Glossary of Terms
T to Z

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T

• TA (Telecommunications Administration) A body, usually a company, that provides public telecommunications services.
• 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 standards.
• 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 SMTP.
• 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 IBM's 3270.
• 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 Cheapernet.
• 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 examples.
• 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 it.
• 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 users.

  U

• 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 datagrams.
• 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 supply fails.
• 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 form.
• 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 future.
• 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 procedures.
• 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 financial services.
• 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 LAN working.
• 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.3 Packet Assembly/Disassembly (pad).
• X.4 International Alphabet No.5 for character oriented data.
• X.20 Asynchronous communications interface definitions for use over the PSTN.
• X.20 bis V-series compatible modem, asynchronous communications interface definitions for use over the PSTN.
• X.21 Synchronous 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.
• X.50bis Fundamental parameters of 48Kbit/s transmission for the international interface between synchronous data networks.
• X.51 Fundamental parameters of multiplexing scheme for the international interface between synchronous data networks using 10-bit envelope structure.
• X.51bis Fundamental 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 in PDNs.
• X.110 international routing principles and routing plan for PDNs.
• X.121 International 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 these numbers:
• X.402 Overall Architecture.
• X.403 Conformance Testing.
• X.407 Abstract service definition conventions.
• X.408 Encoded information type conversion rules.
• X.411 Message transfer system.
• X.413 Message store.
• X.419 Protocol specifications.
• X.420 Interpersonal messaging system.
• 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.501 Models.
• X.509 Authentication framework.
• X.511 Abstract service definition.
• X.518 Procedures for distributed operation.
• X.519 Protocol specifications.
• X.520 Selected attribute types.
• 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 finalized.
• 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 LAN servers.

Y

Z

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