R-express: Collision-free access protocol for packet radio networks

Abstract

In the paper an extension of the L-EXPRESS protocol, suitable for packet radio networks, is proposed. The modified protocol, PR-EXPRESS, preserves all basic features of the original L-EXPRESS and assures all users an ordered, round-robin collision-free access to the common broadcast radio channel. Similarly to L-EXPRESS, PR-EXPRESS protocol is composed of three mechanisms: ACCESS MECHANISM, CYCLE RESTART MECHANISM, and COLD RESTART MECHANISM. Differently from L-EXPRESS where all stations are ordered along the bus, PR-EXPRESS has to cope with the fact that propagation delays between stations depend on their geographical positions and no unique stations ordering according to the propagation delays is guaranteed. Two versions of the algorithm suitable for fixed and mobile stations environments are discussed.

The throughput analysis carried out in the paper shows that the channel utilization provided by PR-EXPRESS is for a large number of active stations as good as or better than the best CMSA policy in the case of mobile stations. If stations are fixed, the improvement in performance is much better and it favourably compares with networks based on coaxial cable operating either with CSMA-CD or L-EXPRESS.

A further advantage of PR-EXPRESS with respect to CSMA or ALOHA is that, because of its round-robin way of operating, the system has no stability problems and different classes of traffic can be easily managed.