Fair DQDB: The Rotating Slot Generator Scheme

We introduce the Rotating Slot Generator (RSG) scheme for dual bus architectures, a medium access control protocol appropriate for high capacity Metropolitan Area Networks (MANs). This scheme uses the looped bus architecture of the Distributed Queue Dual Bus (DQDB) MAN, in which slot generator hardware is incorporated into all stations. Its difference from DQDB is that during its operation the responsibility for generating slots moves periodically from station to station around the loop. Thus, station positions relative to the slot generator change continuously and, therefore, there are no favorable locations on the busses. We first present the system architecture and the RSG protocol. Then, we carry out a thorough investigation of the performance of the RSG scheme through simulation. We demonstrate the fairness of the protocol with respect to throughput and delay, and evaluate its performance in underload and overload conditions, under both a single and multiple priority classes of traffic. We also compare its performance with other variations of DQDB.