Glossary of Terms
T to Z
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
Administration) A body, usually a company, that provides public telecommunications
T1 A committee belonging to
the ANSI whose role is to establish US standards for digital telephony, particularly ISDN.
The committee is in no way responsible for or involved with the Tl (1.5Mbit/s) circuit
T1 A US and Japanese
standard for high-speed data transmission at 1.544Mbit/s - 24 64Kbit/s channels plus
8Kbit/s' control information are provided. Also called a DS1.
T3 A US standard for
high-speed data transmission at 44.736Mbit/s, providing the equivalent bandwidth of 28 T1
circuits. Also called a DS3.
T-Carrier The US standards
for digital transmission lines. The line types are of the form Tn or TIC, and the
corresponding line signal standards of the form DSn or DSIC.
Tap The connecting device on
cable-based LANs like Ethernet, linking to the main transmission medium.
TCP (Transmission Control
Protocol) The standard transport level protocol that provides the full duplex, stream
service on which many application protocols depend. TCP allows a process or one machine to
send a stream of data to a process on another. Software implementing TCP usually resides
in the operating system and uses the IP to transmit information across the network.
TCP/IP (Transmission Control
Protocol/Internet Protocol) The suite of protocols developed and used by DAR PA and
the US DOD. They build up to Laye r Four of the ISO OSI model, but there is no direct
correspondence layer for layer. Three main protocols sit above TCP/IP: Telnet, FTP and
TDM (Time Division
Multiplexer/Multiplexing) Multiplexer which apportions the time available on its
Composite link between its channels, interleaving data from successive channels. The
method divides up digital channels to make maximum use of their bandwidth, by taking input
from each source in turn. TDMs use one of two methods to achieve this, bit interleaving
for synchronous protocols and character interleaving for asynchronous protocols.
TDMA (Time Division Multiple
Access) In LAN technology, a high-speed, burst mode of operation that can interconnect
LANs. First used as a multiplexing technique on shared communications satellites.
Teletex ITU-TS standard for
text and message communications intended to replace telex-operating at 2,400bps, it
accommodates upper and lower case characters and has a well-defined format for
transmission and text presentation.
Teletext Method of
transmitting pages of information using broadcast transmission techniques. Embraces both
standard broadcast transmission systems and in-house/cable systems using this format.
Terminal emulation Software
that allows a PC to mimic the attributes of a dumb terminal normally attached to a
mainframe or mini-computer, giving the user with access to function keys and control
sequences which the host applications normally use when communicating with one of their
own dumb terminals. The most commonly emulated terminals are Dec's VT100 terminal and
TFTP (Trivial File Transfer
Protocol) The TCP/IP standard protocol for file transfer with minimal capability and
minimal overhead. TFTP depends on the connectionless datagram delivery service, UDP.
Thin Ethernet An 802.3 LAN
that uses smaller than normal diameter coaxial cable; often used to link PCs together. It
runs at the same frequency as Ethernet but at smaller distances. Colloquially called
Timeout The expiration of a
pre-defined interval which then triggers some action. For example, in a 30-second, no
activity timeout, disconnection occurs after 30 seconds of no data activity. Simple eh?
Alternatively, the length or existence of such an interval.
Token Unique information in
a packet header which acknowledges that control of the network is to be relinquished upon
receipt of the packet. An empty packet containing a token is forwarded by the recipient to
the next node. The token packet passes round the LAN continuously and, as it goes by, give
each workstation the all-clear to transmit data.
Token Bus A LAN with a bus
topology that uses token passing as its access method. The sequence of nodes is not
governed by physical architecture, but is controlled by software. Arcnet and Map are
Token passing A technique
for restricting access to a network, to a single node at a time. A token is passed from
node to node, granting permission to transmit data. The sender attaches its message to the
token which conveys it across the LAN to its destination, The message is ignored by all
other nodes, and is acted upon by the addressee, When the recipient has accepted the
message, it releases the token so that the next node wishing to send can use it.
