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

The key question in simulating the train workload is what happens when the
adapter queue is full? Does the source keep putting more bursts into the queue or
stops putting new bursts until permitted. We resolve this question by classifying the
application as continuous media (video, etc) or interruptible media (data les). In a
real system, continuous media cannot be interrupted and the cells will be dropped
by the adapter when the network permitted rate is low. With interruptible media,
the host stops generating new PDUs until permitted to do so by the adapter. We are
simulating only interruptible packet trains for ABR tra c.
For interruptible packet trains, the intertrain gap is governed by a statistical
distribution such as exponential. We use a constant interval so that we can clearly
see the e ect of the interval. In particular, we use one-third duty cycle, that is, the
time taken to transmit the burst at the link rate is one-third of the inter-burst time.
In this case, unless there are three or more VCs, the sources can not saturate the link
and interesting e ects are seen with some schemes. In real networks, the duty-cycle is
very small of the order of 0.01� the inter-burst time may be of the order of minutes and
the burst transmission time is generally a fraction of a second. To simulate overloads
with such sources would require hundreds of VCs. That is why we selected a duty
cycle of 1/3. This allows us to study both underload and overload with a reasonable
number of VCs. We used a burst of 50 cells to keep the simulation times reasonable.
Figures 5.22 and 5.23 show simulation results for the transient and the upstream
bottleneck con gurations using the packet train model.
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