Theory, Volume 1, Queueing Systems
Theory, Volume 1, Queueing Systems
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A conventional ALOHA satellite link uses a transponder which blindly echoes all up-channel traffic on the down-channel. An ALOHA channel can never be fully utilized, so an intelligent satellite could statistically multiplex the successful packets from several slotted ALOHA up-channels onto a single down-channel to conserve bandwidth, and hence reduce cost. We refer to this as a concentrated ALOHA system. Throughput, delay and stability effects are considered, varying the number of up-channels per down-channel and the satellite buffer size. Up- and down-channel bandwidths are assigned independent linear costs, and all performance comparisons are between constant cost systems. It is shown that the marginal increase in system performance drops off so quickly that a small number of up-channels maximizes throughput if up-channel bandwidth has a non-zero cost. This small number is a function of the buffer size and the relative cost of up- to down-channel bandwidth. It is also shown that, even if satellite buffer space is free, a small buffer minimizes average delay for some previously studied protocols of this type. A new protocol which improves performance and allows a large buffer to be used effectively is introduced and analyzed. Solving for throughput and delay in concentrated ALOHA systems provides new analytic and numeric results for the G/D/I queue with rest period equal to the service time.