Analysis of an ATM Multiplixer
with Prioritized Service
Abstract
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.
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