بسم الله الرحمن الرحيم

A comprehensive glossary of communications terminology

لا توجد كل المصطلحات بل توجد فقط (192 terms)

3G


Abbreviation for Third Generation: the generic term used for the next generation of mobile communications systems, providing enhanced capacity, quality and data rates and promising exciting new services in all of the areas of voice, text and data.
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ACK

Acknowledgement: notification sent from one network device to another to acknowledge that some event - for example, receipt of a message - occurred.
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ADSL

Asymmetrical Digital Subscriber Line: the most promising of the family of xDSL technologies, in which the data rate from the ISP is greater than the data rate to the ISP. Projections are for capacities of more than 6 Mbit/s to the home and anywhere between 64- and 640 Kbit/s from the home to the ISP.
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AM

Amplitude Modulation: a data transmission
technique that blends the data signal into a carrier by varying (modulating) the amplitude of the carrier.
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Analogue




In an analogue signal numerical values are represented by physical variables such as voltage, current, etc. Analogue video represents an infinite number of smooth gradations between given video levels. Analogue devices are characterised by dials and sliding mechanisms. See also Digital.
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Answer Mode

A modem is put into Answer Mode when a call from another computer is expected. The modem uses "originate mode" when calling another computer. Although a modem can operate in either mode, some modems operate only in originate mode. To communicate with these modems, a modem can switch to answer mode after placing (originating) a call.
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Asynchronous Communication

Communication between computers at irregular intervals. To handle this kind of communication, the modem adds start and stop bits to each character it sends. The start bit lets the receiving computer know when a new character has started. The stop bit indicates the end of the character.

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AT Command Set

The set of industry-standard commands, developed by Hayes, used to control the modem. Most modems are "Hayes compatible".ATMAsynchronous Transfer Mode: a network technology for both LANs and WANs based on transferring data in cells or packets of a fixed size that supports real-time voice and video as well as data. The topology uses a connection-oriented technique similar to the analogue telephone system, maintaining a connection for the duration of a transmission.

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Auto Answer

The modem automatically answers the phone after a certain number of rings. This is in contrast to manual answering, where a person sends an AT command to the modem to cause the modem to answer a ringing telephone.

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Auto-Reliable Mode

The modem automatically negotiates with the remote modem for a connection, determines whether or not it can use error control and data compression during a transmission, and determines which error control and data compression protocol it can use.

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Automatic Dialling

The modem automatically dials the telephone. This is in contrast to manual dialling, where a person dials the number.

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AWG

American Wire Gauge: a standard measuring gauge for certain conductors, including copper. The higher the AWG number the thinner the wire. The origins of the gauge lie in the number of times the wire was run through a wire machine to reduce its diameter. Thus a 24-guage wire was thinner than an 18-guage wire because it had been run through a wire machine 6 more times.

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B Channel

This is an ISDN communication channel that bears or carries voice, circuit or packet conversations. The B-channel is the fundamental component of ISDN interfaces. It carries 64,000 bits per seconds in either direction.

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BABT

British Approval Board for Telecommunications: any modem used in the UK must have this approval. A green circle indicates BABT approval, a red triangle means it has not been approved.
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BACP

Bandwidth Allocation Control Protocol: a protocol that works in conjunction with Multilink PPP to manage bandwidth dynamically. BACP lets two devices negotiate adding or subtracting bandwidth as needed.

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Baseband

Characteristic of a network technology where only one carrier frequency is used and all the available bandwidth is used for each transmission. Ethernet is an example of a baseband network. It is sometimes referred to as Narrowband. Contrast with Broadband.

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Baud Rate

The number of symbols transmitted per second. This is not always the same as the bps rate (see also BPS), because a given symbol, or baud, may have more than one bit.

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BISYNC

BInary SYNChronous: a major category of synchronous communications protocols, developed by IBM and used in mainframe networks. Bisync communications require that both sending and receiving devices are synchronised before transmission of data is started. Contrast with asynchronous transmission.

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Bit/s

Bits Per Second: the speed at which data travels over a communications circuit. For example, a modem that operates at 2400 bits per second can transfer 2400 binary digits each second. A character normally consists of seven or eight of these binary digits, plus the start and stop bits that separate the character from other transmitted characters.

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Bluetooth

Refers to a short-range radio technology aimed at simplifying communications among Net devices and between devices and the Internet. It also aims to simplify data synchronisation between Net devices and other computers. Bluetooth's founding members include Ericsson, IBM, Intel, Nokia and Toshiba.

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BRI

Basic-Rate Interface: the basic ISDN setup, consisting of two 64 Kbit/s B-channels (bearer channels), which carry data and voice in both directions, and one 16 Kbit/s D-channel, which carries call-control information. See also PRI.

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Bridge

A device that operates at the data link layer (Layer 2) of the OSI reference model and whose function is to connect and pass packets of information between two network segments.

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Broadband

Refers to any communications channel with a bandwidth greater than a voice-grade (4 kHz) and generally implies a transmission system that splits the available bandwidth into separate channels to use concurrently. Also referred to as Wideband. Contrast with Baseband.

