-ee", it describes the screen display that
first greets a user and with which the user interacts during the computer session. A GUI makes use of
Windows, Icons, Menus and Pointers - so-called WIMP systems - such as Microsoft Windows, GEM and X-Windows.
Apple's GUI is particularly notable and be-spoke GUIs are becoming popular.
Group In the context of network security, a group is a set of users who share common permissions for
one or more resources. Individually assigned user permissions take precedence over those assigned through
Guard band The unused bandwidth separating channels to prevent crosstalk.
Guard tone A tone generated by a high speed modem dial-up modem to ensure that there is sufficient
bandwidth available on the PSTN circuit for transmission.
H channel The ISDN packet switched channel on Basic Rate Interface, designed to carry user
information streams at different speeds, depending on type: H11=1536Kbit/s, H0=384Kbit/s and H12= 1920Kbit/s.
H.261 A ITU-TS standard for video compression know as Codes for Audiovisual Services at
N x 384Kbit/s. It sets a common algorithm for converting analog video signals to digital, operating at or
Hacker A person who delights in having an intimate understanding of the internal workings
of a system, computers and computer networks in particular. The term is often misused in a pejorative context,
where "cracker" would be the correct term.
Half Duplex A two-way means of transmission, but data can only travel in one direction at a time.
Half-bridge Apple Computer terminology for a device linking LANs over a low-speed link such
as a telephone line or X.25 link. It is termed a half- bridge as one is required at each end of the link.
Handshake Part of the procedure to set up a datacommunications link. The handshake can be
part of the protocol itself or an introductory process: the computers wishing to talk to each other set out the
conditions they can operate under. Some times, the handshake is just a warning that a communication is
Head-end A central point or hub in broadband networks that receives signals on one set
frequency band and retransmits them or another. Every transmission has to go through the head-end in
a broadband network. The head-end is the piece of hardware that enables a network to send and receive
on the same piece of cable. In CATV technology, the head-end is the control center for a cable system
where signals are processed and sent for distribution down the cable system.
Header The control information added to the beginning of a transmitted message. This
may consist of packet or block address, destination, message number and routing instructions.
Hertz A measure of radio frequency. One Hz=one cycle per second. KHz = 1,000Hz; MHz =1,000,000Hz;
HDLC (High level Data Link Control) An ISO standard set of protocols for carrying data
over a link with error and flow control. Similar to IBM's SDLC, versions of HDLC are under development
for multipoint lines. The ITU-TS later adapted HDLC for its Link Access Protocol used for X.25 networks.
It is a bit-oriented data link control procedure under which all data transfer takes place in frames. Each
frame ends with a frame check sequence for error detection. There is a control field at the start of each
frame that allows error detection, and data link setup and data link termination. HDLC is, in fact, a
misnomer, as it is not a high level protocol.
HLMS (Heterogeneous LAN Management Specification) Network management specification
developed by IBM and 3Com. It provides an underlying structure for the development of network management
products which can function with a variety of network operating systems and adapter cards.
Heterogeneous networking The networking of computers from different vendors, or the
running of different operating systems.
Hierarchical network A network with one host at its hub, which is the major processing center,
and one or more satellite processing units. IBM's SNA was originally designed as an hierarchical network.
Host A computer that allows users to communicate with other host computers on a network.
Individual users communicate by using client programs, such as electronic mail, Telnet and FTP.
Host processor A minicomputer or mainframe attached to a network providing services to
Hot fixing The ability to detect and mark bad sectors of a disk, then assign alternate
disk sectors during routine LAN operation. This automatically updates the original defect map.
Hot swap The ability of a device to have parts removed after, for example, a slot-in
card or fan failure, without affecting its operation. Many of today's hubs offer hot swapping without
bringing down the network to charge a component.
HPFS (High-Performance File System) An OS/2 file system that has faster input/output
than the file allocation table file system; it does not restrict file naming to eight characters with a
three character extension; and is compatible with the FAT file system.
HSSI (High speed Synchronous Serial Interface) Interface for transferring data to or from
a WAN leased line (E2, E3), or to and from a LAN via an HSSI-capable DSU and HSSI router.
Hub The center of a star topology network or cabling system. A multi-node network topology that
has a central multiplexer with many nodes feeding into and through the multiplexer or hub. The other nodes
do not usually directly interconnect. LAN hubs are increasingly popular with the growth of structured
cabling and the need for LAN management.
Brouter or Hybrid Bridge/Router Network interconnection device with a standard mode of
operation as a router. It can support multiple OSI Layer Three routing protocols, but also supports
bridging capabilities for network traffic that cannot be routed - non-routable traffic.
Hypertext A link between one document and other, related documents elsewhere in a
collection. By clicking on a word or phrase that has been hightlighted on a computer screen, a user can skip
directly to files related to that subject.
HyperText Markup Language (HTML) The coding that World Wide Web browsers read to create Web pages.
HyperText Transfer Protocol (HTTP) The protocol used to transfer World Wide Web data across