Fairness, priority and predictability of the DQDB MAC protocol under heavy load

In the spring of 1989, the IEEE 802.6 Metropolitan Area Network Standards Committee chose to modify the queue-arbitrated DQDB (Distributed Queue Dual Bus) protocol. The heavy load and overload behavior of this DQDB version is examined. Extensive simulation studies show the protocol behavior to be extremely complex so that this study is restricted to a carefully selected set of scenarios that present dominating behavioral features of the protocol. It is shown that the protocol is unfair, that node throughput is unpredictable during heavy load, and that the priority mechanism cannot be relied upon. It is also shown that network configurations exist which exhibit inverse priority behavior, i.e. less throughput with high priority than with low priority. A comparison of DQDB and reservationless slotted bus operation is presented. It is concluded that the modified DQDB media access protocol exhibits serious deficiencies during heavy load and overload conditions and that at loads low enough to reduce the damage of these adversities sufficiently the queue arbitration shows no advantage over simpler protocols such as slotted bus without reservation