JISC

ADSL  Asymmetric Digital Subscriber Line is a technology for transmitting digital information at a high bandwidth on existing phone lines to homes and businesses. Unlike regular dial-up phone service, ADSL provides continously-available, 'always-on' connection. ADSL is asymmetric in that it uses most of the channel to transmit downstream to the user and only a small part to receive information from the user. ADSL simultaneously accommodates analogue (voice) information on the same line. (source: www.whatis.com)

Analogue  Information represented by a measurable physical quantity with continuous values, as opposed to information in digital form

Beam  This is essentially the signal or group of signals that is sent from the satellite to the surface of the Earth. This surface, defined by the beam, is the satellite 'footprint', ie the area on Earth where the beam can be captured and used. No one satellite can transmit a beam to the entire Earth, though theoretically three equally spaced satellites in a network could cover the entire Earth.

Broadcasting A means of one-way, point-to-multipoint transmission where the end receiver is not known to the broadcaster, ie it is an 'open' system. Broadcasting is typical for radio and television transmission where the audience is only defined by virtue of having the correct receive equipment.

Caching Using a buffer within your own computer's fast memory to hold recently accessed data. It is designed to speed up access to the same data later.

Cellular network Mobile radio system most often used nowadays for telephony, which has rapidly supplemented landline telecommunications as a means of two-way personal communications. Cellular networks works on the principle and use the physics of two-way radio communications and is named after the unit 'cell' into which an area is divided. As a mobile radio telephone moves through this pattern of cells, its user's calls, made as on an ordinary telephone, are switched from one cell to the next by a computerised system.

Compression Reduce the number of bits required for data storage or transmission with special software. Decompression reverses the result of compression.

Contention Contention is the case when multiple users vie for the right to use a communication channel within a multiplexed connection. The contention rate is in that case the number of users that in the worst case have to share the connection. If a connection has a bandwidth potential of 1000 Kbps with a contention rate of 20:1 it is possible that in the worst case users will fall back to a connection speed of 1000/20 = 50 Kbps.

Dark Fibre Dark fibre is optical fibre infrastructure (cabling and repeaters) that is currently in place but is not being used

Data broadcasting Data can be broadcast or transmitted to users over various wireless and cable mediums. The most typical are radio broadcasts (VHF, UHF, satellite) and cable broadcasts (such as simple cable television).

dBW Decibel Watts gives an indication of transmit power: the higher the value, the higher the signal strength

Digital Information represented as discrete numeric values, eg in binary format (zeros or ones), as opposed to information in continuous or analogue form. Binary digits (bits) are typically grouped into 'words' of various lengths - 8-bit words are called bytes.

DVB  DVB stands for Digital Video Broadcasting, the European standard for digital TV. This standard provides a very high-speed, robust transmission chain capable of handling the many megabytes per second needed for hundreds of MPEG-2 digital TV channels.

Encryption  Encryption is the process of altering a video and/or audio signal from its original condition to prevent unauthorised reception. This is done electronically at a place in the supply chain between the contribution point (eg the originating studio) and the uplink towards the satellite. Decryption is the process of returning the video and/or audio to its original condition. Decryption is mostly done at the side of the end-user.

Fixed Wireless Local Loop Local connections that link customer equipment to the switching system in the central office using wireless connectivity - mostly used for broadband data purposes.

Geostationary orbit or Clarke Belt Named after its founder Arthur C. Clarke, the Clarke Belt is an orbit used by satellites at a height of 35,800 km, in which satellites make an orbit in 24 hours, yet remain in a fixed position relative to the Earth's surface.

LNB The Low Noise Block is an essential part of a satellite receiver which receives the signal and amplifies it for use. It is always located with the satellite antenna.

Mirroring servers Network server maintaining an identical copy of its files in (a) another network server, or (b) a redundant drive in the same server. Note: Mirroring can be used as a rudimentary backup system for the original files, but is more often used to spread out the access load for popular sites, eg web sites, by offering users several different locations from which identical files can be accessed.

Mono One or single channel as opposed to stereo or dual channel.

MPEG MPEG is the 'Moving Picture Experts Group', working under the joint direction of the International Standards Organisation (ISO) and the International Electro-Technical Commission (IEC). This group works on standards for digital video compression and file formats. The purpose is to standardise compressed moving pictures and audio. The most notable current MPEG standards are MPEG-1, MPEG-2 and MPEG-4.

Multicasting Transmitting information to a well-defined and controlled group of users on your network

Multiplexer Combines several different signals (eg video, audio, data) onto a single communication channel for transmission. De-multiplexing separates each signal at the receiving end.

Narrow-band A low-bandwidth (low capacity) communications path. Narrow-band networks are designed for voice transmission (typically analogue voice), but which have been adapted to accommodate the transmission of low-speed data

Orbit  The path taken by a satellite. A satellite is usually kept in its orbit through a combination of natural forces, mainly the force of gravity, and on-board resources.

PoP (Point of Presence) The specific physical place where you make connection to the Internet.

Proxy A proxy server can serve several purposes; it can hold the most commonly and recently used content from the World Wide Web for users (versus having to go all the way to the server on which it was originally stored) thus providing quicker access. Also it can filter web content (so it can be used by schools and libraries) and it can convert web pages to match the capabilities of the receiving software and/or hardware.

Rain attenuation Loss of signal at Ku- or Ka-band frequencies due to absorption and increased sky-noise temperature caused by heavy rainfall. Attenuation is the decrease in the amplitude of a signal. In video communications this usually refers to the loss of power of electromagnetic signals between a transmitter and the receiver during the process of transmission. Thus the received signal is weaker or degraded when compared to the original transmission.

Revolution The cycle that normally takes a satellite around Earth.

SoHo Small Office - Home Office Term to describe professional office solutions of the smallest scale: offices for people that work from home like teleworkers, home-based professionals like GPs, lawyers, but also small enterprises (shops, workshops etc). SOHO also exists as a satellite-related term but in this context refers to the scientific SOlar and Heliospheric Observation Satellite.

Stereo Two or more independent channels of information. Separate microphones are used in recording and separate speakers are used in reproduction.

Sub-carrier A second signal sent alongside a main signal to carry additional information. In satellite television transmission, the video picture is transmitted over the main carrier. The corresponding audio is sent via an FM sub-carrier. Some satellite transponders carry as many as four special audio or data sub-carriers whose signals may or may not be related to the main programming.

Thrusters Small axial jets used by the satellite to maintain its orbit. These are often fuelled by drazine or bi-propellant. In time, ion-engines will probably replace such thrusters.

Two-way Operating method in which transmission is possible in both directions of a telecommunication channel

Unicasting Data is delivered to only one user within a network as opposed to multicasting. Each packet in a unicast contains a user ID number. The user's ID must match the ID in the header of the unicast packet, only then can data be received.

VSAT Meaning literally 'Very Small Aperture Terminal', the term refers to any fixed satellite terminal that is used to provide interactive or receive-only communications. VSATs are used for a wide variety of telecommunications applications, including corporate networks, rural telecoms, distance learning, telemedicine, disaster recovery, ship-board communications, transportable 'fly-away' systems and much more.

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