Performance issues with the SAE LTPB and the IEEE 802.4 protocols

The LTPB (linear token passing bus) protocol is a timed token protocol designed by the SAE (Society of Automotive Engineers) to guarantee a specified (and usually low) latency delay for the highest-priority traffic class, even in the case of traffic peaks in other priority classes, fair access among all the stations connected to the bus, and an efficient use of the bus bandwidth. These requirements are met by requiring that each station regulate the token rotation time through a token holding timer and token rotation timers. No station faults, no modification of the logical ring, and no token loss are assumed. The behavior of the LTB protocol is analyzed under various traffic conditions. Performance evaluation is conducted using the simulation tool SPHINX. Rules are established for choosing timer values in order to achieve the required latency delay for any priority class for a given bus load. The optimum message size for file transfers is also determined. The main difference between LTPB and IEEE 802.4 in an error-free environment concerns the use of the token holding timer. How the conclusions can be applied to the IEEE 802.4 protocol is examined