On the Analysis of CSMA-Based Control Nets With Priorities and Multicast

In contemporary control nets, random access schemes like CSMA are commonly used and are often enhanced by fixed priorities for some message types to guarantee an upper bound for the transmission time of high-priority messages. The introduction of priorities has a significant impact on the overall quality of service, which means that messages with a low priority may have long transmission times or high loss rates if the whole network is not adequately dimensioned. Thus, capacity planning includes as one of the first steps a model-based analysis of the network. This paper presents an approach to analyze the quality of service of CSMA-based control nets. Based on open queueing networks, analytical formulas are derived that allow one to compute performance measures like mean throughputs, loss rates, and response times in an extremely efficient way and with limited information about the system load. Particular emphasis is spent on the analysis of priorities in networks using a variant of the CSMA access scheme which has not been analyzed analytically before. Furthermore, the integration of timeouts and multicast communication into the analysis approach is outlined.