Token Ring A 4/16Mbit/s LAN
architecture which uses a token passing access method to allow nodes on the network to
transmit data. Defined in IEEE 802.5 with a ring architecture, a special data packet,
called a token, is passed continuously from node to node (see Token passing, above). The
sequence of nodes is governed by the physical order in which the nodes appear on the ring.
Every node on the ring sees the data, but only the addressed receiving node will accept
Top (Technical and Office
Protocols) A Functional Profile originated by Boeing to separate networking in a
non-shop floor environment. TOP was designed from the outset to be compliant with the ISO
OSI seven-layer model. Development has beer merged with MAP, and the two functional
profiles share a common integration strategy, and have a single (Map/Top) user group.
TP-4/IP A term given to the
ISO protocol suite that closely resembles TCP/IP. Transceiver - A communications device
and software capable of transmitting and receiving (see also MAU). Transmission block - A
sequence of continuous data characters or bytes transmitted as a unit, over which a coding
procedure is usually applied for synchronous or error control purposes.
Transparent Bridging So
named because the intelligence necessary to make relaying decisions exists in the bridge
itself and is thus "transparent" to the communicating workstations. It involves
frame forwarding, learning workstation addresses and ensuring no topology loops exist (in
conjunction with the Spanning Tree algorithm).
Transport driver A network
device driver that implements a protocol for communicating between LAN Manager and one or
more media access control drivers. The transport driver transfers LAN Manager events
between computers on the local area network.
Transport Layer The Fourth
Layer in the OSI model, drawn up by the ISO. The purpose of the transport layer is to act
as an intermediary between the user and the network. All layers above the transport layer
are network independent.
Tree topology A graphic
description of a network topology where there is only one route between any two nodes.
Trellis coding An advanced
method of modulation which combines coding of both amplitude and phase. This gives a
greater throughput and lower error rate for speeds above 9.6Kbit/s.
Trunk in token ring, a trunk
is the cab e running between MSAUs and can be either fiber or shielded twisted pair cable.
STP uses two positive transmit wires in normal mode, with no crossover, while fiber has
one transmit fiber and one receiver fiber. In normal mode, the second pair of wires is not
used it acts as backup and implements the Wrap feature.
Twisted Pair Two insulated
copper wires twisted together with the twists or lays varied in length to reduce potential
signal interference between the pairs. Where cables comprise more than 25 pairs, they are
usually bundled and wrapped in a cable sheath. Twisted pair is the most common medium for
connecting phones, computers and terminals to PABXS. With the IEEE ratification of 10BaseT
for networking 10Mbit/s Ethernet over UTP telephony wiring, twisted pair has become
ubiquitous. As well as performance at Ethernet rates, it offers cost benefits to the end
user through flexibility - ease of relocation. New data-grade and even voice-grade UTP
methods support l00Mbit/s transmission, with 155Mbit/s ATM a probability.
Type A Intelligent Network
term describing IN services evoked by, and affecting, a single user. Most of them can only
be invoked during call setup or teardown.
Type B Intelligent Network
term describing IN services invoked at any point by, and affecting directly, several
UDP (User Datagram Protocol) The
IP standard protocol that allows an application program on one machine to send a datagram
to an application program on another machine. UDP uses the Internet IP to deliver
UPS (Uninterruptible Power
Supply) A battery, attached to a piece of hardware, for example a server, that
provides backup power for conducting an orderly shutdown if the server's normal power
UTP (Unshielded Twisted Pair) The
standard cabling used for telephone lines. The standard IEEE 802.3, 10BaseT, defines use
of Ethernet over UTP for rates up to l0Mbit/s. The general LAN medium of choice for the
1990s, UTP is marketed in brands such as AT&T's PDS (Systimax) and OSCA. V
Van (Value Added Network)/Vans
(Value Added Network Service) A data transmission network which guarantees data
security and integrity through added computer control and communications, from the sender
to the recipient often in the manner of a door-to-door courier or freight forwarder.
Vaporware Products announced
but not yet commercially available. Often used by suppliers to lock users in with the
promise of great things to come.
