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Mạng VSAT (P5)

3 RADIO FREQUENCY LINK ANALYSIS The previous chapter has addressed the networking techniques which allow connection set-up and reliable transfer of information from one user terminal to another. This chapter will address information transfer at the physical level. Figure 5.1 reproduces an excerpt from Figure in order to put into perspective 4.5 the respective topicsof Chapter 4 and the present one. Chapter 4 dealt with the peer layers of the hub andVSAT interface within the VSAT network at datalink control and satellite channel access control levels. The present chapter focuses on the physical layer, which involves forward error correction (FEC), modulation and. | VSAT Networks G.Maral Copyright 1995 John Wiley Sons Ltd ISBNs 0-471-95302-4 Hardback 0-470-84188-5 Electronic 5 RADIO FREQUENCY LINK ANALYSIS The previous chapter has addressed the networking techniques which allow connection set-up and reliable transfer of information from one user terminal to another. This chapter will address information transfer at the physical level. Figure 5.1 reproduces an excerpt from Figure 4.5 in order to put into perspective the respective topics of Chapter 4 and the present one. Chapter 4 dealt with the peer layers of the hub and VSAT interface within the VSAT network at data link control and satellite channel access control levels. The present chapter focuses on the physical layer which involves forward error correction FEC modulation and coding. Indeed the satellite channel conveys information by means of modulated radio frequency carriers which are relayed by the satellite transponder and then received by the destination station. Noise contaminates the received carriers. Therefore the retrieved baseband signals are also contaminated analogue signals are noisy and data may contain erroneous bits. Basically it is not feasible to provide error-free transmission at the physical layer level. The only hope is to limit the bit error rate BER to an acceptable level constrained by cost considerations. It is the job of the upper layers and especially the data link layer to ensure error-free transmission by means of automatic repeat request protocols. The job is easier when the physical layer already provides clean information thanks to a low enough bit error rate. As the BER decreases the performance of the channel improves as illustrated in Figure 4.7. This chapter aims at providing the means to calculate the quality of the information contents delivered to the data link control layer. The quality of digital information is measured by the bit error rate BER which is the ratio of the number of bits received in error to the total number of .