Glossary: NetComm Australia

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AAL (ATM adaptation layer)	The layer of the ATM protocol that adapts different telecommunications service requirements to the ATM network.
ABR (Available Bit Rate)	A service class for ATM in which the peak cell rate is dictated by the network.
AC (Alternating current)	When the voltage source varies in value with time (normally reversing its polarity sinusoidally), the current is alternating current (AC), which varies also varies sinusoidally with time.
Access point	A base station in a wireless LAN. Access points are typically stand-alone devices that plug into an Ethernet hub or server. Like a cellular phone system, users can roam around with their mobile devices and be handed off from one access point to the other. See WLAN (wireless LAN).
Ack (Acknowledgment)	A message sent by a receiver to positively acknowledge the receipt of an agreed amount of data.
Acoustic coupler	A form of modem that does not connect directly to the phone line, but generates audible tones that are then transmitted through the telephone handset
Adaptive routing	A routing algorithm that evaluates the available paths across a network and chooses the one that will provide the best path for a message.
ADC (Analogue-to-digital conversion.)	The conversion of the analogue voltage levels of an analogue signal into ones and zeros of a digital word. It is normally conducted by taking regular samples of the analogue signal and converting the voltage level into a binary value.
Address resolution	Techniques by which the addressing schemes of network-layer (See OSI, Layer 3) addresses are mapped onto data-link layer (See OSI, Layer 2) addresses.
Ad-hoc network	Also known as a peer-to peer network, an ad-hoc network allows all computers participating in a wireless network to communicate with each other without an access point.
ADM (Adaptive delta modulation)	A form of delta modulation in which the step size is varied to improve tracking accuracy.
ADPCM (Adaptive differential pulse code modulation)	A more efficient form of pulse code modulation, in which the previous sample is used to predict the value of the current sample
ADSL	See DSL
AF (Audio frequency)	The range of frequencies audible to the human ear, generally considered as sitting between 15 Hz to 15,000 Hz.
AGC (Automatic gain control )	The mechanism inside a radio receiver that automatically increases or decreases the gain of the various stages of a receiver ensuring that there is constant output regardless of signal fluctuations.
ALE (Automatic link establishment)	A technique used in high frequency (HF) radio communications to choose the best available channel when a connection is established.
AM (Amplitude modulation)	A form of modulation in which information is impressed onto a carrier wave whose amplitude is caused to vary in accordance with the modulating signal.
AMI (Alternate mark inversion)	A digital transmission technique in which binary values are sent by three voltage states.
Ampere (Amp)	The measure of electrical current. Otherwise known as AMP.
Amplification	Increasing the amplitude of a signal. Otherwise referred to as gain.
Amplitude	The size or magnitude of a signal, usually expressed in terms of voltage or current.
AMPS (Advanced Mobile Phone System)	An American standard for first-generation analogue mobile telephones.
Analogue.	A representation of an object that represents the original. Analogue devices monitor conditions such a movement temperature and sound and convert them into analogous electronic or mechanical patterns. For example, telephones turn voice vibrations into electrical vibrations of the same shape. Analogue implies a continuous signal in contrast with digital, which breaks everything down into numbers.
Analogue signal	A continuously varying waveform.
Angle modulation	A modulation in which the angle of a sine wave carrier is varied in accordance with the information contained in the modulating waveform.
Antenna	A device for radiating and receiving electromagnetic waves in the radio frequency (RF) range. Otherwise known as an aerial.
Application layer	(See OSI, Layer 7)
ARQ (Automatic repeat request)	An error-control system in which a request for re-transmission is generated by the receiver when an error in transmission is detected.
ASCII (American Standard Code for Information Interchange ASCII)	A code that uses seven bits to represent standard text characters as well as a number of terminal control characters such as line feed, carriage return and so on.
ASK (Amplitude-shift keying )	A simple version of amplitude modulation used for digital modulation in which the two binary values (one and zero) are represented by two different amplitudes of the carrier frequency (normally, on and off).
Asynchronous	Asynchronous transmission occurs without significant prior coordination between the source and the sink. Each block of data is sent by the source, which receives warning of each block by a start bit and notification of the end of the block with a stop bit (also known therefore as start-stop protocol).
ATM (Asynchronous Transfer Mode).	Cell-switching and multiplexing technology designed to combine the benefits of circuit switching (constant transmission delay and guaranteed capacity) with those of packet switching (flexibility and efficiency for intermittent traffic) through the use of small, fixed-length cells (53 bytes). For wide area networking, Asynchronous Transfer Mode is currently being standardised for use in Broadband Integrated Services Digital Networks (BISDN) by the ITU (International Telecommunications Union) and ANSI(American National Standards Institute). Unlike X.25, Asynchronous Transfer Mode does not provide error control and flow control mechanisms.
Attenuation.	A decrease in magnitude ( a weakening ) of a current, voltage or power of a signal.
ATU (ADSL Tranceiver / Termination unit).	A device that provides ADSL modulation at the telephone line or simply an ADSL modem. The device at the server side is called ATU-C )(central office). The client devices is called AYTU-R (remote). The ATU-C/R are in essence the ADSL modems that sit on both sides of the telco copper loop.

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Backbone.	The part of a communications network that handles the major traffic, highest speed and often longest paths on a network. On the Internet, a backbone is a set of paths providing long distance or local connections for local networks.
Backdoor.	A port or channel that crackers use to access your system. As a rule, it might be easy for a skilled cracker to find a backdoor in a system that is insufficiently protected.
B channel.	An integrated services digital network (ISDN) 64 Kbps bearer channel for pulse-code modulated voice or data.
Balanced transmission	A transmission mode in which signals are transmitted down a transmission line where the current travels down one conductor and returns on the other.
BALUN ( Balanced-to-unbalanced converter )	Device used to convert between a balanced transmission line and an unbalanced line.
Band.	A range of frequencies. See bandwidth.
Band-limited signal.	A signal whose energy is totally contained within a finite frequency range (band).
Bandwidth (BW).	The total information flow over a given period of time, usually measured in Kbps (Kilobits/second or Mbps (Megabits / second). Otherwise understood as the difference between the minimum and the maximum frequencies that can be passed by a communications system.
Baseband	The transmission of signals without modulation. In a digital baseband network, binary values are transmitted directly onto the transmission line as voltage variations between two values.
Baud rate.	Measure of the number of symbols (characters) transmitted per unit of time.
BCD ( Binary coded decimal )	A coding scheme for the storage of data. The code was originally four-bit, latter extended to six-bit before becoming IBM's EBDIC.
BCH ( Bose-Chaudhuri-Hocquenghem code)	A powerful error-correcting code and important subclass of cyclic block codes.
BER (Bit error rate)	The number of bits received in error over a selected period of time. The bit error rate is normally expressed as a fraction.
BFSK ( Binary frequency-shift keying)	In binary frequency-shift keying (binary FSK or BFSK) the carrier is shifted between two frequencies.
Binary.	or "off.
Bis.	he postscript "bis is used to note the second version of an ITU standard (bis is French for two). For example, V.32bis is the second version of the V.32 standard.
BISDN ( Broadband integrated services digital network).	The network normally associated with the future telecommunications systems based on asynchronous transfer mode (ATM).
Bit.	Short for 'binary digit', a 'bit' takes one of two logical values: 1 or 0.
Bit rate.	The rate at which binary digits (bits) are transmitted. The bit rate is measured in bits per second (bps).
Bit stuffing.	The insertion of extra bits into a data stream to avoid the appearance of unintended control sequences that may interfere with a data frame.
Block code	An error detecting code where the data is formed into blocks and processed to produce new data, which is appended to the block and used by the receiver to assist in detecting and correcting errors.
Bluetooth.	A wireless personal area network (PAN) technology from the blue tooth special interest group (www.bluetooth.com),. Founded in 1998 by Ericsson, IBM, Intel, Nokia and Toshiba. Blue tooth is an open standard for short range transmission of digital voice and data between mobile devices (laptops, PDAs, phones and desktop devices). It supports point-to-point and point-to-multipoint applications. Bleu tooth provides up to 720Kbps data transfer within a range of 10 meters, and up to 100 metres with a power boost.
BNC connector (Bayonet nut coupling connector).	A bayonet-style connector for coaxial cable.
BPI (Baseline Privacy Interface)	The DOCSIS-required encryption standard used to protect users and their data. BPI uses a public/private key exchange system to encrypt data that is transmitted between the cable modem and the CMTS. Also: Bits Per Inch A measurement of the recording density of a disk or tape.
Bps ( Bits per second )	The number of bits transferred in one second. A measure of data transmission rate.
BPSK ( Binary phase shift keying )	Binary phase-shift keying (PSK) between two phase states, normally 180 degrees apart.
BRA ( Basic rate access )	An integrated services digital network (ISDN) connection consisting of two independent B channels (each 64 Kbps) and one D channel (16 Kbps). The basic rate access connection is given the notation (2B+D).
Bridge.	A data-link layer (See OSI, Layer 2) device that interconnects two local area networks (LANs). Bridges operate at the data-link layer and therefore can only interconnect local area networks of the same type. Bridges read the destination and source addresses in the packets or frames and forward them on to another local area network depending on the address.
Bridging router (Brouter ).	A term describing bridges that are able to connect two LANs using different data link protocols by providing a limited routing function.
Broadband	A term referring to a wide range of frequencies. The term is often used to describe speeds in excess of T1, or 1.544 Mbps, however, it is most commonly used to describe services that are faster than what was available before.
Broadcast.	To broadcast is to send the same message to a number (normally all) of the network terminals simultaneously.
Broadcast address.	A common address used to direct (broadcast) a message to all terminals in a network.
Broadcast message.	A message directed (broadcast) to all users on a network.
Browser.	A software application used to locate and display content from the Internet.
BSS (basic service Set).	Refers to a group of a wireless access points and group of wireless client computers.
Bus topology.	A bus is a common shared communications medium for transmitting data. A bus LAN topology is a shared transmission line to which terminals are connected.
Byte.	A group of 8 bits.

