Page 57 - DCAP207_NETWORKS_DCAP406_COMPUTER_NETWORKS

Computer Networks/Networks

Notes can be tailored stronger and therefore can better deal with atmospheric distortion. The
antenna at transmitting side is centered in a concave, reflective dish that serves to focus the
radio beam, with maximum effect, on the receiving satellite antenna. The receiving antenna,
similarly, is centered in a concave metal dish, which serves to collect the maximum amount
of incoming signal.
Downlink frequency: It is the frequency used to transmit the signal from satellite to earth
station. In other words, the downlink transmission is focused on a particular footprint, or
area of coverage. The lower frequency, used for the downlink, can better penetrate the
earth’s atmosphere and electromagnetic field, which can act to bend the incoming signal
much as light bends when entering a pool of water.

Table 4.2: Example Uplink/Downlink Satellite Frequencies

Frequency Band Uplink/Downlink Frequency Range Example

C–band 6 GHz/4 GHz TV, Voice,
Videoconferencing
Ku–band 14 GHz/11 GHz TV, Direct Broadcast
Satellite/DSS
Ka–band 30 GHz/20 GHz Mobile Voice

Broadcast: The wide footprint of a satellite radio system allows a signal to be broadcast over a
wide area. Thereby any number (theoretically an infinite number) of terrestrial antennae can
receive the signal, more or less simultaneously. In this manner, satellites can serve a point-to-
multipoint network requirement through a single uplink station and multiple downlink stations.
Recently, satellites have been developed which can serve a mesh network requirement, whereby
each terrestrial site can communicate directly with any other site. Previously, all such
communications were required to travel through a centralized site, known as a head end. Such
a mesh network, of course, imposes an additional level of difficulty on the network in terms of
management of the flow and direction of traffic.

General Properties of Satellite Communication

Configuration: Satellite communication systems consist of antennae and reflective dishes,
much as in terrestrial microwave. The dish serves to focus the signal from a transmitting
antenna to a receiving antenna. The send/receive dishes that make up the earth segment
are of varying sizes, depending on power levels and frequency bands. They generally are
mounted on a tripod or other type of brace, which is anchored to the earth, pad or roof, or
attached to a structure such as building. Cables connect the antennae to the actual transmit/
receive equipment. The terrestrial antennae support a single frequency band for example,
C–band, Ku–band or Ka–band. The higher the frequency bands the smaller the possible
size of the dish. Therefore, while C–band TV dishes tend to be rather large, Ku–band DBS
(Direct Broadcast Satellite) TV dishes tend to be very small. The space segment dishes are
mounted on a satellite, of course. The satellite can support multiple transmit/receive
dishes, depending on the various frequencies, which it employs to support various
applications, and depending on whether it covers an entire footprint or divides the footprint
into smaller areas of coverage through the use of more tightly focused spot beams. Satellite
repeaters are in the form of number of transponders. The transponders accept the weak
incoming signals, boost them, shift from the uplink to the downlink frequencies, and
transmit the information to the earth stations.

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