Efficient CSMA/CD-based protocols for multiple priority classes

Efficient, reservation-based CSMA/CD protocols for handling multiple-priority-class traffic are presented. 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-persistant 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