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Chapter 20:
Multiple Access: Many Users on One Satellite
Satellites are capable of communicating with more than one earthstation at once. The suite of technologies, protocols, and procedures that make this possible - and that avoid (or minimize) the interference of possibly thousands of earthstations simultaneously linking to a single satellite - is called multiple access. Like every other technical decision, the choice of an appropriate multiple-access method depends on a detailed knowledge of the criteria of the communication being performed. As we saw back in Chapter 3, there are many such criteria, such as power restrictions, interference restrictions, spectrum efficiency, reliability, timeliness, and cost, to name a few, that may be important to the people communicating. Again, as with everything, there are trade-offs: no technique can maximize all of the operational characteristics at the same time. In a way, it is rather like the (in)famous “Iron Triangle” of criteria for completion of any project: you can get the job done using three criteria - quality, speed, cost - but you can maximize only two of the three at any one time! One of the most important issues that determines a good multiple-access design is the topology of the network: because different multiple-access methods have advantages and disadvantages relative to each other, different methods may be used on different links of the same communications applications. Furthermore, methods may be combined to increase the efficiency of communication.
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