Performance of a dual-bus fiber optic network operating under a probabilistic scheduling strategy

Abstract

Recent advances in fiber optic technology (viz. its promise to provide information-carrying capacity in the multi-Gbps range over long repeater-free distances) have triggered tremendous activity in the study of unidirectional bus networks spanning the local and metropolitan areas (because signal flow in the fiber is unidirectional). A popular network structure that has received significant attention is the Dual-bus Unidirectional Broadcast System (DUBS) network topology. Many of the earlier access mechanisms studied on this structure were based on round-robin-type scheduling or some variation thereof. However, since round-robin schemes suffer a loss of channel capacity because of their inter-round overhead (which can be significant for long high-speed buses), mechanisms that overcome this limitation have been proposed, including the IEEE 802.6 metropolitan area network (MAN) working committee's recommendation of the Distributed Queue Dual Bus (DQDB) protocol as a MAN standard. An alternate proposal is a probabilistic scheduling strategy, called the p i -persistent protocol, which has recently been studied for single-channel unidirectional bus systems. Our concern here is to apply this probabilistic scheduling strategy to each bus in DUBS, and study the corresponding network performance. In so doing, we allow stations to buffer multiple packets, represent a station's queue size by a Markov chain model, and employ an independence assumption. We find that the maximum network throughput approaches two pkt/slot with increasing buffer size. Further, the protocol's performance is insensitive to bus characteristics, and it appears to be particularly well suited for fiber-optic network applications requiring long distances and high bandwidth. Simulation results, which verify the analytical model, are also included.