C band	A band of frequencies used for terrestrial radio relay and satellite transmissions.
CA (Collision avoidance)	In a carrier-sense multiple access (CSMA) media-access control protocol, a collision occurs when two stations attempt to transmit at the same time. Since the messages interfere with each other, the collision must be avoided or detected. In collision avoidance, each station listens to ensure that the shared channel is free. If so, a station that desires to transmit waits a fixed time plus a random time before transmitting.
Cable	A cable is one or more electrical conductors in an insulating sheath.
Cable modem	A cable modem is a high-speed modem that transmits and receives data over coaxial cables for TV distribution.
Cambridge ring	A token-passing LAN developed by the University of Cambridge.
Campus network	See metropolitan area network (MAN).
Capacitance	Capacitance is the property exhibited by a capacitor, or any wires or devices that are physically arranged to resemble a capacitor.
Capacitor	A capacitor is obtained by placing two conducting plates near to each other, separated by a non-conducting material called a dielectric.
Carrier	A carrier is a continuous frequency that is at the desired channel frequency. The carrier is modulated or impressed with a second (information carrying) signal to translate that information from baseband frequencies to occupy the desired channel.
Cache	A technique used for fast retrieval of regularly accessed data.
Cassegrain	A form of antenna which uses a parabolic reflector to focus incoming radiation onto a hyperbolic sub-reflector and thence into the waveguide. On the transmit path the opposite occurs.
CATV (Cable access television)	Cable access television systems have a common antenna or source from which signals are distributed to a number of users. Often also called closed circuit TV (CCTV).
CBR (Constant Bit Rate)	Constant bit rate is a service class for asynchronous transfer mode networks (ATM networks) in which the cell rate takes a constant, fixed value specified in the connection contract.
CCIR (Comite Consultatif International des Radiocommunications)	The French name for the International Radio Consultative Committee. Now called the ITU-R.
CCITT (Consultative Committee on International Telegraph and Telephony)	An international standards organization now called ITU-T.
CCS (Common channel signalling)	Common channel signalling is an ITU signalling system that had seven standards, although only CCS No 4, No 5, No 6 and No 7 are in common use today.
CCS#7 (Common Channel Signalling, Number 7)	An ITU common channel signalling standard between all digital exchanges with integrated services. Signalling is by packetised data over 64 Kbps channels.
CCTV (Closed-circuit Television )	A video system running on a proprietary or closed network.
CD (Collision detection)	In a carrier-sense multiple access (CSMA) media-access control protocol, a collision occurs when two stations attempt to transmit at the same time. Since the messages interfere with each other, the collision must be avoided or detected. In collision detection, each station to listen to the network after transmission. If another station has transmitted at the same time, both stations stop transmitting and wait a random period of time before trying to transmit again.
CDDI (Copper distributed data interface (CDDI )	Copper distributed data interface (CDDI) is a version of the fibre-distributed data interface (FDDI) LAN standard adapted for twisted-pair copper-wire LANs.
CDMA (Code-division multiple access (CDMA )	A multiple access technique in which each user is given access to the entire channel all the time, with users separated by giving each a unique spreading code.
Cell	In personal communications systems (cellular mobile phone systems) a cell is the geographic area served by a single base station. Cells are arranged so that base-station frequencies can be reused between cells. In asychronous transfer mode (ATM) networks, a cell is a packet of data.
Cell relay	A fast packet-switching mechanism, which uses fixed-sized packets called cells. ATM is based on cell-relay technology.
Cellular radio	A term for cellular telephony services provided by personal communications systems (PCS).
Centronix	A trade name that has become attached to a 36-pin parallel interface, which is an alternative to serial RS-232 links. The Centronix interface is commonly used to connect a computer to a printer.
CEPT (Conference of European Postal and Telecommunications)	A 27-country European telecommunications standardisation committee.
CEPT-1	A CEPT-1 channel is a 2 Mbps TDM channel. Also called E1.
CEPT-4	A CEPT-4 channel is a 140 Mbps TDM channel. Also known as E4
Channel	Strictly, a channel is a single direction path (or simplex path) along which a signal can be sent. However, in common usage the term is synonymous with circuit.
Channel rate	The channel rate is the data rate on a channel measured in bits per second.
Channel spacing	The distance in frequency between channels.
Check bit	A bit attached to a block of data resulting from some form of block coding such as one of the common parity schemes in use.
Checksum	A value calculated by applying a specific algorithm to a file or datagram. The algorithm is design so that even a single bit change in the source file/ data causes a completely different check sum value. Check sums are of fixed length (32 bits for CRC32, 128 bits for MD5, etc.) regardless of the size of the file or datagram they are applied to. See CRC.
Cipher text	The output of an encryption algorithm, which is the encrypted version of a message or data.
CIR (carrier-to -interference ratio)	The ratio of the power in the carrier to the power of the interference signal. The ratio is normally expressed in dB.
Circuit	The medium connecting two communicating devices, or a path between a transmitting and receiving device (there may be one or more intermediary nodes).
Circuit switching	It is very inefficient for a circuit to be established between any two users who require to converse or transfer information. Circuit switching is a much more efficient use of resources in which each user is connected to a central node or switch that provides the capability of interconnecting any two users for the period of the call. The telephone system uses circuit switching.
CLEC (Competitive Local Exchange Carrie )	An American term for an organisation other than a traditional telephone company offering local telephone services.
CMTS (cable modem termination system)	A system of devices located at the head-end (transmission source) that exchanges digital signals with cable modems on cable network.
CNR (Carrier-to-noise Ratio)	The ratio of the carrier power to the noise power. It is a very useful measure in communications because it is not sufficient to simply have a strong signal, since a weak signal in the presence of weak noise may well be as useful as a strong signal in the presence of strong noise.
Coaxial cable (coax)	A transmission medium normally consisting of one central conductor enclosed within a conductive sheath, separated by a dielectric. Coax has a much wider bandwidth than twisted pair cable and can therefore carry high data rates.
Codec (Coder-decoder)	A device used to transform analogue data into a digital bit stream (coder), and digital signals into analogue data (decoder).
Collision	In a carrier-sense multiple access (CSMA) media-access control protocol, a collision occurs when two stations attempt to transmit at the same time. Since the messages interfere with each other, the collision must be avoided or detected.
Companding	Many analogue signals are not best digitised using uniform quantisation steps. For example, in speech small signal amplitudes are more common that large ones. For these signals the quantisation noise can be reduced by using smaller steps for lower amplitudes and larger steps for higher amplitudes. In effect, this compresses the signal range before transmission and expands it at the receiver. This compressing and expanding results in the term companding.
Compression	The process by which redundancy is removed from data before transmission or storage. In lossless compression the original data can be reconstructed precisely. In lossy compression some information is discarded so that the reconstructed data is not identical to the original.
Conditioning	Changes made to a signal before transmission to prepare it for the range of distortions that occur in the transmission channel.
Conductance	When a voltage potential is placed across a wire, the number of electrons that will flow will depend on the degree of freedom of electrons in the conducting wire. The ability of the wire to conduct is called conductance, but this property of the wire is normally considered in terms of how it restricts the flow of electrons, that is its resistance. Resistance is measured in ohms (W).
Congestion control	One of a range of techniques implemented within a network to ensure that the network does not become overloaded at any particular point.
Connection-oriented	Describes a protocol in which a logical connection is established between the transmitting and receiving devices before transmission occurs.
Connectionless	Refers to the exchange of data without prior coordination between the transmitting and receiving devices.
Contention	The result of two or more devices attempting to access the same resource at the same time.
Convolutional code	A form of channel coding that adds patterns of redundancy to the data in order to improve the signal to noise ratio for more accurate decoding at the receiving end. The Viterbi algorithm is used to decode a particular type of convolutional code. See Viterbi decoder.
CRC (Cyclic redundancy check)	An efficient error detection algorithm that uses a polynomial function to generate the block check characters.
Crossover	and "receive pairs are crossed so it can be used to connect similar network devices such as PCs without a hub/switch between them.
Crosstalk	The undesired effect that a signal transmitted on one circuit or channel of a transmission system creates in another circuit or channel.
CSMA/CD (Carrier-sense multiple access/collision detection)	A refinement of carrier-sense multiple access (CSMA) in which a station ceases transmission if it detects a collision using a collision detect (CD) algorithm. It is the most common of the access strategies for bus architectures.
Current	The average flow of charge (electrons) and is measured in amps.
Cyclic code	An important sub-class of linear block codes for error detection, where a new codeword in the code can be formed by shifting the elements along one place and taking one off the end and putting it on to the beginning. Instead of being generated by a matrix, a cyclic code is generated by a polynomial so that the codes are sometimes called polynomial codes. Importantly, cyclic codes have a structure that makes it possible for the encoding and decoding to be performed by simple feedback circuitry.