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Cable Modem

A modem that uses part of the capacity of the local cable system to transmit data rather than TV channels to the home. It works much like a Local Area Network. Unlike the typical cable system, where TV signals can only be broadcast to the home, information is allowed to be transmitted in both directions.

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CAP

Carrierless Amplitude Phase: a multilevel multiphase encoding method and one of the two main modulation methods that can be used with ADSL technology. CAP uses frequency modulation techniques for sending signals over standard copper twisted-copper wire, giving data bit combinations a form of both amplitude and phase. Unlike DMT, CAP uses the whole frequency range from 4KHz up to 1.1MHz as a single channel. It is used in the V.32/V.32bis modem communication standard.

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Carrier

The base signal used to transmit data across a telephone line. The modem modulates this signal (alters its frequency or phase) to encode the data to be transmitted.

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CCITT

Consultative Committee for International Telephone and Telegraph: an international standards organisation dedicated to creating communications protocols that will enable global compatibility for the transmission of voice, data, and video across all computing and telecommunications equipment. Changed its name to the International Telecommunications Union (ITU) in 1993.

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CDMA

Code Division Multiple Access: a technology for digital transmission of radio signals that uses digital encoding and spread-spectrum RF techniques to let multiple users share the same RF channel. In CDMA, a frequency is divided using codes, rather than in time or through frequency separation.

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CDPD

Cellular Digital Packet Data: a wireless communications protocol - widely used by law enforcement agencies - which enables users to transmit packets of data over the cellular network using a portable computing device and a CDPD modem.

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Clear To Send

CTS: an RS-232C signal that tells the computer it can start sending information. See also Request To Send (RTS).

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Client-Server

A network architecture in which each computer or process on the network is either a "client" or a "server". Servers are powerful computers or processes dedicated to managing disk drives (file servers), printers (print servers), or network traffic (network servers ). Clients are typically PCs or workstations on which users run applications. Contrast Peer-to-peer.

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CO

Central Office: a facility that serves local telephone subscribers. In the CO, subscribers' lines are joined to switching equipment that allows them to connect to each other for both local and long distance calls.

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Coaxial Cable

Cable consisting of a hollow outer cylindrical conductor that surrounds a single inner wire conductor. Two types of coaxial cable are currently used in LANs: 50-ohm cable, which is used for digital signalling, and 75-ohm cable, which is used for analogue signalling and high-speed digital signalling.

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Collision

The situation that occurs when two or more devices attempt to send a signal along the same channel at the same time. The result of a collision is generally a garbled message. All computer networks require some sort of mechanism to either prevent collisions altogether or to recover from collisions when they do occur.

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Command Mode

One of the two operating modes of the modem, sometimes called local mode or terminal mode. In command mode, the modem interprets any information it receives from the local computer (or terminal) as modem commands. It tries to perform the commands sent to it, and it returns result codes indicating the results of the commands. See also On-Line Mode.

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Comms Software

A program that sets up a modem and provides a user interface to the various modem functions.

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Configuration Profile

The current operating characteristics of a modem, stored in its S-registers. Whenever a modem command to change one of the operational characteristics (such as setting the volume control or turning the speaker on or off) is issued, the modem changes the values in the S-registers to reflect the changes.

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CSMA/CD

Carrier Sense Multiple Access with Collision Detection: the media access mechanism used by Ethernet and IEEE 802.3 wherein devices ready to transmit data first check the channel for a carrier. If no carrier is sensed for a specific period of time, a device can transmit. If two devices transmit at once, a collision occurs and is detected by all colliding devices. This collision subsequently delays retransmissions from those devices for some random length of time.

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D-Channel

This is an ISDN communication channel used for sending information between the ISDN equipment and the ISDN central office switch. The D-channel can also carry "user" packet data at rates up to 9.6 Kilobits.

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Data Carrier Detect

DCD: an RS-232C signal that indicates the modem is receiving a carrier signal from a remote modem.

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Data Compression

The encoding of text or data so that it takes up less space (fewer bits). CCITT V.42bis and MNP Class 5 are two different data compression protocols. Data compression allows a modem to transmit more information in a shorter period of time and thus increases its data throughput. Decompression by a receiver reverses the process.

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Data Set Ready

DSR: an RS-232Ccircuit that is activated to let a DCE know when a DTE is ready to send or receive data.

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Data Terminal Ready

DTR: an RS-232C signal that tells the modem the local computer (or terminal) is ready for data transmission.

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DCE

Data circuit terminating equipment: a device used to connect two DTEs over a network. A modem is a DCE.

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Dial Modifiers

AT commands that instruct the modem in dialling the telephone. Dial modifiers specify things like whether to use pulse or tone dialling, when and how long to pause between numbers, and whether to dial a stored number.

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Digital

A method of signal representation by turning a voltage on or off. Each on or off state represents a binary 1 or 0, respectively. Unlike analogue signals, digital signals can be repeatedly regenerated without introducing noise or distortion. See also Analogue.

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DirecTV

A satellite television provider, offering Internet access by satellite dish.

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DMT

Discrete MulitiTone: one of the two main modulation methods that can be used with ADSL technology. DMT divides the frequency spectrum supported by standard copper twisted-pair wire into 256 sub-frequencies from 64Khz to 1.1MHz. Each sub-frequency is an independent channel and has it own stream of signals. See also CAP.

