A Conditional Probability Approach to Performance Analysis of Optical Unslotted Bus-Based Networks

Abstract

This paper presents a novel approach to the performance analysis of optical packet switching bus-based networks with unslotted Carrier Sense Multiple Access with Collision Avoidance (CSMA/CA) protocol. Because of the interdependence among bus nodes, an accurate performance analysis of such networks has been an open question for a long time. We model the bus as a multiple-priority M/G/1 queuing system with preemptive-repeat-identical (PRI) service discipline. To solve this model, we use a recurrent level-by-level analysis technique where the interference from higher levels (upstream nodes) is taken into account in terms of reappearance and disappearance rates for the server. The key features of this method are as follows. First, it specifically accounts for the distribution of voids seen by a node (via the number of attempts before a successful transmission) as a function of the node's position and offered network load. Second, it approximately computes the queue-length distribution at each node, and thus provides a tool for buffer size selection so as to meet a loss rate criterion. A comparison of our approximate model solution with network simulations indicates that our model generally offers good accuracy in assessing the performance of the network, including in terms of the queue-length distribution. Occasionally, the results of our model may deviate from simulation results. A discussion of the causes of such deviations, most likely to occur when nodes are close to saturation, offers additional insight into the properties of the bus-based network and its approximate solution.