Approximate Performance Model and Fairness Condition of the DQDB Protocol

The Distributed Queue Dual Bus (DQDB) proposal is the emerging candidate for the Metropolitan Area Network standard of the IEEE 802.6 committee. General properties of the DQDB access scheme such as fairness or priority issues are now well known. Therefore, different versions of the protocol have been released to address the existing problems. We develop a simple probabilistic model of the DQDB protocol under saturated condition in order to avoid excessive simulation runs. The model is based on a two step analysis taking into account the initial utilization state of the network. The accuracy of the model has been checked by comparaison with simulation and found to be very accurate. Moreover, we derive the fairness condition of the DQDB protocol and discuss some policies to achieve it.