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DOCSV

Data Over Circuit-Switched Voice: an ISDN adapter feature that allows data to be sent over a B-channel normally provisioned for voice, avoiding per-minute tariffs often applied to ISDN data calls.

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DSSS

Direct-Sequence Spread Spectrum: one of two types of spread spectrum radio (the other being FHSS) that continuously change frequencies or signal patterns. DSSS multiplies the data bits by a very fast pseudo-random bit pattern that "spreads" the data into a large coded stream that takes the full bandwidth of the channel.

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DTE

Data terminal equipment: an end device on a communications circuit, such as a computer terminal or PC.

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Duplex

The way modems exchange data: half duplex or full duplex. With half duplex transmissions, only one modem can send data at a time. Full duplex transmissions allow both modems to send data simultaneously.

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E1

A four-wire European telephone company standard that carries data at 2.048 Mbit/s. The European-equivalent of a US T1 line.

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EDGE

Enhanced Data for GSM Evolution: a technology that gives GSM and TDMA similar capacity to handle services for 3G. EDGE was developed to enable the transmission of large amounts of data at rates of 384 Kbit/s.

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EIRP

The Effective Isotropic Radiated Power of a transmitter (uplink) is the power that the transmitter appears to have if the transmitter was an isotropic radiator, i.e., if it radiated equally in all directions. By virtue of the gain of a radio antenna, dish, radio telescope or optical telescope, a beam is formed that preferentially transmits the energy in one direction. The EIRP is given by the product of the gain and the transmitter power.

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EPP

Enhanced Parallel Port: a parallel port that conforms to the EPP standard developed by the IEEE 1284 standards committee. The EPP specification transforms a parallel port into an expansion bus that can handle up to 64 disk drives, tape drives, CD-ROM drives, and other mass-storage devices.

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Error Control

The encoding of text or data so that a receiving modem can detect and sometimes correct errors in data transmissions. LAPM and MNP classes 1 through 4 are two different error control protocols.

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Escape Sequence

A sequence of three characters (normally "+++") that switches the modem from the on-line mode to the command mode without breaking the telephone connection.

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Ethernet

Developed by Xerox, Digital and Intel (IEEE 802.3) , this is the most widely used LAN access method. Normally, all stations on a segment share the total bandwidth, which is either 10 Mbit/s (Ethernet), 100 Mbit/s (Fast Ethernet) or 1000 Mbit/s (Gigabit Ethernet). With switched Ethernet, each sender and receiver pair have the full bandwidth.

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ETSI

European Telecommunications Standards Institute: a non-profit membership organisation founded in 1988, dedicated to standardising telecommunications throughout Europe. It promotes worldwide standards, and its efforts are co-ordinated with the ITU.

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FCC

Federal Communications Commission: the U.S. Government agency that supervises, licenses, and regulates electronic and electromagnetic transmission standards.

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FDDI

Fibre Distributed Data Interface: an ANSI standard token passing network that uses optical fibre cabling and transmits at 100 Mbit/s up to two kilometres. Typically used as backbones for wide area networks (WANs).

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FDM

Frequency Division Multiplexing: a technique that uses different frequencies to combine multiple streams of data for transmission over a communications medium. Each signal travels within its own unique frequency range (carrier), which is modulated by the data (text, voice, video, etc.). Many carrier frequencies are then combined for transmission.

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FDMA

Frequency Division Multiple Access: a mobile communications technique in which radio spectrum is divided into frequency bands.

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FHSS

Frequency Hopping Spread Spectrum: a radio transmission method that continuously changes the Center frequency of a conventional carrier several times per second according to a pseudo-random set of channels, thereby making illegal monitoring extremely difficult, if not impossible. See also DSSS.

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FIFO

First In-First Out: a storage method that retrieves the item stored for the longest time. See also LIFO.

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Firewall

A firewall provides a buffer - implemented either in hardware or software, or combination of both - that resides between an internal network and the Internet. It can be configured to allow only specific kinds of messages from the Internet to pass to the internal network, thereby protecting it from intruders or hackers who might try to use the Internet to break into those systems.

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Flash ROM

A type of memory used for firmware in modems and other digital devices. Unlike conventional ROM (read-only memory), flash ROM can be erased and reprogrammed, making it possible to update a product's firmware without re-placing memory chips.

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Flow Control

The mechanism that regulates the flow of data between two devices. Modems typically have two methods of flow control software flow control (XON/XOFF) and hardware flow control (CTS/RTS).

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FSK

Frequency Shift Keying: a data transmission technique that blends a data signal into a carrier by varying (modulating) the frequency of the carrier.

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FTP

File Transfer Protocol: a set of rules that allows two computers to communicate with each other as a file transfer is carried out.

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Full-Duplex

Usually used in reference to communications channels or devices. Means that data can be simultaneously sent and received. Also used to escribe a soundcard's ability to record and playback digital audio simultaneously.

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Gateway

A hardware bridge device that serves as a junction between two different types of network and that contains the necessary protocol translation software to enable them to exchange information.