Videoconferencing Video and
audio communication between two or more parties via a video-codec (coder/decoder) at
either end linked by digital circuits. Formerly needing in excess of 300Mbit/s bandwidth,
systems are now available offering acceptable quality for general use at 128Kbit/s and
high-quality 71KHz audio. Factors influencing the growth of videoconferencing are improved
compression technology, reduced cost through VLSI chip technology, low-cost switched
digital networks - particularly ISDN - the emergence of standards and applications. Main
players include BT, PictureTel and Compression Labs Inc.
Videoconferencing standards ITU-TS
H.261 was the standards watershed. Announced in November 1990, it relates to the decoding
process used when decompressing videoconferencing pictures, providing a uniform process
for codecs to read the incoming signals. Originally defined by Compression Labs Inc. Other
important standards are H.221: communications framing; H.230 control and indication
signals and H.242d: call set-up and disconnect. Encryption, still-frame graphics coding
and data transmission standards have still to be developed.
Videotex Term invented by
the ITU to describe TV equipment used to display computer-based data, whether sent via a
telephone (often called viewdata) or a broadcasting charnel (Teletext). ITU distinguishes
between interactive or broadcast videotex.
VIM (Vendor Independent
Messaging) Standard for the application interface to e-mail from Lotus, WordPerfect
and others. It will include MAPI compliance.
Vines Banyan Systems'
Virtual Networking operating system is based on Unix system V. This network operating
system provides transparent communication across heterogeneous networks and is more
expansive in concept, although attracting far fewer users, than Novell's NetWare.
Virtual circuit A link that
seems and behaves like a dedicated point to point line or a system that delivers packets
in sequence, as happens on an actual point to point network. In reality, the data is
delivered across a network via the most appropriate route. The sending and receiving
devices do not have to be aware of the options and the route is chosen only when a message
is sent. There is no prearrangement, so each virtual connection exists only for the
duration of that one transmission.
Virtual Container (VC) SDH
defines a number of Containers, each corresponding to an existing plesiochronous rate.
Information from a plesiochronous signal is mapped into the relevant container along with
control information known as the "path overhead". The container plus path
overhead form a VC.
Virtual LAN A logical rather
than a physical LAN comprising workgroups drawn together for business reasons or for a
particular project irrespective of each member's actual location. Members are likely to
belong to several such LANs as their job function dictates. Such LANs await the maturity
of high-speed transmission technologies such as ATM before they can exist in any viable
Virtual teams Ad hoc groups
of users formed to solve particular problems without taking them away from their desks. A
useful option made feasible with groupware.
Virtual company Company
built on the basis of teleworking with limited central office administration. Made
possible by improved communications and groupware software, it is a phenomenon of the
VPN (Virtual Private Network) The
provision of private voice and data networking from the public switched network through
advanced public switches. The network connection appears to the user as an end-to-end,
nailed-up circuit without actually involving a permanent physical connection, as in the
case of a leased line. VPNs retain the advantages of private networks but add benefits
like capacity on demand.
Virus Code that attaches
itself to a program and makes copies of itself. It may or may not cause accidental or
malicious damage, but is a serious nuisance particularly in a networked environment and
where dependence on IT is heavy. Anti-virus applications such as Norton are available, but
security should be augmented by procedures and regular backups.
Voice-grade channel, Voice-grade
line A channel or line offering the minimum bandwidth suitable for voice frequencies,
usually 300bit/s to 3.4Kbit/s.
Voice Mail A system that
records, stores and retrieves voice messages; either a standalone device or those that
integrate to some extent with a user's phone system. Standalone voice mail is similar to a
collection of answering machines but able to instruct each machine (voice mailbox) to
carry out a range of features such as call forwarding. Messages can be delivered at a
pre-arranged time, tagged and edited. Integrated systems indicate messages waiting via a
light on a user's phone and/or an alphanumeric display. If the phone rings for a specified
number of rings, it can default to a mailbox which delivers its invitation to leave a
message and records the results.
