Guaranteeing Synchronous Message Deadlines with the Timed Token Medium Access Control Protocol

We study the problem of guaranteeing synchronous message deadlines in token ring networks where the timed token medium access control protocol is employed. Synchronous bandwidth, defined as the maximum time for which a node can transmit its synchronous messages every time it receives the token, is a key parameter in the control of synchronous message transmission. To ensure the transmission of synchronous messages before their deadlines, synchronous capacities must be properly allocated to individual nodes. We address the issue of appropriate allocation of the synchronous capacities. Several synchronous bandwidth allocation schemes are analyzed in terms of their ability to satisfy deadline constraints of synchronous messages. We show that an inappropriate allocation of the synchronous capacities could cause message deadlines to be missed, even if the synchronous traffic is extremely low. We propose a scheme, called the normalized proportional allocation scheme, which can guarantee the synchronous message deadlines for synchronous traffic of up to 33% of available utilization.