Principles Digital Communication System & Computer Networks (Charles River Media Computer Engineering)

20.6 TCP/IP OVER SATELLITE LINKS

The TCP/IP protocol stack can be used in any network—the transmission medium can be cable, optical fiber, terrestrial radio, or satellite radio. However, when TCP/IP is used in satellite networks, the stack poses problems. This is due to the characteristics of the satellite channels. The problems are as follows:

• The satellite channel has a large propagation delay. Large delay causes timeouts in the flow control protocol. The source assumes that the packets have not reached the destination and resends the packets. As a result, the destination receives duplicate packets. This causes congestion in the network.

• The satellite channels have a larger Bit Error Rate (BER) than the terrestrial channels. As a result, packet losses will result in more retransmissions. When retransmission of packets is required, TCP automatically reduces the window size, though the network is not congested. As a result, the throughput of the channel goes down.

Satellite communication systems are characterized by large propagation delay and high Bit Error Rate. In such systems, using TCP/IP creates problems because of the timeouts for acknowledgements and retransmissions. TCP/IP protocols are suitably modified to work on satellite channels.

To overcome these problems, a number of solutions are proposed, which include the following:

• To improve the link performance, error-correcting codes are used. Errors can be corrected at the destination, and retransmissions can be reduced.

• Instead of using the flow control protocols at the transport layer, these protocols can be implemented at the datalink layer, so that the TCP layer does not reduce the window size.

• Instead of using a default window size of 16 bits in the TCP segment, 32 bits can be used to increase the throughput.

• For bulk transfer of information from the source to the destination, multiple TCP connections can be established.

• Another interesting technique used is called spoofing. A small piece of software will run at the source, generating the acknowledgements locally. The local TCP layer is cheated by the spoofing software. The spoofing software in turn receives the actual acknowledgement from the destination and discards it. If a packet is to be retransmitted because it was received in error at the destination, the spoofing software requests the TCP layer to resend the packet.

Many improvements in the TCP/IP protocol layers are required for its use in satellite networks.