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Gigabit Ethernet

The latest version of Ethernet. It offers 1 Gbit/sec raw bandwidth - 100 times faster than the original Ethernet - yet is compatible with existing Ethernets, because it uses the same CSMA/ CD and Media Access Control (MAC) protocols.

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GPRS

General Packet Radio Service: an enhancement for GSM and TDMA core networks that introduces packet data transmission. GPRS uses radio spectrum very efficiently and provides users with "always on" connectivity and greater bandwidth.

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GPS

Global Positioning System: refers to satellite-based radio positioning systems that provide 24 hour three-dimensional position, velocity and time information to suitably equipped users anywhere on or near the surface of the Earth (and sometimes off the earth). GPS technology is used in a wide range of applications, including maritime, environmental, navigational, tracking and monitoring.

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GSM

Global System for Mobile Communications: first introduced in 1991, GSM is the largest digital mobile standard in use today. Implemented in 400MHz, 900MHz, 1800MHz and 1900MHz frequency bands.

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Guard Time

A period of time during which the modem must not receive characters. The escape sequence has a guard time to ensure that data sent from a remote modem isn't interpreted as an escape sequence.

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Half-Duplex

Data transmission in 2 directions, but only 1 direction at a time.

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HDLC

High-level Data Link Control: An ISO communications protocol used in X.25 packet switching networks. The HDLC protocol embeds information in a data frame that allows devices to control data flow and correct errors at the data link layer.

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HomePNA

Home Phoneline Networking Alliance: an association of industry-leading companies working together to ensure adoption of a single, unified phoneline networking standard and the development of a range of interoperable home networking solutions.

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HomeRF

Home Radio Frequency: a wireless personal area network (PAN) technology from the HomeRF Working Group, founded in 1998 by Compaq, IBM, HP and others. HomeRF uses the Shared Wireless Access Protocol (SWAP) and provides an open standard for short-range transmission of digital voice and data between mobile devices (laptops, PDAs, phones) and desktop devices.

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HSCSDHigh Speed Circuit Switched Data: the final evolution of circuit switched data within the GSM environment. HSCSD enables the transmission of data over a GSM link at speeds of up to 57.6kbit/s. This is achieved by cocatenating consecutive GSM timeslots, each of which is capable of supporting 14.4kbit/s. Up to four GSM timeslots are needed for the transmission of HSCSD.

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Hub

A common connection point for devices in a network. Often used to describe the device that serves as the centre of a star-topology network.

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IMT-2000International Mobile Telecommunications-2000: an ITU initiative to define a standards framework for 3G mobile systems providing access - by means of one or more radio links - to a wide range of telecommunications services supported by the fixed telecommunication networks (e.g. PSTN/ISDN/IP) and to other services which are specific to mobile users.

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IrDA

Infrared Data Association: a standard for transmitting data via infrared light. IrDA ports enable the transfer of data between IrDA devices such as computers and printers without using a cable.

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ISDN

Integrated Services Digital Network: the CCITT standard that defines a completely digital telephone/telecommunications network which carries voice, data, and video over existing telephone network infrastructure. ISDN provides two 64 Kbit/s channels, which can be combined or used independently for both voice and data. It is designed to provide a single interface for hooking up a phone, fax machine, PC, etc.

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ISP

Internet Service Provider: a company that provides access to the Internet. For a monthly fee, subscribers are provided with the necessary software, a username, password and access phone number. Using a modem or ISDN terminal adapter they can then log on to the Internet, browse and download from the WWW and send and receive e-mail. An amount of free Web space is generally provided, allowing the subscriber to create a Web site and thereby have a presence on the Web.

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ITU

International Telecommunications Union: the United Nations agency for telecommunications. The ITU combines the standards-setting activities of the predecessor organisations formerly called the International Telegraph and Telephone Consultative Committee (CCITT) and the International Radio Consultative Committee (CCIR), being charged with establishing and co-ordinating standards for electronic communications worldwide.

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K56flex

A protocol, jointly developed by Lucent Technologies and Rockwell International Corp., to achieve 56 Kbit/s modem transmissions over ordinary phone lines. K56flex allows downloads at up to 56 Kbit/s; uploads are limited to the normal V.34 speed of 33.6 Kbit/s. See also X2.

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Kbit/s

Kilobits Per Second: a measure of data transfer speed. Modems, for example, are measured in Kbit/s. Note that one Kbit/s is 1,000 bits per second, whereas a kilobyte is 1,024 bytes. Data transfer rates are measured using the decimal meaning of K whereas data storage is measured using the powers-of-2 meaning of K. This measure is often expressed as "Kbps". However, this is also sometimes used to mean "Kilobytes per second" (as well as the the more common "KBps"). To avoid confusion the PC Technology Guide uses "Kbit/s" throughout.

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LAN

Local Area Network: a computer network technology designed to connect computers separated by a short distance. The local group of linked computers are generally connected to a single, shared server.

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LAPM

Link Access Procedure For Modems: one of the two protocols specified by V.42. LAPM provides error control when a modem is communicating with another modem that supports LAPM.

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LIFO

Last In First Out: a queuing method in which the next item to be retrieved is the item most recently placed in the queue. See also FIFO.

