It is imperative for the correct functioning of an SDH-based system to employ techniques capable of spreading out errors and performing enhanced error monitoring activities. (See the discussion on error control below.) Satellite Link AccessAccess methods typically seen in Local and Metropolitan Area Networks are not suited for use with satellite systems due to the high propagation delays created by the long distances to the satellites. LAN and MAN performance is dependent upon short transmission times whereas satellite systems are effective when utilized at maximum capacity. Therefore, an access method must be used in this system that “keeps the pipe full”. There are presently three basic access methods used in satellite systems. Unfortunately, none of these schemes are optimized for use with ATM technology. These three methods, Frequency Division Multiple Access (FDMA), Time Division Multiple Access (TDMA), and Demand Assignment Multiple Access (DAMA) can be modified from their present form to a configuration more suited for use in an ATM over satellite implementation.The FDMA access method divides the total available satellite bandwidth into equally sized portions. Each portion is assigned to one earth station for exclusive use by that station. This scheme thus eliminates errors and collisions since there is no signal interference between individual earth stations. In addition, FDMA can be used with smaller antennas. Unfortunately, however, FDMA requires guard bands for signal separation which is not conducive to the goal of maximum capacity usage in the system. (FDMA is also considered to be rather inflexible.)Unlike the subchannel frequency division of FDMA, the conventional TDMA access method divides the bandwidth into time slots. These time slots are usually equal-sized, however, variable time slots or allocation on demand configurations are also possible. Using a round-robin scheme, earth stations e...