D channel	An integrated services digital network (ISDN) 16 or 64 Kbps packet-switched channel that carries signalling information to control circuit-switched calls (B channels). The D channel can also be used for packet switching or low-speed telemetry when no signalling information is being used.
DAC (Digital-to-analogue conversion)	The opposite of analogue-to-digital conversion (ADC) in which the digital signal is converted to an analogue form.
DAMA (Demand-assigned multiple access)	One of the two main techniques for allocating channels to users. In fixed-assigned multiple access (FAMA), each user is allocated a channel permanently, whether they use it or not. This is inefficient and most multiple-access systems use DAMA in which the available channels are allocated on an as-required basis to users. When the user is finished with the channel it is relinquished and made available for another user.
Dark fibre	Fibre that has been laid in anticipation of future requirements, but is currently unused.
Data link layer	(see OSI, layer 7)
Datagram	In packet switching a datagram is a small packet of data that carries sufficient information to allow it to be routed from the originating data terminal equipment to the destination data terminal equipment without having to establish a connection between the data terminal equipments across the network.
DB (Decibel)	A measure of the relative strength of two signals. The number of decibels is 10 times the log of the ratio of the power of two signals, or 20 times the log of the ratio of the voltage of two signals.
DC (direct current )	When the voltage is supplied by a constant source such as a battery, the current supplied is constant and is called direct current (DC).
DCE (Data circuit-terminating equipment)	The DCE provides the signal conversion and coding between the data terminal equipment (DTE) and the line. The DCE may be separate, integral to the DTE, or part of an intermediate equipment.
De-emphasis	See emphasis / de-emphasis.
Decryption	The reverse of encryption in which encrypted data (called cipher text) is translated into the original data (called plain text). Also called deciphering.
DECT (Digital European Cordless Telecommunications)	A standard for cordless telephony which provides voice and data services to slow-moving mobile users.
Degradation	Decline in service or quality.
Delay distortion	Distortion of a signal that arises when the propagation delay for the transmission medium is not constant over the frequency range of the signal. All frequencies do not therefore arrive together, resulting in delay distortion.
Delta modulation	An incremental pulse-code modulation (PCM) system where, rather than transmitting the absolute amplitude at each sampling, only the changes in signal amplitude from sampling instant to sampling instant are transmitted.
Demodulation	The opposite of modulation in which the original baseband signal is recovered from the modulated carrier.
Demultiplexer	Performs opposite function to a multiplexer in which a multiplexed channel is broken out into its constituent individual channels.
DES (Data Encryption Standard)	DEC is an algorithm that encrypts plain text data into an encoded form, called cipher text. - Deviation. In an FM signal, deviation is the extent by which the carrier frequency is shifted from its resting frequency.
DHCP (Dynamic Host Configuration Protocol)	Software that automatically assigns IP addresses to client stations logging onto a TCP/IP network.
Digital signal	A discrete or discontinuous signal.
Digitise	The conversion of an analogue signal into a digital signal.
Dipole	A simple antenna structure where the two lines of a transmission line are opened up at the end to form an antenna with two arms in a T shape, where each arm of the T is one-quarter of a wavelength long.
Dispersion	The velocity of propagation of an electromagnetic wave through a medium is frequency-dependent. Lower frequencies travel more slowly than high frequencies meaning that high frequencies arrive first and cause distortion of the received signal, known as dispersion. Ideally, the propagating medium and the channel bandwidth should be chosen so that the velocity of propagation of the highest and lowest frequencies is similar.
DLC (Digital Loop Carrier)	Technology that uses digital techniques to bring a wide range of services from a carrier to uses via twisted pair copper phone lines.
DMZ (demilitarised zone)	A computer/device on an internal protected network that is placed outside the firewall but is still available from the LAN. The advantage of a DMZ computer is that it has all communications ports open and can use any special application that communicates on non-standard ports, such as many VoIP programs and online games. The disadvantage is that the device might be vulnerable to attacks, not being protected by the firewall.
DNS	Domain Name System.
Doppler effect	The change in frequency of an electromagnetic signal that results from the relative speed of the source and the sink.
Downlink	The signal from the satellite transponder to a satellite ground station.
DPSK (Differential phase-shift keying)	A modulation technique that uses phase modulation. DPSK changes phase each time a 1 bit is transmitted and does not change phase when a 0 bit is transmitted.
DQDB (Distributed queue dual bus)	An IEEE 802.6 standard for MAN using a twin optic-fibre loop extending up to 50 km at throughputs of 140 Mbps.
DSB (Double sideband)	The power consumed in an AM carrier transmits no information and is effectively wasted. It would make sense, therefore, to transmit only the side-frequencies, which do contain the information. When both sidebands of an AM modulated carrier are transmitted without the carrier, the transmission is called double-sideband suppressed-carrier (DSBSC) or, more commonly, DSB.
DSL (Digital Subscriber Line)	A communications technology that increases the digital capacity of ordinary telephone lines (the local loops) into the home or office by several orders of magnitude depending on distance between the customer and the telco central office. DSL is geared to two types of usage. Asymmetric DSL (ADSL) is for Internet access, where fast downstream is required, but slow upstream is acceptable. Symmetric DSL (SDSL, HDSL, etc.) is designed for short haul connections that require high speed in both directions. Unlike ISDN, which is also digital but travels through the switched telephone network, DSL provides "always-on operation. At the telco central office, DSL traffic is aggregated in a unit called the DSL Access Multiplexor (DSLAM) and forwarded to the appropriate ISP or data network. Although DSL only arrived in the very late 1990s, there have been more versions than most other new transmission technologies. The major DSL flavors are summarized below. See PPPoA.
* ADSL (Asymmetric DSL).	Uses the same line the telephone because it uses higher frequencies than the voice band. However, a POTS splitter must be installed on the customer's premises to separate the line between voice and ADSL. A version of ADSL, known as G.lite, Universal ADSL, ADSL Lite and splitterless ADSL, is geared to the consumer. It eliminates the splitter and associated installation charge, but all phones on the line must plug into low-pass filters to isolate them from the higher ADSL frequencies. ADSL is available in two modulation schemes: Discrete Multitone (DMT) or Carrierless Amplitude Phase (CAP). See CDSL, G.shdsl, ATU-C and ATU-R.
* HDSL (High Bit Rate DSL).	A symmetric technology, which provides the same transmission rate in both directions. HDSL is the most mature DSL, because it has been used to provide T1 transmission over existing twisted pair without the additional provisioning typically required for setting up T1 circuits such as the removal of bridged taps and the installation of repeaters. HDSL requires two cable pairs and goes up to 12,000 feet, while HDSL-2 requires only one cable pair and supports distances up to 18,000 feet. HDSL does not allow line sharing with analogue phones.
* IDSL (ISDN DSL).	ffers relatively low speed (144 Kbps) in both directions compared to other symmetric versions, but it does provide 16 Kbps more than standard ISDN, because the 16 Kbps "D channel is used for data rather than call setup. It also offers the longest distance of 26,000 feet. Unlike standard ISDN, IDSL does not support analog phones, and signals are not switched through the telephone network. Since IDSL uses the same 2B1Q line coding as ISDN, ISDN customers can use their existing equipment (ISDN BRI terminal adapters and routers) when connecting to IDSL.
* RADSL (Rate Adaptive DSL).	A version of ADSL that adjusts speeds based on signal quality. Many ADSL technologies are actually RADSL.
* SHDSL (Symetric DSL).	Otherwise known as G.991.2, G.SHDSL is an international, ITU standard for symmetric DSL. G.SHDSL allows high-speed symmetrical data streams to be sent and received over a ingle pair of copper wires at rates between 192 kbps and 2.31 Mbps. The first DSL technology to be developed from the ground up as an international standard, G.SHDSL will transport T1,E1, ISDN, ATM and IP signals.
* VDSL/ VHDSL (Very High Bit Rate DSL).	A asymmetric version of DSL that is used as the final drop from a fiber optic junction point to nearby customers. VDSL lets an apartment or office complex obtain high-bandwidth services using existing copper wires without having to replace the infrastructure with optical fiber. Like ADSL, VDSL can share the line with the telephone.
* VoDSL (Voice Over DSL).	Sending voice over a DSL line. Using compression, a large number of voice channels can be placed on DSL channels, which makes the technology very attractive. For example, up to 150 voice channels can be transmitted over a 1.5 Mbps DSL line. DSL signals at the customer side are delivered into an integrated access device (IAD), which forwards them over twisted pair to the carrier. The signals go to the carrier's DSLAM and then to an access switch that forwards voice to a voice gateway and then the PSTN and data to the appropriate data network.
DSSS (Direct Sequence Spread Spectrum)	See Spread spectrum.
DTE (Data terminal equipment)	Used to convert user information into data signals for transmission, and then converts the received data signals into user information.
DTMF (Dual-tone multiple frequency)	The touch-tone dialling system where a dual-tone signal is sent to indicate the desired number dialled from a telephone.
Dumb terminal	An input/output terminal that does not perform any local processing.
Duplex transmission	A data transmission mode in which a transmission path can carry information in both directions simultaneously.
Dynamic range	The difference between the maximum signal level in an analogue communications system and the lowest signal level (normally determined by the noise present).

EAP (Extensible Authentication Protocol)	An extension to the PPP protocol that enables a variety of authentication protocols to be used. EAP is not tightly bound to the security method. It passes through the exchange of authentication messages, allowing the authentication software stored in a server to interact with its counterpart in the client. EAP is also used in 802.11 wireless LANs.
EBCDIC (Extended Binary-coded Decimal Interchange Code)	An eight-bit code developed by IBM to represent 256 characters and numbers of information.
Echo	The reflection of a transmitted signal, normally from the circuit termination at the receiving end.
Echo cancellation	Modern long-distance telephone circuits employ echo cancellation to prevent the energy in a speaker's voice being reflected from the receiving end and returning to the speaker to be heard as an echo of their own voice. Echo cancellation must be turned off to allow modems to communicate.
Echo suppressor	A device that allows a transmitted signal to pass in one direction only as a method of minimising the effect of echo.
EDC (Error detecting code)	A code by which data can be encoded so that the receiver can detect the presence of the errors.
Edge (Enhanced Data rates for Global Evolution)	An enhancement to the GSM and TDMA wireless communications systems that increases data throughput to 384 Kbps. See UWC-136, GSM and TDMA.
EHF (Extremely high frequency)	A radio-frequency (RF) frequency in the band 3-30 GHz. The super-high frequency (SHF) and extremely-high-frequency bands are known as the microwave bands and their wavelengths are short enough to be propagated by highly directional antennas and waveguides. Propagation ranges are limited to line-of-sight but long-distance communication can be achieved by employing a series of radio relay stations. These portions of the spectrum are used for television and high-speed data services requiring large bandwidths.
EIRP (Effective isotropic radiated power)	The equivalent power of a transmitted signal in terms of an isotropic (omnidirectional) radiator. Normally the effective isotropic radiated power equals the product of the transmitter power and the antenna gain (reduced by any coupling losses between the transmitter and antenna.)
ELF (Extremely low frequency)	The radio-frequency (RF) band between 30-300 Hz. The ELF band is not of significant use in communications due to the extremely small bandwidth available and the enormous antennas required.
EMI (Electromagnetic interference)	The interference caused by the presence of electromagnetic radiation.
Encryption	A process in which transmitted data (plain text) is scrambled (encrypted) into cipher text at the transmitter and reconstructed into readable data (plain text) at the receiver.
Equalisation	One of a range of techniques employed to compensate for distortion introduced in a channel.
Error detection and correction	A critical aspect in data transmission is the determination of whether the received data is error-free. To do this some form of error protection, or error detection and correction is utilised. The numbers of errors at the output of the receiver can be reduced by adding additional information to the message to increase the receiver's decision-making ability. This additional, redundant information increases the overhead that is included in the transmission.
Error rate	The number of data bits received in error to the total number of data bits transmitted.
Ethernet	or "network card generally implies Ethernet by default. All Macs and many PCs come with 10/100 Ethernet ports for home use, not just to create a small home network, but to connect to the Internet via a DSL or cable modem, which requires it. A 10/100 port means that it supports both 10BaseT at 10 megabits per second (Mbps) and 100BaseT at 100 Mbps. Ethernet was invented by Bob Metcalfe and David Boggs at Xerox PARC in 1973, which first ran at 2.94 Mbps. Metcalfe later joined Digital where he facilitated a joint venture between Digital, Intel and Xerox to collaborate further on Ethernet. Version 1 was finalized in 1980.