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Line Noise

Random signal disturbances that sometimes occur over telephone lines. Noise can disrupt communications and corrupt the transmitted data. The ratio of the usable signal to unusable noise on a communications link is referred to as the signal-to-noise ratio. Fibre optic cables are far less susceptible to noise than metal wire cables.

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Local Loop

The lines between a customer and the telephone company's central office, often called the "last mile." Local loops use copper-based telephone wire.

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Loop Qualification

This is a test done by the phone company to make sure the customer is within the maximum ISDN distance of 18,000 feet from the central office that services that customer.

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Loopback Test

Diagnostic test where characters that are sent to the modem are immediately sent back from the modem so the computer can compare the characters sent with the characters received.

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MAC

Medium Access Control: general standard for the data link layer in the OSI Reference Model. The IEEE divides this layer into two sub-layers - the logical link control (LLC) layer and the media access control (MAC) layer. The MAC sub-layer is the lower of the two and is responsible for moving data packets to and from one Network Interface Card (NIC) to another across a shared channel. It varies for different network types and is defined by the IEEE 802 family of standards.

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MAN

Metropolitan Area Network.

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Manual Dialling

Dialling a remote modem from a telephone connected to the modem. This is in contrast to automatic dialling, where the modem dials the number.

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MAPI

Messaging Application Programming Interface: an API developed by Microsoft and other computer vendors that provides Windows applications with an implementation-independent interface to various messaging systems.

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Mbit/s

The measure of data transfer speed to mean 1 million bits per second. This is often expressed as "Mbps". But this is also sometimes used to mean "Megabytes per second" (as well as the the more common "MBps"). To avoid confusion the PC Technology Guide uses "Mbit/s" throughout.

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Mid Span Repeater

A device that amplifies the signal coming or going to the central office. This device is necessary for ISDN service if you are outside the 18,000 feet distance requirement from the central office.

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MKK

Musen Kensa-kentei Kyokai: the Government of Japan's authorised radio terminal equipment inspection institute.

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MNP

Microcom Networking Protocol: a series of standards, running from MNP Class 1 to MNP Class 10, designed to improve communications between modems but now superseded by LAPM. They do not stand alone, but operate in conjunction with other modem standards.

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Modem

MOdulator/DEModulator: a modem transforms (modulates) digital information into an analogue signal that can be sent across a telephone line. It also demodulates an analogue signal it receives from the telephone line, converting the information contained in the signal back into digital information.

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Modulation

Converting a data stream into sounds to be sent down a phone line. The opposite is demodulation. See also Modem.

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MP

Multilink PPP: a protocol that allows the combination of both B-channels for a total of 128 Kbit/s using synchronous PPP framing. Unlike the older hardware-based BONDING specification, MP is implemented in software and requires only that the communications software support the protocol.

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Multiplexer

A device that integrates serial digital waveforms into a single channel by partitioning the inputted data into segments and combining them together into a bitstream.

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Napster

Created in 1999, Napster is a controversial music indexing service that gives individuals access to one another's MP3 files by creating a unique file-sharing system via the Internet.

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NIC

Network Interface Card: a card that is installed in a computer system to provide network communication capabilities to and from that computer.

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Node

Endpoint of a network connection or a junction common to two or more lines in a network. Nodes can be processors, controllers, or workstations. The term is often used generically to refer to any entity that can access a network, and is frequently used interchangeably with device.

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NOS

Network Operating System: an operating system that includes special functions for connecting computers and devices into a local-area network (LAN). Some operating systems, such as UNIX and the Mac OS, have networking functions built in. The term NOS, however, is generally reserved for software that enhances a basic operating system by adding networking features.

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Off-Hook

The condition of a telephone line that corresponds to picking up the telephone receiver. A modem creates an off-hook condition when it tries to communicate on a telephone line.

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OFTEL

The UK government regulator for telecommunications, first established in the mid-1980s to oversee the introduction of competition in a market dominated by British Telecom.
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On-Hook

The condition of a telephone line that corresponds to hanging up the telephone receiver. A modem creates an on-hook condition to break its connection to a telephone line.

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On-Line Mode

One of the two operating modes of the modem, also called data mode. In on-line mode, the modem interprets all information sent to it as data. The only exception is the escape sequence (normally "+++"), which returns the modem to command mode without breaking the connection.

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OSI

Open System Interconnection: an ISO standard for worldwide communications that defines a framework for implementing protocols in seven layers. Information is passed down through the layers until it is transmitted across the network, where it is passed back up the stack to the application at the remote end.

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PABX

Private Automatic Branch eXchange: an in-house telephone switching system that interconnects telephone extensions to each other, as well as to the outside telephone network. Modern PBXs use all-digital methods for switching and can often handle digital terminals and telephones along with analogue telephones.

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Packet

A logical grouping of information that includes a header containing control information and (usually) user data. Packets are most often used to refer to network layer units of data.

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PAN

Personal Area Network: refers to the ability of small devices such as cellphones, pagers, PDAs, and wearable computers to exchange data wirelessly within the physical limits of someone's personal space.

