Traffic Management for the Available Bit Rate (ABR) Service in ...

Figure 5.19: Network con guration with upstream bottleneck.
5.7 Results with Packet Train Workload
The most commonly used tra c pattern in congestion simulations is the so called
"in nite source model." In this model, all sources have cells to send at all times. It is a
good starting con guration because, after all, we are comparing schemes for overload
and if a scheme does not work for in nite source it is not a good congestion scheme. In
other words, satisfactory operation with in nite source model is necessary. However,
it is not su cient. We have found that many schemes work for in nite source models
but fail to operate satisfactorily if the sources are bursty, which is usually the case.
In developing the OSU scheme, we used a packet train model to simulate bursty
tra c [47]. A packet train is basically a \burst" of k cells (probably consisting of
segments of an application PDU) sent instantaneously by the host system to the
adapter. In real systems, the burst is transfered to the adapter at the system bus
rate which is very high and so simulating instantaneous transfers is justi ed. The
adapter outputs all its cells at the link rate or at the rate speci ed by the network in
case of rate feedback schemes. If the bursts are far apart, the resulting tra c on the
link will look like trains of packets with a gap between trains.
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