Major performance characteristics of the DQDB MAC protocol

Abstract

Major performance characteristics of the DQDB (Distributed Queue Dual Bus) MAC (medium access control) protocol in different traffic environments are shown. First, the author addresses throughput unfairness in the default protocol during heavy load periods, and the fact that under such a condition the vital priority mechanism cannot be guaranteed to be effective. He then focuses on the optional bandwidth balancing mechanism which prevents throughput dominance by a single user when other users have heavy-traffic demands, but which still exhibits a degree of unfair behavior owing to the rather long transients needed to achieve balancing. This particularly affects the low-traffic users. Finally, it is shown that DQDB behaves quite fairly up to high bus loading when sources have equal traffic characteristics, but that again unfairness is observed in asymmetric traffic environments.