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Parity

A data encoding scheme that computers (and terminals) use to check the validity of transmitted characters. This scheme adds an extra bit to each character, which the transmitting computer sets or clears based on the type of parity the computers agree to use (odd or even). For example, if the computers use even parity, the transmitting computer sets or clears the parity bit so that there are an even number of bits set in each character it transmits. The receiving computer checks each character and flags a parity error if any character has an odd number of bits set.

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PCM

Pulse Coded Modulation: a technique for converting an analogue signal with an infinite number of possible values into discrete binary digital words that have a finite number of values. The waveform is sampled, then the sample is quantised into PCM codes. PCM is a digitisation technique used by the CCITT V.90 standard , not a universally accepted standard in its own right.

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PCS

Personal Communications Services: the collective term for US mobile telephone services in the 1900MHz frequency band.

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PDCPersonal Digital Cellular: a Japanese standard for digital mobile telephony in the 800MHz and 1500MHz bands.

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Peer-to-Peer

A network architecture in which each workstation has equivalent capabilities and responsibilities. Contrast Client-Server.

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PM

Phase Modulation: a data transmission technique that blends a data signal into a carrier by varying (modulating) the phase of the carrier.

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POTS

Plain Old Telephone Service: the basic analogue (nondigital) telephone service - standard single line telephones, telephone lines, and access to the public switched network. There are no added features, such as call waiting or call forwarding, with POTS.

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PPP

Point to Point Protocol: a protocol that allows a computer to connect to the Internet through a dial-in connection and enjoy most of the benefits of a direct connection; including the ability to run graphical front ends such as Internet Browsers. PPP is generally considered superior to SLIP, because it features error detection, data compression, and other elements of modern communications protocols that SLIP lacks.

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PRI

Primary-Rate Interface: the enhanced ISDN setup, consisting of 23 B-channels (30 in Europe) and one D-channel. See also BRI.

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PSK

Phase Shift Keying: a data transmission technique that blends a data signal into a carrier by varying (modulating) the phase of the carrier by a certain number of degrees for each succeeding signal.

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PSTN

Public Switched Telephone Network: refers to the international telephone system based on copper wires carrying analogue voice data. This is in contrast to newer telephone networks base on digital technologies, such as ISDN and FDDI.

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Pulse Dialling

A method of dialling the telephone where the modem sends pulses (which you hear through the handset as clicks) to represent the telephone numbers (one pulse for a one, two pulses for a two, etc.). Pulse dialling is normally associated with rotary-dial phones. See also Tone Dialling.

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QAM

Quadrature Amplitude Modulation: modulation technique used by high speed modems combining amplitude and phase modulation of the data signal. QAM generates four bits out of one baud. For example, a 600 baud line (600 shifts in the signal per second) can effectively transmit 2,400 bit/s using the technique.

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Quantisation

The process of representing a voltage with a discrete binary digital number. Approximating an infinite valued signal with a finite number system introduces an error called quantisation error or noise.

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RADIUS

Remote Authentication Dial-In User Service protocol: a client/server security protocol that allows network managers to reduce the risk of distributing security information across many devices by centralising authentication and permission attributes in a single server.

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RADSL

Rate Adaptive Digital Subscriber Line: an implementation of ADSL that automatically adjusts the connection speed on start up to adjust for the quality of the telephone line, thereby allowing the service to function over longer distances than does ordinary ADSL.

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Reliable Connection

A connection between two modems where they communicate using an error control protocol (such as LAPM or MNP).

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Request To Send

RTS: an RS-232C signal that requests the modem to send data. It initiates any data transmission between the computer (or terminal) and the modem. It is answered by a Clear To Send (CTS) signal.

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RF

Radio Frequency: the range of electromagnetic frequencies above the audio range and below visible light. All broadcast transmission, from AM radio to satellites, falls into this range, which is between 30KHz and 300GHz.

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RJ11

A common jack type most often used for connecting analogue phones, modems and fax machines to a communications line.

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RJ45

The most commonly used connection standard in networks. Its connector looks almost identical to a standard telephone jack, only slightly bigger. Twisted pair cabling is used - comprising two independently insulated wires twisted around each other. One wire carries the data while the other wire is grounded and absorbs any signal interference. This can come in unshielded (UTP) and shielded (STP) versions. Nodes are connected individually to the network, so if one connection fails, the rest of the network can continue to operate.

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Router

A device that operates at the network layer (Layer 3) of the OSI reference model and whose function is to use one or more metrics to determine the optimal path along which network traffic should be forwarded. Routers forward packets from one network to another based on network layer information. Occasionally referred to as "gateway" - although this definition of gateway is somewhat outdated.

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RS-232

A standardised connection system for connecting a device to the serial port of a computer or terminal. This is the recommended standard of the Electronic Industries Association (EIA) for exchanging information between DTE (such as computers) and DCE (such as modems).

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S-RegistersThe RAM in a modem that is used to store its current configuration profile (operating characteristics).

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SAN

Storage Area Network: a high-speed special-purpose network that interconnects different kinds of data storage devices - such as tape libraries and disk arrays- with associated data servers on behalf of a larger network of users.

