Selective reject ALOHA/FCFS: An advanced VSAT channel access protocol

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Abstract
This paper proposes and evaluates selective reject (SREJ) ALOHA/FCFS, a highly efficient, flexible and robust multiaccess protocol for very small aperture terminal (VSAT) applications. SREJ‐ALOHA with locally synchronous FCFS retransmission scheduling is a ‘semi‐compatible’ enhancement of asynchronous SREJ‐ALOHA, which has earlier been shown to be an attractive satellite channel sharing protocol for low volume bursty terminals with variable length messages. The FCFS retransmission scheduling policy proposed here broadens the applicability of SREJ‐ALOHA by providing higher maximum throughput, particularly in the important source traffic scenario with bimodal message length distributions. Improved channel‐sharing efficiency and delay are achieved by exploiting time‐of‐arrival feedback naturally embedded in an unslotted SREJ‐ALOHA channel to develop an implicit time‐of‐arrival based (FCFS) ordering for conflict‐free, locally synchronous retransmission of most contending messages. An analytical procedure for evaluation of stability, throughput and delay for a general finite source, variable message length traffic model is described. The model is used to demonstrate that random access SREJ‐ALOHA/FCFS typically achieves a maximum throughput in the region of 0·4–0·45 along with excellent stability and delay properties. In addition, a natural extension of the protocol which supports implicit reservation for long messages is described and shown to offer a high maximum throughput (>0·6), comparable to that achieved by highly co‐ordinated slotted systems, for example mixed interactive/file‐transfer traffic scenarios.

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