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.

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