Thin route telecommunications

INTELSAT and EUTELSAT transponders which are not required for trunk telecommunications may be leased to telecommunications operating organisations for national networks and for onward leas­ing to users for their private networks. A considerable number of other organisations have brought satellites into operation primarily to provide for national networks and private networks, and many more satellites are currently in the planning stage or under construc­tion, their frequency assignments and orbital locations being coor­dinated with those of other satellites which are already in operation. These national satellites are broadly similar to those of INTELSAT and EUTELSAT. They use the same frequency bands, although there is a growing tendency to concentrate on the frequency bands which are allocated exclusively to the fixed satellite service (see Table 51.1) in order to avoid constraints arising from sharing spec­trum with terrestrial radio systems. Their transponders are typically 36MHz in bandwidth and with a single carrier saturated power rating between 6 and 20 watts. They differ mainly in their use of antennas with higher gain and footprints which are limited, more or less, to national boundaries.

With few exceptions the earth stations which use these satellite facilities are simple and relatively low in cost. Some have antennas of substantial size, up to 10 metres in diameter and requiring active satellite tracking but most antennas are less than 5 metres in diameter, making them less environmentally objectionable, and their relatively broad beams do not need to track actively a satellite which is in an accurately maintained geostationary orbit.

These points of similarity apart, these earth stations and the facilities that they provide show considerable variety. The following can perhaps be taken to cover the majority of cases:

1. In a number of large countries where high quality terrestrial communication facilities do not link all centres of population, satellite networks have been set up to provide trunk connections between provincial centres. Elsewhere, where the population is sparsely distributed over inaccessible terrain, amongst moun­tains or across archipelagos, satellites may provide reliable communications to isolated communities. The trunk network configuration will usually resemble the INTELSAT global net­work, although on a smaller scale, using FDMA and with analogue telephone channels in frequency division multiplexed basebands frequency modulating the radio carriers, or digital channels in time division multiplexes phase shift modulating the carriers. Networks serving isolated communities are likely to use voice switched carriers for single telephone channels, analogue or digital.

2. Big corporations operating in well separated localities have found it worth while to lease satellite transponders for private telecommunications networks linking their establishments. In principle these networks resemble the national trunk networks described in (1) above, but they may be more flexible in their configuration, being designed to cater for needs such as tele­conferencing and high speed data transfer which are not well supplied at present by the public telecommunications networks.

3. The largest single use of satellite transponders in the fixed satellite service, at present, is for the distribution of television the head ends of cable broadcasting networks.

4. In recent years extensive use has developed, in particular in the USA, of very low cost earth stations for data networks. These earth stations use very small antennas, typically less than 2.5 metres in diameter; they are the so called Very Small Aperture Terminals (VSATs). Their use may become more widespread in the future.

Дата добавления: 2015-07-20