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Exact Delay Analysis of Packet-Switching Concentrating Networks

We present an exact delay analysis for a packet-switching concentrating system consisting of an arbitrary number of active stations connected in tandem by unidirectional, asynchronous transmission lines, all with identical transmission rate. Packetized messages from exogenous sources enter the system at every station, are handled from station to station in a store-and-forward fashion, and exit at the downstream end; there are no intermediate departures. The service discipline at each station is either FCFS, fixed priority, or alternating priority, the transmission of a packet is not interrupted while in progress. We assume that the packets have identical length (in bits), that the sources are independent and that each source generates batch Poisson traffic. The FCFS case was solved by Kaplan [7] and Shalmon and Kaplan [19]. Here we analyze the priority disciplines. For the packets and messages from each source, we obtain the steady-state moment generating functions for their end-to-end waiting times. We offer simple formulas for the corresponding mean waiting times, and show that the Poisson approximation for departures is unreliable. The analysis for all three disciplines generalizes to the case where the line capacities are nonincreasing in the direction of the flow, and for the priority disciplines, it further generalizes to a concentrating tree network, and to an arrival process somewhat more general than batch Poisson.