Dual Bus MAN's with Multiple-Priority Traffic

AbsfructThe IEEE 802.6 standard for metropolitan area networks does not provide multiple priority traffic for connectionless data services. A priority mechanism that was considered in earlier versions of 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 preemptive priorities and, at the same time, can satisfy minimal fairness requirements for transmissions at the highest priority level. In this study, a protocol with strictly preemptive priorities, i.e., a protocol that does not admit low-priority traffic if the load from highpriority traffic exceeds the capacity of the transmission channel, is presented. In addition, the protocol guarantees fairness for transmissions at the highest priority level. By introducing a general characterization of bandwidth allocation schemes for dual bus networks, existing priority mechanisms can be categorized according to the provided quality of service. The unique existence of a bandwidth allocation scheme for multiple priority traffic is shown with a full utilization of the channel capacity, with a fair distribution of bandwidth respective to traffic from a particular priority level and with preemptive priorities. The performance of the presented protocol is compared to existing proposals for multiple priority mechanisms. It is shown that adopting the new protocol results in shorter access delays for high-priority transmissions. The protocol allows the stations of the network to react quickly to load changes. It is shown that the effectiveness of the priority scheme, compared to priority schemes using the bandwidth-balancing mechanism, is less dependent on increasing the transmission speed of the network.