F (Farad)	The Farad (F) is the SI unit of capacitance.
Fade.	A slow change in signal strength.
FAMA (Fixed-assigned multiple access).	One of the two main techniques for assigning channels to users. In FAMA, each user is allocated a channel permanently, whether they use it or not. This is inefficient and most multiple-access systems now use demand-assigned multiple access (DAMA) in which the channels are allocated as-required.
FBR (Fixed bit rate)	Another term for constant bit rate (CBR) in asynchronous transfer mode (ATM).
FCS (Frame check sequence)	The product of an error-detecting code, normally inserted as the final field in a block of transmitted data that can be used upon reception of the data to detect errors.
FDD (Frequency division duplex)	A technique in which one frequency band is used to transmit and another used to receive.
FDDI (Fibre-distributed data interface)	An ANSI (American National Standards Institute ) standard for fibre optic LANs employing token passing on a ring topology and providing speeds of 100 Mbps.
FDMA (Frequency-division multiple access)	Divides a channel amongst users who are each given their portion of the available channel bandwidth for their permanent use.
FDM (Frequency-division multiplexing)	A technique that divides the available bandwidth of a channel into sub-channels, each of which is assigned to one device. Frequency-division multiplexing is directly applicable to analogue sources and essentially involves forming a composite signal by 'stacking' several information channels side-by-side in frequency. This composite frequency-multiplexed signal is then used to modulate a carrier in some conventional manner. Recovery of the individual messages after reception and initial demodulation is accomplished by bandpass filtering and frequency selection of the channels.
FEC (Forward Error Correction)	A technique for allowing a receiver to correct errors itself, without reference to the transmitter. It does this by using additional information transmitted along with the data and employing one of the error detection techniques. The receiver can correct a small number of the errors that have been detected. If the receiver cannot correct all detected errors, the data must be re-transmitted.
FH (Frequency hopping)	Using frequency hopping, data is transmitted at one frequency, the transmitter changes frequency and data is transmitted at the new frequency. Employed as a multiple access technique as well a mechanism for low probability of intercept and resistance to jamming.
Fibre optic cable.	A cable made by enclosing a thin glass fibre core in a glass cladding and surrounding the result in a protective jacket. Electrical signals are translated into light pulses by modulating a laser, and are detected at the receiver by photo-electric diodes. A waveguide for the optical frequencies is provided by the difference between the refractive indices of the cladding and the glass core.
FM (Frequency modulation)	A form of modulation in which the modulating signal is used to vary the frequency of a carrier wave. Frequency modulation is a form of angle modulation.
Frame	A formatted packet of data (bits), usually associated with the data-link layer.
Frame Relay	A form of packet switching based on the use of variable-length, data-link-layer frames. Many of the basic functions have been streamlined or eliminated to provide for greater throughput, and there is no network layer.
Frequency	The rate of signal variation (normally sinusoidal oscillation) in cycles per second. Frequency has the units of hertz (Hz).
Frequency-shift keying (FSK)	The simplest (binary) form of frequency modulation (FM), which is used for digital modulation in which the two binary values are represented by two different frequencies near the carrier frequency. Normally the carrier is shifted low for a 1 (called a mark in old telegraph terms) and high for a 0 (space).
Full-duplex transmission	See duplex transmission.

Gain	The ratio of the input power to the output power of a circuit or device. Gain is normally measured in dB.
Gateway	An application layer (see OSI Layer 7) device that interconnects two local area networks (LANs). Gateways operate at the application layer and therefore can interconnect local area networks of the different types, even those using different network protocols. Because they operate at the application layer, gateways can understand data formats and perform the functions of a firewall.The gateway runs two protocol stacks, corresponding to the two stacks in the two disparate networks being connected.
Gaussian distribution	Normal distribution.
GBPS (GigaBits Per Second)	A billion bits per second.
GEO (Geostationary earth orbit)	A type of a geosynchronous orbit coordinated so that the satellite's period of rotation equals the Earth's period of rotation, and the satellite is always positioned over the same location above the Equator.
Geosynchronous orbit	A satellite orbit in which the period of rotation of the satellite is a multiple of the Earth's period of rotation. When the satellite's period of rotation equals the Earth's period of rotation, the satellite is always positioned over the same location and is called geostationary.
Go-back-N automatic repeat request (Go-back-N ARQ)	An error correction technique in which the transmitter sends blocks whose size is determined by a window size. Not every block requires acknowledgement and a NACK from the receiver has the transmitter go back a predetermined number of blocks and begin to retransmit those blocks.
GPRS (General Packet Radio Service)	A 2.5G add-on to GSM and other second generation cellular communications systems that provides an always-on packet data capability.
GSM (Global System for Mobile Communication – GSM).	A European-based digital cellular communication based on time-division multiple access (TDMA).
Guardband	Used to separate sub-channels in frequency-division multiplexing (FDM) to avoid crosstalk.

H (Henry)	The SI unit of inductance.
Half-duplex transmission	A data transmission mode in which a transmission path can carry information in both directions but in only one direction at a time.
Hamming distance	The number of digits that are different between two numbers. The Hamming distance between the two binary numbers 110010 and 110100 is 2, since two of the binary digits are different.
Harmonic	An integer multiple of a reference (fundamental) frequency.
Header	Carries control information that precedes user data in a data frame or packet.
HeartBeat	Telstra Cable uses an authentication system nicknamed “the heartbeat” which is similar in concept to a "ping. Every five minutes or so, a message is sent to your computer to see if you're still there. Several applications like firewalls, Win98/ME ICS and VPNs block this message, causing the connection to drop out regularly. While there are plans to ditch it, it was originally used provide people with multiple, independent connections using the one modem between several computers and maintain independent usage statistics for each one.
HEO (Highly elliptical orbit)	An elliptical satellite orbit that has a closest point to the earth is much closer than the farthest point away from the earth.
Heterodyne	Describes the process of 'beating' two frequencies together to generate a frequency that is the difference between them.
HF (High frequency)	The radio-frequency (RF) band the range 3-30 MHz. The HF band provides relatively reliable propagation over long distances. Reasonable efficient antennas can be built requiring low radiated power.
Hub	The centre point for the wiring of a star-wired network.
Hz (Hertz)	The SI unit of frequency; one hertz is one cycle per second.

IANA (Internet Assigned Numbers Authority)	The Internet body that was responsible for managing Internet addresses, domain names and protocol parameters. It was later superseded by ICANN (Internet Corporation for Assigned Names and Numbers), which was formed in 1998.
ICANN (Internet Corporation for Assigned Names and Numbers)	A non-profit, international association founded in 1998 and incorporated in the U.S. It is the successor to IANA (Internet Assigned Numbers Authority), which manages Internet addresses, domain names and the huge number of parameters associated with Internet protocols (port numbers, router protocols, multicast addresses, etc.). ICANN provides a list of accredited registrars in addition to Network Solutions that accept domain registrations.
ICO (Intermediate circular orbit)	A circular satellite orbit of heights between 10,000 km and 15,000 km above the earth. Also called medium earth orbit (MEO).
IEEE Institute of Electrical and Electronic Engineers)	The Institute of Electrical and Electronic Engineers is a US-based professional society for electrical and electronic engineers.
IEEE 802.3	The IEEE for carrier-sense multiple access/collision detection ( CSMA/CD) networks.
IEEE 802.4	The IEEE for token bus networks.
IEEE 802.5	The IEEE 802.5 standard for token-ring networks.
IETF (Internet engineering task force)	An open international technical committee that standardises most Internet communications protocols.
IF (Intermediate frequency)	The basis of a superheterodyne radio receiver is the conversion of the wanted signal frequency into a constant frequency known as the intermediate frequency (IF). It is at this intermediate frequency that most of the gain and the selectivity of the receiver is provided without affecting the bandwidth of the receiver. Impulse noise. Impulse noise is noise that is caused by a high-amplitude, short-duration noise pulse.
Inductance	The property exhibited by an inductor, or any wires or devices that are physically arranged to resemble an inductor.
Inductor	A current passing through a wire will produce a magnetic field around the wire. Winding the conductor into a coil strengthens the magnetic field. The resulting element is called an inductor, whose inductance (L) is measured in the unit henry (H). The voltage across an inductor is directly proportional to the time rate of change of the current through it. An inductor therefore acts as a short circuit to DC.
Internet	The Internet is a specific collection of interconnected networks spanning the world and is the most used medium of electronic communications between people and business.
Internetwork	A collection of local area networks ( LANs) normally connected together with routers.
Internetworking	The interconnection of local area networks (LANs), normally through the use of routers.
Interoperability	The ability of one piece of communications equipment to exchange information with another.
Inverse multiplexer	A multiplexer that provides a high-speed circuit between two devices by aggregating multiple lower-speed circuits.
IP (Internet protocol)	The internetworking protocol that underlies the Internet, providing connectionless service across multiple packet-switching networks. Part of the TCP/IP protocol suite.
IP V.4 (Internet Protocol version 4)	Is still the most commonly used IP protocol. Initially deployed in 1983, IP V 4 addresses are 32 bit number often expresses as four octets in dotted decimal notation ( for example 123.01.456.456).
IP V.6 (Internet Protocol Version 6)	The next generation IP protocol. Started in 1991, the specification was completed in 1997 by the IETF. It is backward compatible with and designed to fix the shortcomings of V4 , providing better security and virtually unlimited room for networks and systems as it increases the address space from 32 to 128 bits. IP V6 is expected to slowly take over from IP V 4 with the two expected the run side-by-side for some years.
IR (Infrared)	The electromagnetic radiation of wavelengths between visible light and radio waves (in the approximate range of 770 nm to 1 mm).
ISB (Independent sideband)	The number of single-sideband (SSB) channels that can be transmitted over a given communication system is determined by the minimum frequency separation of the channels.
IRDP (ICMP router discovery protocol – RFC 1256)	Using router discovery, clients dynamically discover routers and can switch to backup routers if a network failure or administrative change is required.
ISDN (Integrated Services Digital Network)	A network which provides for the integration of voice and data (as well as other formats such as video and graphics images) over a digital transmission network.
ISO (International Standards Organization)	An international standards organisation, covering a range of areas including telecommunications.
ISO	OSI reference model. See Open Systems Interconnection (OSI) reference model.
Isotropic	Radiating equally in all directions.
ISP (Internet Service Provider)	A generic term to describe an organisation that provides access to the Internet.
ITU (International Telecommunications Union)	An international telecommunications standards organisation. ITU-R: ITU-Radio. ITU-T: ITU-Telecommunications.

Jabber	An open source instant messaging system based on XML with client software available for Windows, Mac, Linux and Palm.
Java	When it runs as a stand-alone, non Web-based program on a user's machine, it is simply a "Java application.
JavaScript	A scripting language that uses a similar syntax to Java, but it is not compiled into bytecode. It remains in source code embedded within an HTML document and must be translated a line at a time into machine code by the JavaScript interpreter. JavaScript is very popular and is supported by all Web browsers. JavaScript has a more limited scope than Java and primarily deals with the elements on the Web page itself.

