A fair distributed queue dual bus access method

The distributed queue dual bus (DQDB) has emerged as a proposed draft standard of the IEEE 802.6 Metropolitan Area Network Working Group. DQDB nodes using the queued arbitrated access mechanism are shown to access the transmission medium in a way that depends on their physical locations with respect to other nodes. The fundamental reasons for this asymmetric access property are analyzed in detail and simulation modeling results are shown that verify the analysis. A modified access control mechanism is proposed that augments the normal DQDB information about downstream nodes with status information about upstream nodes, thereby allowing the distributed queue to behave like a centralized queue with a round-robin queueing discipline. This method is based on distributed control of the slot reservation requests in order to minimize the effects of propagation delay and allow multiple simultaneous entries in the distributed queue. For each DQDB priority level, this may be accomplished by the addition of two bits in the access control field and two counters per bus. The proposed mechanism is shown to be robust in the presence of transmission errors in the access control bits