VSAT (Very Small Aperture
Terminal) One-meter diameter satellite dishes used by remote sites, as opposed to the
3m dishes used by head office, in a satellite-linked network. The central office is able
to broadcast or multicast data to the remote sites. In Europe, regulatory restrictions are
gradually being lifted to allow two-way transmission. The US does not labor under the same
protectionism and so has a flourishing VSAT community that allows the remote site to reply
using the same link.
V.Fast Forerunner to the
V.34 modem standard due for ratification by 1995. V.Fast modems send a 1Mbyte file
typically in under 1.5 minutes.
V Series A group of ITU-TS
recommendations governing data transmission over telephone lines. Series includes:
V.21 300bit/s duplex modem
for use over PSTN
V.22 1200bit/s duplex modem
for use over PSTN and leased lines
V.22 bis 2.4Kbit/s duplex
modem for use over PSTN and leased lines.
V.23 600/1200bit/s modem for
use over PSTN.
V.24 Definitions of
interchange circuits between DTE and DCE.
V.25 bis Automatic calling
and answering equipment on the PSTN.
V.26 2.4Kbit/s modem for use
over leased lines
V.26 bis 2400/1200bit/s half
duplex modem for use over PSTN.
V.26 ter 2400/1200 bit/s
full duplex modern for use over PSTN.
V.27 4.8Kbit/s modem for use
over leased lines.
V.27 bis 4800/2400bit/s
modem for use over leasedline.
V.27 ter 4800/2400bit/s
modem for use over PSTN
V.29 9.6Kbit/s modem for use
over leased lines.
V.32 Up to 9.6Kbit/s bps
modern for use over PSTN or leased lines.
V.32 bis Up to 14.4Kbit/s
modem for use over PSTN or leased lines.
V.42 Error control
V.42 bis Data compression
technique for use with V.42
VTAM (Virtual Telecommunications
Access Method) An IBM software routine: the virtual access method for 3270 systems. W
WAN (Wide Area Network) A
network which covers a larger geographical area than a LAN and where telecommunications
links are implemented, normally leased from the appropriate PTO(s). Examples of WANs
include packet switched networks, public data networks and Value Added Networks.
Wiring closet Location,
usually a physical box in which the cabling on a particular floor is terminated, typically
in a wiring frame.
Wiring frame Frame used to
organize and manage the termination and connection of multiple cables.
Windowing Generic method of
displaying data on screen, mimicking looking at several pieces of paper at once. Each
window can be resized, moved and otherwise manipulated. It lies at the heart of making
multiuser systems user-friendly and points the way for presetting tomorrow's applications.
Windows The now ubiquitous
Microsoft standalone operating system with integral graphical user interface, running on
top of MS Dos. Version 4.0 is imminent.
Windows for Workgroups (WFWG) A
Microsoft operating system running on each PC that allows between two and 20 users to
share information such as files and e-mail by clicking on icons. There is no server as
such, but WFWG is compatible with NetWare and LAN Manager.
Windows NT (New Technology) Microsoft's
scalable 32-bit version of Windows aimed at high-end workstation "power" users.
It is a standalone operating system that is also a "network ready" system
capable of being a small application server for a workgroup of Windows based PCs.
Windows NT Advanced Server
(NTAS) An extension of Microsoft's Windows NT and incorporating all its features, NTAS
is a server operating system offering centralized management and security, fault tolerance
and multiple connectivity options. Geared to client/server computing on practically any
network, not least NetWare and Vines as well as LAN Manager.
WOSA (Windows Open Service
Architecture) A framework of open-ended interfaces allowing Microsoft Windows and
applications running under it to integrate with enterprise computing environments. It
includes APIs for messaging (MAPI), standard access to databases (ODBC) and extensions to
Workflow automation The flow
of documents around an organization in a prescribed order (workflow) can be automated,
delivering an hierarchical and controlled form of workgroup computing. Workgroup computing
- Method of organizing a business around productive teams using computer support to enable
cooperative working and to eliminate time/space restrictions. An extension of conventional
Workstation Term used freely
to mean a PC, node, terminal or high-end desktop processor (for CAD/CAM and similar
intensive applications) - in short, a device that has data input and output and operated
by a user.
Wrap Redundancy measure in
IBM token ring LANs. Trunk cabling used in token ring TCUs contains two data paths: a main
and back-up (normally unused). If the trunk cable is faulty, the physical disconnection of
the connector at a TCU causes the signal from the main path to wrap onto the back-up and
maintain the loop. X
X Series Recommendations
drawn up 51 by the ITU-TS to establish interfaces for Terminal Equipment (DTE) and Data
Circuit Terminating Equipment (DCE) and public data networks (PDN). The series includes:
X.1 International user
classes of service.
X.2 International data
transmission services and optional facilities.
X.4 International Alphabet
No.5 for character oriented data.
communications interface definitions for use over the PSTN.
X.20 bis V-series compatible
modem, asynchronous communications interface definitions for use over the PSTN.
communications interface definitions for use over the PSTN.
X.21bis V-series compatible
modem, synchronous communications interface definitions for use over the PSTN.
X.25 Interface between DTE
and DCE for terminals using packets over public data networks.
X.28 DTE/DCE interface for
asynchronous DTE accessing a pad.
X.29 Definition of handshake
protocol for use between pads and between the pad and DTE.
X.30 Support of X.20 bis,
X.21 and X.21 bis DTEs by ISDN.
X.31 Support of packet mode
DTEs by ISDN.
X.32 Interface between DTE
and DCE for terminals operating in packet mode accessing public data networks via PSTN,
ISDN or circuit switched PDN.
X.50 Fundamental parameters
of multiplexing scheme for the international interface between synchronous data networks.
parameters of 48Kbit/s transmission for the international interface between synchronous
X.51 Fundamental parameters
of multiplexing scheme for the international interface between synchronous data networks
using 10-bit envelope structure.
parameters of a 481Kbit/s transmission scheme for the international interface between
synchronous data networks using a 10-bit envelope structure.
X.58 Fundamental parameters
of multiplexing scheme for the international interface between synchronous data networks
using a 10-bit envelope structure.
X.60 Common channel
signaling for circuit switched data applications.
X.61 Signaling system no.7 -
data user part.
X.70 Terminal and transit
control signaling for asynchronous services on international circuits between
anisochronous data networks.
X.71 Decentralized terminal
and transit control signaling on international circuits between synchronous data networks.
X.75 Packet switched
signaling between public networks providing data transmission services.
X.80 Interworking of
inter-exchange signals for circuit switched data services.
X.92 Hypothetical reference
connections for synchronous PDNs.
X.96 Call progress signals
X.110 international routing
principles and routing plan for PDNs.
numbering plan for PDNs.
X.130 Call set-up and
clear-down times for international connection to synchronous PDNs.
X.132 Grade of service over
international connections to PDNs.
X.400 A message handling
system standard that permits the electronic exchange of text as well as other electronic
data like graphics and fax. The X.400 standard is an overview which is broken down under
X.402 Overall Architecture.
X.403 Conformance Testing.
X.407 Abstract service
X.408 Encoded information
type conversion rules.
X.411 Message transfer
X.413 Message store.
X.500 A directory standard
that lets applications like e-mail access information which can either be central or
distributed. The benefit of a directory is the ability to minimize the impact on the user
of changes to a network. The standard is broken down under subsequent numbers:
X.511 Abstract service
X.518 Procedures for
X.520 Selected attribute
X.521Selected object types.
Xapia (X.400 Application
Programming Interface Association) Body standardizing the interface to X.400 e-mail
services. Other APIS, like VIM and MAPI, are likely to comply with Xapia when it is
X/Open A body comprising of
computer vendors, responsible for researching, defining and publicizing open systems.
X.Windows A networked GU I
based on a client/server architecture, it displays information from multiple networked
hosts on a single workstation. Available on PCs as X.terminal emulation and emulation on
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