Kbps (KiloBits Per Second)	One thousand bits per second. Kbps is used as a rating of relatively slow transmission speed compared to the common Mbps or Gbps ratings. Upper case "B" in KBps means kilobytes per second, but "b" for bit and "B" for byte are not always followed and often misprinted. KBps or KB/s would be used for earlier disk and tape transfer ratings as data are transferred in parallel, not serial.
KVM (Keyboard, Video Mouse) switch	A hardware device that allows a single keyboard, video (monitor) and mouse to be used with a number of computers, when a single user interacts with all of the computers (one at a time).

L2TP (Layer 2 Tunnelling Protocol)	A protocol from the IETF that allows a PPP session to run over the Internet or an ATM or Frame Relay network. L2TP defaults to using IPSec encryption in order to provide security for a VPN from the remote user to a corporate LAN.
LAN (Local area network)	A communication network that interconnects communicating devices within a small area.
Latency	Delay, normally in propagation time across a network.
Layer	Protocols are normally structured using layers to reduce complexity. Dividing communication software into multiple protocol layers also increases flexibility because it allows subsets of protocols to be used as needed. See OSI.
Leased line	A private communications channel leased from a common carrier.
LEO (Low earth orbit) satellite.	A satellite that at operates at heights of between 500 and 2,000 km above the earth's surface.
Low frequency (LF)	The radio-frequency (RF) band in the range 30-300 kHz. LF suffers from the same difficulties associated with ELF: small bandwidths are available; and antennas are large and inefficient, leading to low radiated powers.
Link	The circuit established directly between two nodes.
Linux	Developed in 1990 by Finnish computer science student Linus Torvalds, Linux is a version of Unix that runs on a variety of hardware platforms including x86PCs, Alpha, PowerPC and IBM's product line. Linux is open source software meaning that its core code is freely available to software developers and technology companies on condition that the enhancements made to the code are made publicly available also. The program has earned a reputation for strong application and network stability in the corporate environment.
LLC (Logical Link Control)	A sub-layer of the data link layer of the OSI reference model, which forms the interface between the network layer and the media access control protocols.
Local loop	A transmission path (generally twisted pair cable) between an individual subscriber (normally a domestic subscriber) and the nearest switching centre of a public telecommunications network.
LSB (Lower sideband)	When only one sideband is transmitted, the transmission is called single-sideband suppressed carrier (SSBSC) or more commonly SSB. When the upper sideband is transmitted the transmission is called upper side band (USB) and when the lower sideband is transmitted it is called LSB.

MAC (Media access control protocol)	A sub-layer of the data link of the OSI reference model. The media access control sub-layer defines how a transmitting station gains access to the network media. Common MAC protocols are carrier-sense multiple access/collision detection (CSMA/CD) and token-passing.
M-ary frequency-shift keying (M-ary FSK)	In binary frequency-shift keying (binary FSK or BFSK) carrier is shifted between two frequencies. A convenient way of trading bandwidth for signalling speed is to increase the spread between the lowest and highest frequency (increasing the bandwidth) but to increase the signalling speed by extending the number of frequencies to a number M. Such a system is called an M-ary frequency-shift keying system.
Man-machine interface (MMI)	The interface presented by a machine to a human operator.
Manchester encoding	A digital line-coding technique in which a transition is guaranteed in the middle of each bit period, which assists the receiver in retaining synchronisation.
Mbps (Megabits per second - not to be confused with MBps)	Stands for millions of bits, or mega-bits per second and is a measure of bandwidth.
Media / medium	The physical carrier of data signals. Common LAN media include twisted-pair wires, coaxial cables, and fibre optic cables, although RF and infra-red frequencies are also used.
MEO (Medium earth orbit)	A circular satellite orbit of heights between 10,000 km and 15,000 km above the earth. Also called intermediate circular orbit (ICO).
Mesh topology	A network topology in which each communicating node is directly connected to all other nodes.
Message switching	A message is a discrete data communication. Message switching is a store-and-forward concept where a message with an appropriate destination address is sent into the network and is stored at each intermediate switching point (network node) where its integrity is checked before it is sent onto the next stage of its journey.
MF (Medium frequency)	A radio-frequency (RF) band in the range 300-3,000 kHz. The lower portion of the MF band is useful for communications systems requiring reasonably stable transmission over moderately long distances.
MIB (Management Information Base)	A database containing ongoing information and statistics on each device in a network. Is it used with SNMP to keep track of each device's performance and verify whether it is functioning properly.
Microwave	Electromagnetic waves in the frequency range of about 2-40 GHz. - Millimetric wave. Electromagnetic waves in the frequency range 30-300 GHz.
MLPPP (MultiLink PPP)	An extension to the point-to-point protocol (PPP) allowing for two channels to be linked together so as to double the throughput. It is used for ISDN transmission and channel bonding. See PPP and ISDN.
MNP (Microcom network protocols)	A set of commonly used proprietary modem protocols supporting data compression and error checking.
Modem	Short for modulator/demodulator, or device which transforms a digital bit stream into an analogue signal (in the modulator), and vice versa (in the demodulator) so that the digital signal can be passed over an analogue channel.
Modulation	The shifting or translation of a signal from one frequency band to another is accomplished by the process of modulation. In this process, the baseband signal is impressed in some way on a carrier signal at a higher frequency such that one or more of the characteristics of the carrier signal are altered. In this manner, baseband frequencies are translated in frequency up to the carrier frequency. The three characteristics of the carrier signal that can be altered are the amplitude, the frequency and the phase. This chapter describes the three corresponding CW modulation methods: amplitude modulation (AM), frequency modulation (FM) and phase modulation (PM).
Monopole	A simple antenna structure where the two lines of a transmission line are opened up at the end to form an antenna with two arms in a T shape and one of the arms of the T is removed, where the arm of the T is one-quarter of a wavelength long.
MPEG (Moving Pictures Expert Group)	A committee formed by the ISO to set standards for digital compression of full-motion video.
MPLS (MultiProtocol Label Switching)	An IETF standard for routing packets over the Internet. MPLS uses labels, or tags with forwarding information, which are the attached to IP packets by a router that sits at the edge of the network usually called a label edge router (LER). The LERs perform the complex packet analysis and classification, but do it only once before the packet enters the core of the network. The routers within the core, known as label switch routers (LSRs), examine the label quickly and forward the packet without having to make any forwarding decisions. The receiving LER removes the label.
MRU (Maximum Receive Unit)	Indicates to the peer (receiver) of the PPP connection, the maximum size of the PPP information field that the device can receive.
MSAU (Multistation access unit)	A network hub in Token Ring local area networks (LANs).
MSS (Maximum Segment Size)	The largest size of datat that TCP will send in a single IP packet.
Multicast address	An address that designates a group of entities within a domain.
Multipath	Propagation that arrives at a receiver having travelled over different paths, and consequently arrives at slightly different times, with potentially slightly different phases.
Multiple access	The ability of nodes to remotely access a transmission medium such as a satellite transponder.
Multiplexer (mux)	A hardware device that performs multiplexing.
Multiplexing.	A communication system needs to convey multiple messages simultaneously from a number of information sources in one location to a number of users at another. Multiplexing schemes economically meet this need by combining the messages from several information sources to be transmitted as a complete group over a single transmission facility with provision at the receiver(s) for separation (demultiplexing) back to the individual messages. There are two generic forms of multiplexing: frequency-division multiplexing (FDM) and time-division multiplexing (TDM).
Multipoint connection	A connection in which more than two terminals share one communications link.

NACK	Negative acknowledgement
NAP (Network Access Point)	A major public network exchange facility for interconnecting traffic between ISPs.
NAPT (Network Address Port Translation).	Otherwise referred to as Port Mapping, NAPT refers to an extension to NAT in which multiple client computers communicate through the same IP and also have unique port numbers assigned to transmission in order to distinguish between them.
Narrowband	The term narrowband is used to indicate a small amount of bandwidth. Often used to refer to baseband, and particularly voice, frequencies.
NAT (Network Address Translation)	An IETF standard that enables LANs to use one set of private IP addresses for internal traffic and another set of IP addresses for external traffic. Typically a NAT device makes all IP address translations where the LAN meets the WAN. NAT permits a larger number of LAN users to share one external IP address and add some network security since private IP address ranges are not routable outside the LAN.
NAS (Network Attached Storage)	A hard disk storage unit set up with its own IP address and attached directly to the LAN rather than being attached to a computer, for serving applications on workstations.
NetWare	A local area network (LAN) operating system (NOS) software provided by Novell.
Network	The interconnection of two or more computers by a communications medium (line, coaxial cable, fibre, IR or RF), together with appropriate hardware and software.
Network layer	( see OSI, layer 3)
Network manager	Individual responsible for administering, configuring, planning, monitoring, and establishing standards and procedures for a network.
NIC (Network interface Card)	NICs provide the interface between the network device (say the PC) and the network. Different cards are required depending on the media access technique (such as Ethernet or Token Ring) and the type of network device, as well as the local area network (LAN) physical medium (coaxial cable, twisted pair or fibre optic cable).
Node	A processing point (normally a switching or routing point) in a network.
Noise	Random and unpredictable electrical signals produced by natural processes both internal and external to the communications system. These unwanted signals combine with and distort the wanted signal.
NOS (Network operating system)	The core software on a network. The NOS runs on the network server and provides the basis for resource sharing by the other devices on the network. Largely, the network operating system does for the network what an operating system (OS) does for a personal computer.
NRZ (Non-return to zero)	A digital signalling technique in which the signal is at a constant high level (above zero volts) for a binary 0 and a low value (below zero) for a logical 1.

Octet	A group of eight bits. Commonly synonymous with byte, although there is no strict definition of a byte as an 8-bit group, so octet is preferred when a precise description is required.
Odd parity	See parity.
OFDM (Orthogonal frequency division multiplexing)	A technique by which a data stream with a higher data rate is broken up into a number of smaller data streams and modulated on a series of adjacent frequencies.
Optical fibre	See fibre optics.
OS (Operating system )	An integrated set of specialised programs that is used to manage the resources and overall operations of a computer.
OSI reference model (Open Systems Interconnection reference model)	A seven-layered set of protocols specified by the International Standards Organization (ISO) for transmitting data from one terminal to another.
Layer 1 (physical layer)	The services in the OSI protocol stack (layer 1 of 7) that provide the transmission of bits over the network medium. Also known as the "PHY layer.
Layer 2 (data link layer)	The services in the OSI protocol stack (layer 2 of 7) that manage node-to-node transmission.
Layer 3 (network layer)	The services in the OSI protocol stack (layer 3 of 7) that provide internetworking for the communications session.
Layer 4 (transport layer)	The services in the OSI protocol stack (layer 4 of 7) that provide end-to-end management of the communications session.
Layer 5 (session layer)	The services in the OSI protocol stack (layer 5 of 7) that initiates and manages the communications session.
Layer 6 (presentation layer)	The services in the OSI protocol stack (layer 6 of 7) that provide conversion of codes and formats for the communications session.
Layer 7 (application layer)	The software in the OSI protocol stack (layer 7 of 7) that provides the starting point of the communications session.

