Channel access protocols for half duplex satellite terminals

The author shows why carrier-sense multiple access (CSMA) is unusable with the combination of half duplex terminals and the success/failure channel. Distributed round-robin and distributed reservation protocols are given as examples of two classes of usable protocols. The author shows how to convert the success/failure channel to a something/nothing channel-(a channel in which accesses can be distinguished from no accesses)-so that group-testing techniques can be used to make reservations efficiently. The round-robin protocols are relatively simple but are generally not satisfactory for large terminal populations. The more complex reservation protocols using group testing perform satisfactorily over a wide range of conditions. Even with this sophisticated protocol, half duplex networks are not as efficient as full duplex networks. For full duplex terminals, ALOHA can be used to obtain relatively short reservation cycles that are almost independent of the number of terminals