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An effective scheme for pre-emptive priorities in dual bus Metropolitan Area Networks

The IEEE 802.6 standard for Metropolitan Area Networks does not provide multiple priority traffic for connectionless data services. A priority mechanism that was considered for the standard showed to be not effective. As of now, there exists no protocol for multiple access dual bus networks that is able to implement pre-emptive priorites and, at the same time, can satisfy minimal fairness requirements for transmissions at the highest priority level. In this study, a protocol with strictly pre-emptive priorities, i.e., a protocol that does not admit low priority traffic if the load from high priority traffic exceeds the capacity of the transmission channel, is presented. The protocol is derived from a unique bandwidth allocation scheme with a full utilization of the bus capacity, with a fair distribution of bandwidth respective to traffic from a particular priority level and with pre-emptive priorities. The performance of the presented protocol is compared to a priority mechanism that is based on the bandwidth balancing mechanism. It is shown that adopting the new protocol results in shorter access delays for high priority transmissions.