PABX (Private access branch exchange).	A term normally referring to an organisation's internal telephone exchange.
Packet	A group of data bits that include the information as well as overhead including source and destination addresses, identification information, and frame check sequence. The term packet is usually associated with the network layer.
Packet Filter	Blocks traffic based on a specific web address (IP address) or type of application ( eg email, FTP, Web etc) which is specified by a port number.
Packet switching	The method of packet switching provides both the response of circuit switching and the efficient link utilisation of message switching by imposing a maximum length on the transmitted units by splitting all messages into packets. Long messages are therefore prevented from blocking links and a system of time division multiplexing is forced onto the network.
PAD (Packet assembly/disassembly)	The data terminal's interface with a packet-switching network that breaks messages into packets for transmission and reassembles the packets into messages at receiver. This terminology is usually associated with X.25 networks.
PAM (Pulse amplitude modulation).	The simplest form of pulse modulation and is either transmitted by itself or, more commonly, is used to develop other pulse-modulated waveforms. In pulse-amplitude modulation, the signal is sampled at regular intervals to obtain a pulse whose amplitude is proportional to the amplitude of the signal at the instant of sampling.
PAP/CHAP (Password authentication protocol and Challenge Handshake Authentication Protocol).	The standard method for authentication users to ISP networks.
PAT	See NAPT.
Parity bit	An additional check bit appended to a group (normally seven) of binary digits to make the sum of all the binary ones, including the check bit, always odd (odd parity) or even (even parity).
Patch	A piece of software code designed as a short-term fix to a programming problem or error. By definition the patch is an intermediate bug fix, which is not necessarily the best overall solution, however, offers a quick repair until amended versions of the software are released.
PBX (Private branch exchange).	An organisation's internal telephone exchange.
PCM (Pulse code modulation).	Similar to pulse-position modulation (PPM) in that the received information is not determined by the shape of the pulse, but has the additional advantage that the precise location of the pulse is not important either. The analogue signals are first sampled using pulse amplitude modulation. The pulse amplitude modulation pulses are then encoded into a binary code that is transmitted as a digital stream. At the receiver these pulse code modulation (PCM) codes are decoded into pulse amplitude modulation (PAM) pulses that are then used to reconstruct the analogue waveform. In adaptive differential pulse-code modulation (ADPCM), the binary number that is transmitted is not the number that corresponds to the actual voltage level, but rather the difference between the current sample value and the last. The difference values have a smaller dynamic range and compression (of the order of one-half) results).
PDA (Personal Digital Assistant).	Generic term describing a wide variety of handheld computers.
PDM (Pulse duration modulation)	Also known as pulse-length modulation (PLM) or pulse-width modulation (PWM), is a pulse modulation technique that transmits amplitude information of an analogue signal, not by the height of the pulse (as in pulse-amplitude modulation (PAM)), but by its duration.
Peer-to-peer networking	A means of communications which allows computers to communicate with each other without a central server and still share files, applications and devices.
Phase	The difference in angle between two sinusoidal wave forms. - Physical layer (See OSI, layer 1).
Picocell	The smallest of the cells in a cellular personal communications systems (PCS)-normally covering an office area. Picocell is most commonly used in connection with third-generation personal communications systems.
Ping (Packet Internet Groper)	A basic Internet utility program that sends a small amount of data (ICMP packet) to test connectivity between network devices. The verb ping refers to using the ping command to test whether a particular IP address exists and can accept requests.
PKA (Public key authority)	A PKA issues public keys for use in exchanging data using public key encryption.
Plain text	The unencrypted (or decrypted) version of a message.
PLM (Pulse length modulation)	Also known as pulse-width modulation (PWM) or pulse-duration modulation (PDM), is a pulse modulation technique that transmits amplitude information of an analogue signal, not by the height of the pulse (as in PAM), but by its duration.
PM (Phase modulation)	A form of modulation in which the carrier wave's phase is caused to vary in accordance with the modulating signal.Phase modulation is a form of angle modulation.
Point-to-point	Refers to a communications line that provides a path from one location to another (from point A to point B). Contrast with multipoint.
Polarisation	The orientation of the electric field in an electromagnetic wave. Commonly used types of polarisation include horizontal, vertical and circular.
Polling	The process in which data terminals are asked whether they have any data to transmit.
POP (Point of Presence).	A node providing a unique access point to the Internet. For example an ISP or carrier may provide several different POPs spanning a certain geographic area.
POP3 (Post Office Protocol V 3)	A protocol that allows network nodes to retrieve email from a server via a temporary connection. It is useful for nodes that are not permanently connected to the Internet.
POTS (Plain Old Telephone Service)	One of many terms to describe low bandwidth communications over the standard coper wire telephone system.
Power (P)	When the flow of electrons is slowed by a resistance, they must give up some of their energy. The dissipation of energy over time is called power (P). The power dissipated in a resistance is given by the product of the voltage and the current: that is, P=VI.
PPP (Point-to-point protocol)	The most popular method for transporting IP packets over a serial link between the user and the ISP. Developed in 1994 by the IETF and superseding the SLIP protocol, PPP establishes the session between the user's computer and the ISP using its own Link Control Protocol (LCP). PPP supports PAP, CHAP and other authentication protocols as well as compression and encryption.
PPPoA (Point-to-Point-Protocol over ATM)	standard for incorporating the popular PPP protocol, widely used for dial-up Internet connections, into a DSL connection that uses ATM as its transport to the carrier's facilities. From the PC, IP packets travel over an Ethernet connection to the DSL modem, officially called an "ADSL transceiver unit-remote (ATU-R). The ATU-R adds the PPP protocol to the IP packets and transports them to the carrier's DSLAM over ATM.
PPPoE (Point-to-Point Protocol Over Ethernet)	A standard for incorporating the popular PPP protocol, widely used for dial-up Internet connections, into a cable modem connection that uses Ethernet as its transport to the carrier's facilities. Used by a large number of cable modem providers, PPPoE supports the protocol layers and authentication widely used in PPP and enables a point-to-point connection to be established in the normally multipoint architecture of Ethernet. A discovery process in PPPoE determines the Ethernet MAC address of the remote device in order to establish a session.
PPM (Pulse Position modulation).	A pulse modulation technique that uses pulses that are of uniform height and width but displaced in time from some base position according to the amplitude of the signal at the instant of sampling. PPM is also sometimes known as pulse-phase modulation. Pulse position modulation has the advantage over pulse amplitude modulation (PAM) and pulse duration modulation (PDM) in that it has a lower noise immunity since all the receiver needs to do is detect the presence of a pulse at the correct time; the duration and amplitude of the pulse are not important.
PPTP (Point-to-Point-Tunnelling).	A protocol from Microsoft that is used to create a virtual private network (VPN) over the Internet. It uses encryption based on RSA's RC4, known as Microsoft Point-to-Point Encryption (MPPE). Remote users can access their corporate networks via any ISP that supports PPTP on its servers.
Pre-emphasis / de-emphasis	A technique of emphasising certain frequencies over others to provide better signal delineation in non-linear communications mediums.
Preamble	A bit pattern at the beginning of a data transmission, usually used to synchronise the transmitter and receiver.
Presentation layer	See OSI, Layer 6.
Print server	A computer oer standalone device that allows a number of users to direct printed output to the same printer.
Promiscuous mode.	A mode in which a network device listens to all addresses, not just its own.
Propagation delay.	The amount of time it takes for a signal to travel from its source to its destination.
Protocol.	A set of rules or conventions that govern one or more of the many aspects of communication.
Proxy server.	An intermediate gateway sitting between a client PC and the Internet and providing forwarding/caching services as well as security logging and administrative control.
PSK (Phase-shift keying).	A form of modulation in which the phase of the carrier signal is shifted to represent digital data.
PSTN (Public switched telephone network)	The formal name for telephone networks operated by telecommunications carriers.
Public key	One of the two keys used in an asymmetric encryption system. The public key is made public, to be used in conjunction with a corresponding private key.
Public-key encryption	A form of asymmetric encryption.
PVC (Permanent virtual circuit)	Provides a permanent link between two nodes across a packet-switched network. Packets on the permanent virtual circuit always follow the same route across the network.
PWM (Pulse width modulation).	Also known as pulse-length modulation (PLM) or pulse-duration modulation (PDM), is a pulse modulation technique that transmits amplitude information of an analogue signal, not by the height of the pulse (as in PAM), but by its duration.

QAM (Quadrature amplitude modulation)	A modulation technique in which both the phase and amplitude of a carrier is varied by the symbols of the message.
QoS (Quality of Service)	A term used to monitor and describe the quality of a communications link.
QPSK (Quadrature phase-shift keying)	Phase shift keying (PSK) between four phase states, normally 90 degrees apart.
Quantisation	The process of converting a continuous variable (an analogue signal) into discrete samples (a digital number). Only a limited number of amplitudes are allowed in the discrete samples-for example, if three bits are used, only eight amplitudes can be represented. The difference between the sample value and the original waveform value is called quantisation error, or quantisation noise.
Quantum	Quantum is derived from the Latin word quantus meaning 'how many'. The difference between the discrete set of amplitudes allowed during the quantisation of an analogue waveform.
Quaternary	The term quaternary is used to describe a system having four states.

