Glossary of Terms
A to D
GOTO: A B C D E F G H I J K L M N O P Q R
S T U V W X Y Z
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
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
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
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
Alternate buffer Two buffers
are sometimes used to handle data I/0. These are a alternated to achieve continuous
Alternate routing - Safety
technique enabling communication to continue iii the event of node failure or congestion.
The network design allows for alternate paths through the network to arrive at the same
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 transmission, whereas a digital signal changes on y
between two set values without intermediate variations.
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.
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.
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
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.
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
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 I in use today, connecting
a large number of universities in the US and Europe, as well as commercial users.
ASCII The American Standard
Code for Information Interchange developed by ANS I 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.
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 consumption 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, FTAM, 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 separate 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 extra 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 also 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 digital 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. B
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 running 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 typical y 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 or routers.
Background Task or Mode A
secondary function perforated by a computer without interrupting its current or primary
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 pa r. Baluns are often used so that IBM 3270 terminals can run off
twisted pair, or to allow co-axial Ethernet 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 the data.
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
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 I EEE 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
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 16 Kbit/s "D" channel (2B + D), carrying
user traffic and signaling information respectively to the user via twisted pair local
Baud A unit of s gnarling
speed, expressed in terms of the number of discrete conditions or signal events per
second. It is on y the same as bit/s, when one discrete signaling condition is used to
transmit a single bit of data.
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 transmits (beacons) a special MAC frame used to isolate the
location of the error using beacon transmit and beacon repeat modes.
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
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
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 I 0
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.
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 mil ion 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.
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
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
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
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
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
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 that 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. C
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 on a telepoint network. Vendors with CAI compliant systems
include Northern Telecom and GPT.
CATV (Cable TeleVision) Cable
system covering defined areas, such as the U K'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 representative organization.
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 the 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 t 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 communications 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 powerful 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
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
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.
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
Communications Controller A
switching unit central to the implementation of host-based IBM S NA 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 decision makers.
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 s 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
CPE (Customer Premises
Equipment) Telecommunications- communications equipment, including PBXs and wiring,
located in a user's premises.
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; termination 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.
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 ink 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
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. A so known by older, proprietary names CIT (Computer Integrated
Telephony) and CSTA (Computer Supported Telephony Applications). D
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 OS I 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 switching nodes.
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 system.
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 transaction 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
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 cordless
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
Demodulation Technique for
retrieving information from a modulated signal. Demonstrated by the eponymous modem
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.
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
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 remote 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 System Node.
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 ideal 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 memory.
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 layers 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 vendors 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
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 communications 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 technologies.
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, functionally 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 El 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
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
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