Efficient CSMA/CD-Based Protocols for Multiple Priority Classes

Efficient, reservation-based CSMA/CD protocols for handling mulliple-priority-class traffic are presented. Improving upon previous proposals, which require a reservation for each transmission attempt, the new protocols allow the transmission of a varying-length stream of packets with just a single reservation. Two protocols are proposed, one for a system in which the number of active users (contenders) in the currently highest priority class can be determined during the reservation period, and the other for a system in which this number cannot be determined. If the number of active users is known, then optimal p-persistence is used to transmit the packets; otherwise, a dynamically-determined combination of one-persistent and p-persistent periods is used to transmit the packets in the reserved priority group and to prevent lower priority users from interrupting the reserved priority transmission stream. Preemptive and nonpreemptive versions of the protocols are described. A simple analytical model is developed and used to obtain channel efficiency as a function of priority group size. Using this model, it is shown that the new protocols allow higher channel utilization than previous, reservation-per-attempted-transmission protocols, not only when arrivals within a priority class are clustered together, but also when just one packet is present in a priority class. The cost of preemptions is also analyzed, and the maximum number of preemptions per transmission group is found that still yields channel utilization higher than reservation-per-attempted-transmission protocols.