Rayleigh fading	Fading in a channel due to the interference caused to the main signal by the same signal arriving over many different paths, resulting in out-of-phase components incident at the receiver.
Relay	Used to pass on a signal, normally after retiming and regenerating it.
Repeater	A physical layer (see OSI, Layer 1) device that interconnects two local area networks (LANs). Repeaters operate at the physical layer and therefore can only interconnect local area networks of the same type. A repeater does not understand any network protocols; each bit received on one port is regenerated and retransmitted on all other ports.
Resistance	When a voltage potential is placed across a wire, the number of electrons that will flow will depend on the degree of freedom of electrons in the conducting wire. The ability of the wire to conduct is called conductance, but this property of the wire is normally considered in terms of how it restricts the flow of electrons, that is its resistance. Resistance is measured in ohms.
Resistor	A circuit component that has resistance.
Resting frequency	The resting frequency is the frequency of an FM carrier when no modulating signal is present. The presence of a modulating signal will cause the carrier to shift away from the resting frequency.
RF (Radio frequency).	The range of frequencies used for radio communications (across a radio channel). The radio-frequency range is divided by the ITU-R into a number of bands of frequencies, where all frequencies in a band exhibit similar properties. The bands are ELF, VF, VLF, LF, MF, HF, VHF, UHF, SHF and EHF.
RFI (Radio frequency interference).	See electromagnetic interference (EMI).
Ring topology	A local area network (LAN) topology in which the medium forms a loop to which workstations are attached. Data is transmitted in a simplex form from one station, normally employing a token-passing. - RJ # connector. A four-wire modular connector used for telephone and modem connection. RJ-45 is the larger 8-wire modular connector used for 10BaseT Ethernet.
Roaming	The ability of a wireless device to move from one access point's ( eg mobile phone and phone tower) range to another without losing connection.
Router	A network layer (see OSI, Layer 3) device that interconnects two local area networks (LANs). Routers operate at the network layer and therefore can interconnect local area networks of the different types. A router can only connect two networks using the same network-layer protocol. Routers form the basis of routing in the Internet.
Routing algorithm	One of several algorithms used to determine how to transmit a message from its source to its destination across a network (that is, between network nodes).
Routing table	A database containing network node and terminal addresses as well as the path to be used in transmitting data between nodes.
RS code (Reed-Solomon code).	An important subclass of non-binary BCH error-correcting codes in which the encoder operates on multiple bits rather than individual bits. Reed-Solomon codes are therefore ideally suited to use with m-ary modulation.
RSA algorithm (Rivest-Shamir-Adleman algorithm).	A popular public-key encryption algorithm.
Rx.	Rx is an abbreviation for 'receive'.

SackOpts (Selective Acknowledgements - RFC 2018).	Allows TCP to recover fro IP packet loss without resending packets that were already received by the client.
Sampling rate.	The rate at which samples are taken form an analogue waveform during the analogue-to-digital conversion process.
SAN (Storage Area Network).	A high-speed, special purpose network that interconnects different types of data storage devices with associated data severs.
SCSI (Small Computer System Interface - pronounced "scuzzy).	ntroduced in 1986, SCSI is hardware interface that allows for the connection of up to 15 peripheral devices to a single board called a "SCSI host adapter that plugs into the motherboard, typically using a PCI slot. SCSI peripherals are daisy chained together. They all have a second port used to connect the next device in line. SCSI host adapters are also available with two controllers that support up to 30 peripherals.
Selective-reject ARQ (Selective-reject automatic repeat reques).	An error control technique in which the transmitter only retransmits blocks that are in error.
Serial transmission.	The transmission of bits one after the other.
Server	A network server, (more traditionally called a file server), is normally a fast processor with large storage and memory capacity, which manages the network and the network file system and provides a number of functions such as: access to printers, disk space, application programs, back-up tape drives, as well as running the network operating system (NOS).
Session layer.	See OSI, layer 5. - SHF (Super high frequency). The radio-frequency (RF) band in the range 3-30 GHz. SHF and EHF are known as the microwave bands and their wavelengths are short enough to be propagated by highly directional antennas and waveguides. Propagation ranges are limited to line-of-sight but long-distance communication can be achieved by employing a series of radio relay stations. These portions of the spectrum are used for television and high-speed data services requiring large bandwidths.
Shift	The amount by which the FSK carrier frequency changes from 1 (mark) to 0 (space) and vice versa.
Shortwave	See HF.
Sideband	A sideband is a band of frequencies either side of the carrier in amplitude modulation (AM).
Sidelobe	A slidelobe is the response of a receive antenna to a signal from other than its main axis (and similarly for a transmit antenna).
Simplex transmission	Simplex transmission is a mode of data transmission in which a transmission path can carry information in one direction only-from the source to the sink.
Single mode fibre	A single-mode fibre is a fibre optic cable in which the light is guided down the centre of a very narrow core (approximately 8 microns thick). Supports the fastest optical fibre transmission. - Sink. The sink converts the electrical signal from the receiver back into the form of original message information. For voice, the sink will be a speaker to produce sound waves for aural reception. For text, a teletypewriter will produce words on a piece of paper, or a computer will display words on a screen or store them in some form. A television will be used to display on its screen the series of images contained in video signals.
Sliding window	Sliding window is a method of flow control in which a source may send more than one packet without acknowledgement from the sink. The number of packets is provided by a 'window' size that can often be increased and decreased to control congestion within a network.
Slip.	A communications protocol used for dial up access to TCP/IP networks predating PPP>
SMS (Short Message Service).	A common method of transmitting short messages over mobile phone networks.
SMTP (Simple Mail Transfer Protocol).	TCP/IP protocol used in sending and receiving email.
SNMP (Simple network management protocol).	The SNMP is used to pass network-management information in TCP/IP networks.
SNR (Signal-to-noise ratio).	The ratio of the signal power to the noise power.
Source.	The originator of the information that is to be exchanged via the communication system.
Spectrum.	The range of electromagnetic frequencies.
SPI (Stateful Packet Inspection).	A firewall technology that monitors the state of a transaction to verify that the destination of an inbound packet matches the source of the previous outbound request. SPI is a popular means of ensuring network security.
Spread spectrum.	A spread-spectrum signal occupies a bandwidth much greater than that of the message signal it conveys. There are two main types of spread spectrum technique: direct sequence spread spectrum, and frequency hopping.
SSB (Single sideband).	Suppressing the carrier in an amplitude-modulated (AM) waveform produces double sideband (DSB). This increases the efficiency of the transmission since only the sidebands are amplified. A degree of inefficiency remains, however, as the transmission bandwidth needs to be double the message bandwidth due to the transmission of both the upper and lower sidebands. These two sidebands are redundant since all the necessary information can be conveyed by only one sideband. When only one sideband is transmitted, the transmission is called single-sideband suppressed carrier (SSBSC) or more commonly SSB. When the upper sideband is transmitted the transmission is called upper side band (USB) and when the lower sideband is transmitted it is called LSB. This reduces the bandwidth of the modulated wave by half and makes it equal to that of the baseband signal. All of the transmitted power then goes toward transmitting the baseband information.
SSID (serer Set Identifyer).	A 32-byte set of characters that uniquely name a WLAN. This name allows stations to connect to a desired network when multiple independent networks operate in the same physical area.
Star topology.	A star topology is a network topology in which all nodes are connected to a central node through which all data are transmitted.
Star-wired LAN (Star-wired local area network).	A local area network (LAN) that has a physical star topology even though it may employ a logical bus or ring topology.
Static routing.	Static routing is a form of routing in which paths between nodes are always used in a pre-defined way.
STDM (Statistical time division multiplexing).	Statistical time division multiplexing is a form of time-division multiplexing (TDM) that provides improved channel-sharing efficiency by allowing some of the multiplexed channels to increase in data rate above their share if other stations are not using theirs to the full capacity. Also known as a stat mux.
Stop-and-wait ARQ (Stop-and-wait automatic repeat request).	An error correction technique in which the sender transmits a block of data and then waits for an acknowledgment before transmitting the next block.
Store-and-forward system.	See message switching.
STP (Shielded twisted pair).	A form of twisted pair cable in which a separate shielding conductor surrounds each pair.
Sub-sampling.	Sub-sampling is the process of sampling a signal that has already been sampled to reduce the sampling frequency. - SVC (Switched virtual circuit). Provides a semi-permanent link established across a packet-switched network for the duration of the session between source and sink. Like a permanent virtual circuit, packets on the switched virtual circuit always follow the same route across the network, but unlike a permanent virtual circuit (PVC), a switched virtual circuit does not remain after the users have finished.
Symmetric encryption.	A form of cryptography in which encryption and decryption are performed using the same key.
Synchronous transmission.	Data transmission in which source and sink have a common view of time and both run clocks that are continually synchronised by coordinating pulses extracted from the incoming data stream.

TCP/IP (Transmission Control Protocol/Internet Protocol (TCP/IP).	TCP/IP is the most common suite of internetwork protocols.
TDD (Time-division duplex).	Refers to duplex communications links where the uplink is separated from the downlink by the allocation of different time slots in the same frequency band.
TDM (Time division multiplexing).	A technique that shares a transmission channel between users by dividing transmission time by allotting to each device a time slot during which it can send or receive data. Time division multiplexing involves interleaving in time the narrow pulses of pulse-code modulation (PCM) or delta modulation (DM) to form one composite digital signal for transmission. De-multiplexing of the time-multiplexed pulse streams at the receiver is accomplished by gating appropriate pulses into individual channel filters.
TDMA (Time-division multiple access).	A mechanism for sharing a channel, whereby a number of users have access to the whole channel bandwidth for a small period of time (a time slot). The difference between time-division multiplexing (TDM) and time-division multiple access is that time-division multiplexing requires users to be collocated to be multiplexed into the channel. In that regard, time-division multiple access can be considered as a remote multiplexing technology.
Telegraphy.	iterally, telegraphy means "writing at a distance. First major form of communication using Morse code to transfer information via the on-off keying of a carrier. More sophisticated forms include telex. Largely overtaken by higher-capacity data techniques.
Telephony.	iterally, telephony means "speaking at a distance. The transmission of voice frequencies (in analogue or digital form) across a telecommunications network.
Ter.	Ter is a postscript used to note the third version of an ITU standard (ter is French for three). For example, V.32ter is the third version of the V.32 standard.
Ternary.	Ternary means having three states.
TFTP (Trivial File Transfer Protocol).	A simplified version of FTP that uses UDP, rather than TCP for data transport.
Timestamp.	Technique for establishing temporal relationships.
Throughput.	The amount of information carried by a communication system. Data rate equals information rate plus overhead.
Token bus.	A medium access technique using token passing on a logical bus local area network (LAN) topology.
Token ring.	A medium access technique using token passing on a logical ring local area network (LAN) topology.
Token-passing.	A media access technique in which a small set of bits called the token is passed between network devices.
Topology.	The way in which the devices on a LAN are connected together. There are three main LAN topologies: bus, star and ring.
Transceiver.	A contraction of transmitter/receiver-a device that is able to act as both a transmitter and a receiver.
Transmission medium.	The physical path between transmitters and receivers in a communications system.
Transponder.	In satellite communications, a transponder receives the transmission from earth (uplink), amplifies the signal, changes frequency and retransmits the data to a receiving earth station (downlink).
Transport layer.	See OSI, layer 4.
Trojan.	A program which masquerades as something it is not in order to gain unauthorised access to a PC. Usually a piece of malicious code contained in another seemingly harmless program. Derived from ancient Greek author Homer's renowned epic, the Iliad which relates the story of how the Greeks won the Trojan war by concealing their warriors in a huge, hollow wooden horse.
Twisted pair.	A coper transmission line consisting of two insulated wires twisted together.

