Ad-hoc wireless networking: contention free multiple access

In the relatively new field of wireless LAN, the choice of MAC protocol can affect the performance of the system as much as the myriad of possibilities for the physical layer. The chosen MAC must be able to cope with the anomalies of the transmission medium and successfully deal with instructions from higher layers. Suggestions for an ad-hoc network have the result that the medium access (MAC) layer protocol must be capable of giving each user equal access to the radio channel with the minimum amount of interference and without the use of a controlling base station. This paper is concerned with two issues in ad-hoc networking: assessing the effects of variable coverage; and the possibility of multiple-access using a contention-free approach. Variable coverage will cause the participating systems to split and merge into groups of those with mutual “audibility”, thus frustrating the reliable propagation of multicast traffic, this is necessary to automate the construction of a network. A contention-free multiple access method is attractive at higher bit rates (in the order of 10s of Mbit/s) where multiple channels may be used together with equalisation to achieve an acceptable raw throughput. In such circumstances, collisions arising from a contention-based access technique will tend to lead to excessive retraining and reduce throughput