A MULTICLASS PRIORITY-BASED SLOTTED-RING LAN AND ITS ANALYSIS

We consider a slotted-ring LAN that handles multiple classes of jobs with different priority requirements. We take n classes of jobs where class i jobs have preemptive priority over class j jobs, 1 less-than-or-equal-to i < j less-than-or-equal-to n and propose a new protocol that allocates slots in such a way that, on the average, the response time of a class i job is better than that of a class j (j > i) job. The protocol is distributed in the sense that any node can locally determine if it can use a slot, without exchanging any message with other nodes. The protocol is adaptive to the load distribution of the system; in other words, the lower classes of jobs automatically get a larger share of the slots when the load due to the higher classes of jobs decreases. An approximate analysis of response time distribution is carried out for two classes of jobs. Response time of higher priority traffic remains unaffected by lower priority traffic. As a side benefit to this analysis, the response time distribution of a single class slotted-ring LAN can be readily obtained. Extension to the analysis for n classes of jobs is discussed. We propose an extension to the original protocol that improves the response time of lower classes of jobs with only a marginal increase in the response time of higher classes of jobs. Simulation is carried out to confirm the analysis.