UBR (Unconstrained Bit Rate).	A service class for asynchronous transfer mode networks (ATM networks) in which the cell rate may be varied arbitrarily by a source.
UDP (User Datagram Protocol).	An alternative to TCP which uses IP to get a data unit (datagram) from one network node to another.
UHF (Ultra High Frequency).	The radio-frequency (RF) band in the range 300-3,000 MHz. UHF can be used for fixed communications systems that transmit large amounts of bandwidth over short distances and often line-of-sight. Common applications include television, ground-to-air communications and mobile services. Other non-communications services include: radar, space research, radio astronomy, and telemetry.
Unbalanced transmission.	Unbalanced transmission is a transmission mode in which signals are transmitted on a single conductor and the return path for the current is through the ground.
Unicode.	A 16-bit ISO 10646 character set. It can accommodate more characters than ASCII, allowing for easier internationalisation. Unicode has also been used as a hacking exploit on some webservers.
Uplink.	The signal from a satellite ground station to the satellite transponder.
UPnP (Universal Plug and Play).	A standard that defines the connected environment, an automatic network of different, self-configuring devices, such as PCs, peripherals, wireless devices, and intelligent appliances. In a UPnP environment, all those devices would be able to configure themselves, acquire IP address automatically, announce their presense and location on the network (using HTTP), and be able to communicate using Internet and Web protocols. URL. (Universal resource locator). The Internet standard for the specification of a file by defining the type of service as well as the location of the file.
USB (Universal Serial Bus). 1/	A hardware interface for low-speed peripherals such as the keyboard, mouse, joystick, scanner, printer and telephony devices. It also supports MPEG-1 and MPEG-2 digital video. USB has a maximum bandwidth of 12 Mbits/sec (equivalent to 1.5 Mbytes/sec), and up to 127 devices can be attached. Fast devices can use the full bandwidth, while lower-speed ones can transfer data using a 1.5 Mbits/sec subchannel. USB's hot swap capability allows everything to be plugged in and unplugged without turning the system off. USB ports began to appear on PCs in 1997, and Windows 98 fully supports it. USB Version 2.0 dramatically increases capacity to 480 Mbits/sec, which challenges FireWire (IEEE 1394) as the serial interface of the future.
2/ USB (Upper sideband).	When only one SSB sideband is transmitted, the transmission is called single-sideband suppressed carrier (SSBSC) or more commonly SSB. When the upper sideband is transmitted the transmission is called upper side band (USB) and when the lower sideband is transmitted it is called LSB.
UTP (Unshielded twisted pair).	A form of twisted pair cable in which no separate shielding conductor is provided.

V.90	An ITU standard (1998) for a modem that communicates at %^Kbps downstream and 33.6 Kbps upstream. It is intended for use only with ISPs and online services that are digitally attached to the telephone system. Most service providers are typically connected with high-speed digital T1 or T3 circuits.
V.92	An improved version of the V.90 modem that boosts the upstream rate from 33.6Kbps to 48Kbps. Introduced in 2000, V.92 reduces connection time (handshaking) by remembering the previous settings negotiated when dialing the same telephone number.
VBR (Variable Bit Rate)	A service class for asynchronous transfer mode networks (ATM networks) in which the cell rate is allowed to vary in accordance with the connection contract.
VC (Virtual circuit)	See switched virtual circuit.
VF (Voice frequency)	A frequency in the range produced by the human voice. The sounds produced in speech contain frequencies that lie within the frequency band 100-10,000 Hz. However, the main useful voice frequencies are contained within the band 300 Hz to 3.4 kHz.
VHF (Very high frequency)	The radio-frequency (RF) band in the range 30-300 MHz. VHF band can be used for communications systems such as television, which require the transmission of large bandwidths over short distances (generally line-of-sight). Small, directional antennas are economical and effective. The VHF band is used for fixed communications services, ground-to-air communications, mobile services, and television.
Viterbi decoder	Developed in the late 1960s by Andrew Viterbi to decode a particular convolutional code, Viterbi decoders provide among the most effective ways of decode wireless voice communications in satellite and mobile phone transmissions. Viterbi outputs a 0 or a 1 based on its estimate of the input bit.
Virus	Software used to infect a computer. After the virus code is written, it is buried within an existing program. Once that program is executed, the virus code is activated and attaches copies of itself to other programs in the system. Infected programs copy the virus to other programs. The effect of the virus may be a practical joke that pops up a message on screen out of the blue, or it may destroy programs and data right away or on a certain date. It can lie dormant and do its damage once a year. For example, the Michaelangelo virus contaminates the machine on Michaelangelo's birthday.
VLF (Very low frequency)	A radio-frequency (RF) band in the range 3-30 kHz. VLF shares limitations associated with with ELF: only small amounts of bandwidth are available; antennas are large and inefficient, leading to low radiated powers. However, VLF has similar advantages to ELF including low attenuation through the atmosphere and seawater making it useful for communications and maritime radio navigation systems. The band has more use for communications than ELF since it has a bandwidth that is large enough to provide reliable communications. Propagation is primarily by surface wave and has little seasonal or diurnal variation. However, communications are prone to static and to interference from other radio frequencies.
Vocoder.	A common type of source coding is performed by the voice coder or vocoder, which removes sufficient redundancy from speech so that the bit rate is reduced to 2.4 Kbps.
VoIP (Voice over IP)	The two-way transmission of audio over a packet-switched IP network (TCP/IP network). When used in a private intranet or WAN, it is generally known as "Voice over IP," or "VoIP." When the transport is the public Internet or the Internet backbone from a major carrier, it is generally called "IP telephony" or "Internet telephony." However, the terms IP telephony, Internet telephony and VoIP are used interchangeably. IP telephony uses two protocols: one for transport and another for signalling. Transport is provided by UDP over IP for voice packets and either UDP or TCP over IP for signals.
VPN (Virtual Private Network)	A network of secure links over a public IP infrastructure. Technologies which fall into this category include PPTP, L2Tpand IPSec.
VSB (Vestigial side band)	Single sideband (SSB) modulation is bandwidth efficient for single channel voice applications because the low frequencies in voice are not required. There are many types of communication, however, such as television, video, facsimile and high-speed data signals that have a large bandwidth incorporating significant low-frequency content. SSB systems have a poor low-frequency response so vestigial-sideband (VSB) modulation is used, which is derived by filtering double-sideband (DSB) so that one sideband is completely passed and just a trace, or vestige, of the other sideband is included. The bandwidth of vestigial-sideband systems is therefore only slightly larger than that of SSB systems but with the increased low-frequency performance of DSB systems.

W3C (World Wide Web Consortium).	An international consortium founded in 1994 that unites over 450 organisations involved with the Internet and the World Wide Web. It's purpose is to "lead the World Wide Web to its full potential by developing common protocols that promise its evolution and ensure its interoperability. The W3C publishes and develops open standards for the Web.
WAN (Wide Area Network).	A communications network providing connectivity across a wide geographic area as distinct from a LAN (local area network) or MAN (metropolitan area network). WANs typically utilise higher speed network protocols.
WAP (Wireless Application Protocol).	A specification for a set of communications protocols designed to standardise the way that handheld devices such as mobile phones and PDAs (personal digital assistants) can speak to each other and connect to the Internet.
Wavelength.	The distance between peaks (or troughs) of a sinusoidal waveform.
WDM (Wavelength division multiplexing).	The sharing of an optical fibre channel by providing multiple channels, each of which modulates a laser of different wavelength. A modern form of frequency-division multiplexing (FDM).
WDS (Wireless Distribution System).	A wireless access point node that enables wireless bridging in which WDS APs communicate with each other only (without allowing for wireless clients or stations to access them), and/ or wireless repeating in which APs communicate both with each other and with wireless stations at the expense of half the throughput.
WEP (Wired Equivalent Privacy).	An IEEE standard specified in the Wi-FI (Wireless Fidelity or 802.11b) standard to provide WLANs (wireless local area network) with a level of security equivalent to a fixed LAN by encrypting data.
WiFi	The popular name for publicly available networks based on the pervasive IEEE-ratified 802.11b standard for wireless communications.
Wireless.	Generically refers to transmission of data or voice communications using radio waves as opposed to physical cables or wiring. The most pervasive forms of wireless communication stem from the various IEEE ratified 802.11 standards for wireless. 802.11 was first introduced in 1997. The first standard was 802.11b, which specifies from 1 to 11 Mbps in the unlicensed 2.4GHz band using direct sequence spread spectrum (DSSS) technology.sing the orthogonal FDM (OFDM) transmission method, there are two subsequent standards that provide from 6 to 54 Mbps: 802.11a transmits in the higher 5GHz frequency range and is not backward compatible with the slower 802.11b running in the 2.4GHz frequency range; 802.11g works in the same range and is compatible. The 11a and 11b standards are endorsed and branded as "Wi-Fi by the Wi-Fi Alliance.n 802.11 system works in two modes. In "infrastructure mode, wireless devices communicate to a wired LAN via access points. Each access point and its wireless devices are known as a Basic Service Set (BSS). An Extended Service Set (ESS) is two or more BSSs in the same subnet.also known as "peer-to-peer mode, wireless devices can communicate with each other directly and do not use an access point. This is an Independent BSS (IBSS). The speed of 802.11 systems is distance dependent. The farther away the remote device from the base station, the lower the speed. 802.1x. A security protocol from the IEEE for wireless LANs that adhere to the 802.11 standard. It relies on the Extensible Authentication Protocol (EAP) to pass messages to any of a variety of authentication servers such as RADIUS or Kerberos
WISP (Wireless Internet Service Provider).	An ISP that allows subscribers to connect to a server at designated access points using a wireless connection such as WiFi. This type of ISP offers broadband service and allows subscriber computers, called stations, to access the Internet and the Web from anywhere within the zone of coverage provided by the server antenna(s).
WML (Wireless Markup Language).	A tag-based language similar to HTML, used to create web pages that can be delivered using WAP to handheld wireless devices. WML is designed for the smaller displays of these devices, using less memory than HTML and Java Script.
Worm.	A computer program able to replicate itself over a network. Usually, but not always malicious, worms typically exploit vulnerabilities in automatic file sharing features found on PCs. Worms are not like viruses in that they tend not to actually alter or infect files rather, simply replicate themselves.
WPA (Wi-Fi Protected Access).	A security protocol from the Wi-Fi alliance for 802.11 wireless networks which uses Temporal Key Integrity Protocol (TKIP) to provide stronger encryption than the earlier WEP (Wired Equivalent Privacy) method. Derived from, and a subset of, the IEEE 802.11i security standard, WPA includes 802.1x authentication.
WYSIWYG (What you see is what you get – pronounced "wizzy-wig).	Software that displays what a particular computer task will look like while it is being created.