13.3 PROBLEMS IN SATELLITE COMMUNICATION

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The main attraction of satellite communication is that it provides communication facilities to any part on the earth—satellites are insensitive to the distance. However, the problems associated with satellites are:

Propagation delay: In a star network, the total delay from one VSAT to another VSAT is nearly 0.5 seconds if the VSAT has to communicate via the hub. This type of delay is not acceptable particularly for voice communication, because it results in echo and talker overlap. Propagation delay also causes problems for many data communication protocols such as TCP/IP. Special protocols need to be designed for data communication networks that use satellites.

If the VSAT communicates directly with another VSAT, the propagation delay is nearly 0.25 seconds. We will discuss multiple access techniques that facilitate direct communication from one VSAT to another VSAT.

Low bandwidth: As compared to the terrestrial media, particularly the optical fiber, the bandwidth supported by satellites is much less. Though present satellites provide much more bandwidth than the satellites of the 1970s and 1980s, the bandwidth is nowhere comparable to the optical fiber bandwidth.

Noise: Satellite channels are affected by rain, atmospheric disturbances, etc. As a result, the performance of satellite links is generally poor as compared to terrestrial links. If data is received with errors, the data has to be retransmitted by the sender. To reduce retransmissions, forward error correcting (FEC) codes are implemented.

The problems associated with satellite communication are: high propagation delay, low bandwidths as compared to terrestrial media, and noise due to the effect of rain and atmospheric disturbances.

Note

The large propagation delay in satellite networks poses problems for voice communication. High delay causes echo and talker overlap. Echo cancellers need to be used to overcome this problem.

The TCP/IP protocol stack used in computer communication will not perform well on satellite networks. The stack is suitably modified to overcome the problems due to propagation delay.

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