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SDLC

Synchronous Data Link Control: Proposed by IBM in the 1970's, SDLC is the primary data link protocol used in their SNA networks. It is a bit-oriented synchronous protocol that is a subset of the HDLC protocol.

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Serial Port

The circuits and connector that facilitate communication between a computer and serial devices such as printers, modems, plotters, mice, and custom laboratory equipment. On a PC, this socket is a DB-9 or DB-25 male connector. It is a full-duplex device, using separate lines for transmitting and receiving data at the same time. Maximum throughput is 115.2 Kbit/s. Also called a COM or communications port.

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Shannon's Law

Defines the relationship between the maximum throughput in any given channel to the presence of noise.

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SLIP

Serial Line Internet Protocol: a protocol that allows a computer to connect to the Internet through a connection and enjoy most of the benefits of a direct connection, including the ability to run graphical front ends such as Internet Browsers. SLIP is also used to run TCP/IP over phone lines. See also PPP.

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SMDS

Switched Multimegabit Data Service: a high-speed, switched data communications service offered by telephone companies for interconnecting separate local area networks (LANs) into a single wide area network (WAN). Prior to SMDS's arrival in 1995, the only way to connect LANs was through a dedicated private line. SMDS is becoming an increasingly attractive alternative because it is more flexible and usually more economical.

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SNA

Systems Network Architecture: a mainframe network topology introduced by IBM in 1974. Originally designed as a centralised architecture with a host computer controlling many terminals, SNA has evolved over the years so that it now also supports peer-to-peer networks of workstations. SNA incorporates data protocols, network interface cards and just about every facet of communication.

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SNR

Signal-to-Noise Ratio: a measure of link performance arrived at by dividing signal power by noise power. Typically measured in decibels. The higher the ratio, the clearer the connection.

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Start/Stop Bits

The bits at the beginning and end of a data block when using asynchronous data transmission. See also Asynchronous Communication.

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STP

Shielded Twisted Pair: telephone wire that is wrapped in a metal sheath to eliminate external interference. See also UTP.

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Switch

A device that operates at the data link layer (Layer 2) of the OSI reference model and whose function is to filter and forward packets of information according to their destination address.

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Switched Ethernet

An Ethernet network that runs through a high-speed switch. Changing to switched Ethernet means replacing the Ethernet hub with a switch. As a result the full bandwidth - 10 Mbit/s for Ethernet or 100 Mbit/s for Fast Ethernet - is made available to each sender and receiver pair.

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T1

A four-wire USA telephone company standard that carries data at 1.544Mbit/s. The US-equivalent of a European E1 line.

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TCP/IP

Transmission Control Protocol/Internet Protocol: a set of communication protocols developed by the U.S. Department of Defense that allows dissimilar computers to share information over a network. TCP checks for lost packets, puts the data from multiple packets into the correct order and requests that missing or damaged packets be resent.

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TDM

Time Division Multiplexing: a data communications technique that interleaves separate data streams into one high-speed transmission by assigning each stream a different time slice in a set. The receiving end then divides the single stream back into its original constituent signals.

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TDMA

Time Division Multiple Access: a mobile communications technique in which a radio frequency channel is divided into time slots, each of which lasts for a fraction of a second. TDMA divides a 30KHz channel into six time slots that are allocated in pairs, resulting in three usable TDMA channels. Any given conversation can use one or more of every third time slot on an ongoing basis during a call.

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Token Ring

A local area network (LAN) technology developed by IBM (IEEE 802.5). Packets are conveyed between network end stations by a token moving continuously around a closed ring that uses twisted wire cable to connect nodes.

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Tone Dialling

One of two methods of dialling the telephone. (The other is pulse dialling.) With tone dialling, the modem sends tones of different frequencies to represent the telephone numbers. Tone dialling is normally associated with push-button (touch-tone) phones and is also called Dual Tone Multi-Frequency (DTMF) dialling.

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Topology

The pattern of interconnection between nodes in a communications network.

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Transceiver

a term used to describe a combination of transmitter and receiver. In the context of networking, a transceiver is an electronic interface or adapter between the Ethernet coaxial cable and the drop cable that attaches to network devices to provide the drive, reception, and collision detection between physical network media.

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Twisted Pair

Two insulated wires, usually copper, twisted together and often bound into a common sheath to form multi-pair cables. In ISDN, these cables are the basic path between a subscriber's terminal or telephone and the PBX or the central office.

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UART

Universal Asynchronous Receiver Transmitter: the chip that drives a serial port. IBM chose the National INS8250, better known simply as "the 8250", for the serial ports in its original PC. The subsequent 16550 UART provided support for speeds of 9,600 bps and greater.

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UMTS

Universal Mobile Telecommunications System: a 3G standard, being developed under the auspices of ETSI, and intended mainly for the evolution of GSM networks.

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USB

Universal Serial Bus: Intel's standard for attaching peripherals to PCs. Designed for low to medium data throughput, it should remove the need to install many devices internally once it gains widespread acceptance. The original 1995 USB1.1 standard supports a rate of 12 MBit/s, the subsequent USB2.0 standard up to 480 Mbit/s.

