Page 56 - DCAP406_DCAP_207_Computer Networks

Unit 4: Physical Layer-2

Cost: The acquisition, deployment and rearrangement costs of microwave can be high. Notes
However, it often compares very favorably with cabled systems, which require right-of-
way, trenching, conduit, splicing, etc.
Applications: Microwave originally was used for long haul voice and data
communications. Competing long distance carriers, microwave was found a most attractive
alternative to cabled systems, due to the speed and low cost of deployment where feasible,
however, fiber optic technology is currently used in this regard. Contemporary applications
include private networks, interconnection of cellular radio switches, and as an alternative
to cabled systems in consideration of difficult terrain.

4.2 Satellite Communication

Satellite radio, quite simply, is a non-terrestrial microwave transmission system utilizing a
space relay station. Satellites have proved invaluable in extending the reach of voice, data, and
video communications around the globe and into the most remote regions of the world. Exotic
applications such as the Global Positioning System (GPS) would have been unthinkable without
the benefit of satellites.

Figure 4.3: Satellites in Geostationary Earth Orbit

Satellite

Satellite dish Satellite dish

Contemporary satellite communications systems involve a satellite relay station that is launched
into a geostationary, geosynchronous, or geostatic orbit. Such satellites are called geostationary
satellite. Such an orbit is approximately 36,000 kms above the equator as depicted in Figure 4.3.
At that altitude and in an equatorial orbital slot, the satellite revolves around the earth with the
same speed as of that the speed of revolution of earth and maintains its relative position over the
same spot of the earth’s surface. Consequently, transmit and receive earth stations can be pointed
reliably at the satellite for communications purposes.

The popularity of satellite communications has placed great demands on the international
regulators to manage and allocate available frequencies, as well as the limited number of
orbital slots available for satellite positioning are managed at national, regional and international
levels. Generally speaking, geostationary satellites are positioned approximately 2º apart in
order to minimize interference from adjacent satellites using overlapping frequencies.
Such high frequency signals are especially susceptible to attenuation in the atmosphere. Therefore,
in case of satellite communication two different frequencies are used as carrier frequencies to
avoid interference between incoming and outgoing signals. These are:
Uplink frequency: It is the frequency used to transmit signal from earth station to satellite.
Table 4.2 shows the higher of the two frequencies is used for the uplink. The uplink signal

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