Cycle Compensation Protocol: A Fair Protocol for the Unidirectional Twin-Bus Architecture

The IEEE 802.6 Standard - Distributed Queue Dual Bus (DQDB) - for metropolitan area networks (MAN's) has been proposed. It is based on a unidirectional twin bus architecture. The DQDB protocol lays more emphasis on the overall channel utilization than the fair sharing of channel bandwidth by all the stations. In this paper, we first describe the unfairness problem in which the upstream stations occupy most of the channel bandwidth while the downstream stations get fewer chances to transmit their packets. Many proposed possible fixes are also discussed. We propose a protocol called Cycle Compensation Protocol (CCP), which ensures fairness regardless of the ratio of end-to-end propagation delay to the slot size and also achieves almost the same throughput and delay as those of DQDB. CCP also guarantees that the channel bandwidth acquired by a station is inversely proportional to the number of busy stations and will reach this state within a limited time delay.