Analogue: A signalling system that can take an infinite number of values. This is often described by a mathematical sine curve. An example of an analogue scale is temperature measurement, c.f. digital
Close this window
Baud: number of signal elements per second. Baud rate is often taken to mean number of bits per second, but this is not necessarily true. E.g. in a binary system, a high could be represented by +5v and a low by -5v. This is one bit per signal element and is equivalent to one bit per second, however, if the coding system allows 4 values instead of two, e.g. +5v, +2.5v, -2.5v and -5v, then you can encode 2 bits into a single signal element, and so on.
Close this window
Circuit switching: In a circuit-switched network, bandwidth is reserved along the network for the duration of each 'conversation' and no data is stored at any point along the network. Packets of data are just passed from node to node without being stored at any point until they reach their destination. The telephone network is circuit-switched. c.f. packet switching.
Close this window
Coax (say co-axe): Coaxial cable
Close this window
CATV: Community Antenna Television – the cable TV system.
Close this window
Crosstalk: Signal noise caused by electronic coupling between paths (e.g. wires). This can occur during a fast transition between voltages in one wire which causes a glitch (i.e. a short and possibly very high change) in the other wire. This can be prevented by having an intermediate wire which is always earthed.
Close this window
Digital:A signaling system that has only a discrete number of values, e.g. the alphabet. c.f. analogue
Close this window
DSL: Digital Subscriber Line
Close this window
Echo-cancellation: A technique by which the entire passband of a channel can be used to send and receive. It works since a sender knows what it is sending out, so it can filter this out of the signal that it recives to get just the incoming signal.
Close this window
Equaliser: This is a device that tries to cancel out distortion on a line by "distorting" the channel in the opposite direction to the actual distortion.
Close this window
Ethernet: The currently dominant local area network technology. This is a 'broadcast' technology, with all messages being broadcast to all receivers, but only the destination computer picks the message up. Each packet of the message has a destination address with a unique 48-bit address that identifies the destination computer.
Close this window
Frequency Division Multiplexing: A means of allowing multiple 'conversations' to occur simultaneously by having different conversations to broadcast at different frequencies in the spectrum.
See also multiplexing.
Close this window
Full Duplex: A mode of communication where both parties can send and receive simultaneously (e.g. the PSTN is full duplex since both participants of a telephone conversation can talk simultaneously, if they choose)
Close this window
GPRS: General Packet Radio Service
Close this window
GSM: Global System for Mobile communications
Close this window
HDTV: High Definition Television
Close this window
Headend: Source of a CATV broadcast
Close this window
HFC: Hybrid fibre/coax
Close this window
ISDN: Integrated Services Digital Network
Close this window
ITU: International
Telecommunications Union. This is a
body of the United Nations that defines standards in the field of
telecommunications.
Close this window
Local Loop: The copper cable connecting a subscriber premises with the local telecoms exchange
Close this window
Modulation: The process of moving a signal's spectrum to a different frequency band centred on some chosen frequency called the carrier frequency, in order to facilitate transmission over a medium.
Close this window
Multiplexing: Generic term for any means of using the same medium (e.g. wire) to carry multiple conversations so that it seems to the end users that they have unique access to the medium. This is commonly achieved by letting different conversations broadcast on different frequencies (FDM) or by allowing each conversation to take control of the medium for a specified (small) amount of time in a pre-determined pattern (TDM – Time Division Multiplexing).
Close this window
Packet: A packet, in data terms, is a 64KB 'lump' of data, which contains a header with information such as the destination address, the source address etc followed by the actual data that was sent in this packet
Close this window
Packet switching: In a packet switched network, no bandwidth is ever reserved at any of the internal nodes for a 'conversation'. This means the nodes must have some kind of storage facility to cope with the fact that data may be coming into the node at a faster rate than it can get rid of it. The Internet is a packet-switched network. c.f. circuit-switching
Close this window
Passband: The range of frequencies along which a medium (such as a wire) will treat all frequencies equally in terms of distortion and signal degradation
e.g. the local loop has a passband of 300-3400Hz
Close this window
PSTN: Public Switched Telephone Network. The telephone network that we are all used to.
Close this window
QAM: Quadrature amplitude modulation. A coding system that allows many bits per signal element
Close this window
QPSK: Quaternary Phase Shift Keying. A code that allows two bits per signal element.
Close this window
Signal Element: This is each 'pulse' of a coding system.
Close this window
Trunk Line: These are the ultrahigh-capacity lines that form the backbone of the telecoms system, connecting together the major centres of communication, nationwide.
Close this window
UMTS: Universal Mobile Telecommunications System
Close this window
WAP: Wireless Application Protocol. A protocol
designed to allow access to information and services from mobile
phones using the Internet as the means of delivery
Close this window