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USB-IF

USB Implementers Forum: a non-profit corporation founded by the group of companies that developed the Universal Serial Bus specification to provide a support organization and forum for the advancement and adoption of USB technology. The Forum facilitates the development of high-quality compatible USB peripherals (devices), and promotes the benefits of USB and the quality of products that have passed compliance testing.

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UTP

Unshielded Twisted-Pair: a four-pair wire medium used in a variety of networks. UTP does not require the fixed spacing between connections that is necessary with coaxial-type connections. See also STP.

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UTRAN

UMTS Terrestrial Radio Access Network: the name of the WCDMA radio network in UMTS.

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V.34

An ITU modem standard for data transmission at up to 33.6 Kbit/s. V.34 is the successor to several earlier ITU standards, and most V.34 modems can interoperate with older, slower modems.

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V.90

An ITU's modem standard, agreed on 4 February 1998, which brought to an end a year-long 56 Kbit/s standards battle between the rival proprietary X2 and K56Flex standards.

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VBR

Variable Bit Rate: maximum throughput set in advance, but data not always sent evenly.

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VoIP

Voice over IP: The technology used to transmit voice conversations over a data network using the Internet Protocol. The data network involved might be the Internet itself, or a corporate intranet, or managed networks used by local or long distance carriers and ISPs. The technique promises drastically reduced costs to carriers and therefore prices to end users. Also referred to as IP Telephony.

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VPN

A virtual private network is a private data network that makes use of the public telecommunication infrastructure (typically the Internet), maintaining privacy through the use of a tunnelling protocol and security procedures. It can be contrasted with a system of owned or leased lines that can only be used by one company, the idea of a VPN being to afford the same capabilities but at a much reduced cost.

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WAN

Wide Area Network: a geographically dispersed network formed by linking several computers or local area networks (LANs) together over long distances, usually using leased long-distance lines. WANs can connect systems across town, in different cities, or in different regions of the world.

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WAP

Wireless Application Protocol: a protocol that enables Internet services to be delivered to small-screen mobile devices. The application via which WAP-enabled devices access Web content is referred to as a "micro-browser".

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WCDMA

Wideband Code Division Multiple Access: a 3G wideband radio technique which makes highly efficient use of radio spectrum and is capable of supporting data rates of up to 2 Mbit/s, sufficient to allow simultaneous access to several voice, video and data services at once.

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WECA

The Wireless Ethernet Compatibility Alliance is a non-profit international association formed in 1999 to certify interoperability of wireless Local Area Network products based on IEEE 802.11 specification.

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WEP

Wired Equivalent Privacy data encryption is defined by the 802.11 standard to prevent access to the network by "intruders" using similar wireless LAN equipment and capture of wireless LAN traffic through eavesdropping. WEP allows the administrator to define a set of respective "Keys" for each wireless network user based on a "Key String" passed through the WEP encryption algorithm. Access is denied by anyone who does not have an assigned key.

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Wi-Fi

Branding that denotes products that have been certified as being interoperable. The scheme was originally operated by WECA - the Wireless Ethernet Compatibility Alliance – and applied to products conforming to the IEEE 802.11b wireless networking standard. WECA was subsequently renamed the Wi-Fi- Alliance and the brand applied also to IEEE 802.11a products.

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WiMAX

Worldwide Interoperability of Microwave Access: an implementation of the IEEE 802.16 standard, WiMAX provides metropolitan area network connectivity at speeds of up to 75 Mbit/sec. WiMAX systems can be used to transmit signal as far as 30 miles.

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WLAN

Wireless LAN: a local area network that transmits over the air typically in an unlicensed frequency such as the 2.4GHz band. Wireless access points (base stations) are connected to an Ethernet hub or server and transmit a radio frequency over an area of several hundred to a thousand feet which can penetrate walls and other non-metal barriers.

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X.10

A communications protocol for remote control of electrical devices designed for operation over standard household electrical wiring. It transmits data using Amplitude Modulation.

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X.25

An ITU standard for packet-switching networks approved in 1976, X.25 defines layers 1, 2, and 3 in the OSI Reference Model. Such networks are widely used for point of sale (POS) terminals, credit card verifications and automatic teller machine (ATM) transactions. New packet-switched networks employ frame relay and SMDS technologies rather than X.25.

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X2

Technology developed by U.S. Robotics for achieving modem transmissions at close to 56 Kbit/s over ordinary phone lines. See also K56flex.

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xDSL

Digital Subscriber Line: it shares the same phone line that the telephone service uses, but because it uses a different part of the phone line's bandwidth, it does not interfere with normal phone service. This is possible because there is a significant amount of unused capacity in current phone wires. The technology will allow subscribers to hook up DSL modems to a local Internet Service Provider (ISPs) and still be able to talk on the phone - all using the same phone line. The "x" represents a variety of possible methods and information rates that can be handled through DSL.

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XON/XOFF


A way of controlling the flow of data between a modem and its host computer and between two modems, also called software flow control. XON stands for "Transmitter On" and XOFF stands for "Transmitter Off". If the modem receiving data needs time to process the data or do some other task, it sends an XOFF signal to the host computer (or sending modem). The host computer (or sending modem) then waits until it receives an XON signal before sending more data.