FQDB: a fair multisegment MAC protocol for dual bus networks

The fair queue dual bus (FQDB) medium-access control (MAC) protocol for dual bus networks, which is intrinsically fair in bandwidth sharing and retains all the positive features of distributed-queue dual-bus (DQDB) systems including simplicity, robustness, flexibility, 100% bandwidth utilization, and zero access delay at very low load, is presented. Features which make this protocol appealing with respect to others DQDB alternatives are represented by the fact that FQDB is compatible with the DQDB standard with respect to the physical arrangement and attachment to the bus, the XOR write tap, the access control field (ACF) length, and the three priority levels. Nevertheless, its properties are attractive also for protocols operating at 1 Gb/s and up. Fairness is formally proved, and it is shown that a bandwidth sharing pattern different from the uniform one can easily be enforced, as may be the case if bridges or other large users are presented. Queueing disciplines, priorities, and implementation issues are discussed, and comparisons obtained by simulation are given