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10/5/2009
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A widely used term for the point between two dissimilar networks. An international gateway is an exchange which interfaces between the network carrying national telephone traffic and the links to other countries. In a cellular system a gateway mobile switching centre (GMSC) is the interface between the traffic within the cellular system and the PSTN. These are consistent with a gateway from a confined network (or walled garden) to the outside world.
Gateways often include the function of protocol conversion, such as between a local area network (LAN) serving PCs and a mainframe computer which uses a different protocol or in a softswitch where an H323 enabled network connects to a non-H323 enabled network.
Generalised Multi-Protocol Label Switching (GMPLS)
A protocol suite which was invented by the IETF to allow transport networks to be controlled in a similar way to IP networks using MPLS methods.
Some applications of satellites, notably for communications, require the satellite to appear stationary above the earth so that it is always in a known position for line-of-sight communication with specific earth stations. This can be accomplished with a geostationary earth orbit (GEO) which is just less than 36,000 km (more than 22,000 miles) above the equator and takes into account the speed of rotation of the earth. In practice the satellite does not appear in a fixed position but moves around in a 'box' or 'window' due to a number of factors including gravitational pulls from the sun and the non-spherical nature of Earth. Maintaining position requires energy which is minimised by defining the window.
Giga - see units and symbols.
This is the proportion of calls that are allowed to fail during the busy hour due to the limitation of switching plant. In any telephony system the theoretical maximum traffic carried would occur if half the customers were talking to the other half. Fortunately for the provision of an economically sustainable telecommunications system this is never likely to happen. The mathematical probability of the number of simultaneous conversations at any particular instant for a telephony system can be determined taking into account calling rates and holding times. The chosen grade of service used for design is always a balance between customer acceptability of the service and its cost on the one hand and the cost of providing that service on the other. See also Erlang.
An aspect of circuit-switching design where the sources of traffic are divided into a convenient number of groups, and arrangements are made to provide individual trunks for the early choices of each group with a progressive system of commoning so that traffic from more and more groups is offered to the later choices. This assumes that the switch always searches the trunks in the same order.
Grid computing makes use of a large number of computers in a network and applies them to a single problem at the same time. To a user it looks like one computer but it is is really a virtual computer using distributed and parallel processing techniques to appear as one set of resources - not just processing but networking and storage capacity as well. It requires the use of software co-operatively working between many, up to thousands, of computers. It makes use of an emerging set of open standards and protocols such as the Open Grid Services Architecture (OGSA). It is usually applied to a problem that requires a great number of computer processing cycles, or access to large amounts of data. It can be a public collaboration (in which case it is also sometimes known as a form of peer-to-peer computing) but it can also be used within organizations who can optimise computing and data resources by pooling them for major tasks.
A ground or surface wave is a form of propagation when the transmitting and receiving antennas are close to the surface of the earth and are vertically polarised. This wave, supported at its lower edge by the presence of the ground, is of practical importance at broadcast and lower frequencies (medium wave/long wave).
A term used in time division multiplex (TDM) systems (analogue) to describe a 48kHz bandwidth containing 12 voice channels. Groups were aggregated into Supergroups (5 Groups making 60 channels). A flexibility frame where groups were routed to and from particular equipment racks and shelves was called a Group Distribution Frame (GDF).
Group delay - see delay
A term formerly used in the UK network, mainly before digitalisation, to describe a telephone exchange which handled trunk lines (known as junction circuits) from several local exchanges and interconnected them to other GSCs, usually through other trunk level switches. A GSC was typically co-located with a local exchange (i.e. one which terminates access lines).
The global system for mobile communications.
A special version of GSM adapted for railway use.
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