Analysis of an ATM Multiplixer
with Prioritized Service

J. F. Meyer, S. Montagna, R. Paglino, and A. Puglisi


We examine an ATM multiplexer that is able to support two classes of service having qualities specified at the cell level. The first is a Guaranteed Bandwidth (GB) service which ensures very small cell loss probabilities and cell delays. The amount of statistical multiplexing permitted for such connections is therefore quite limited. The second is one suited to non-real-time Variable Bit Rate (VBRnr) sources where much longer delays can be tolerated. In this case, larger buffers and higher levels of statistical multiplexing can be exploited to increase the utilization of the output link. To accommodate these differences, we consider a multiplexer with two buffers, one for each service class, where the GB connections have service priority. Specifically, the low-priority VBRnr traffic is served only when there are no cells in the buffer for the GB connections. Both types of traffic are modeled by On/Off sources. The principal aim of the investigation is to analyze the steady-state behavior of the low-priority buffer with respect to two hypotheses concerning the guaranteed traffic. The first presumes that the GB connections are allocated according to their peak bit rates; in this case, the analysis is exact. The second hypothesis permits partial statistical multiplexing of the GB traffic; here, the exact method is extended so as to provide an approximate solution. A simpler model is then considered for this purpose, which suffices if the low-priority buffer is sufficiently large. In conclusion, some attention is devoted to the transient behavior of this buffer, where we find that congestion probabilities can exceed those experienced under steady-state conditions.

Keywords: ATM, multiplexer, service priority, analysis.

Compressed PostScript File: