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

In the real world, the transient response is almost as important as steady state
performance. Jain and Routhier [47] point out that most real world tra c is bursty
because most sources are transient. Transient response can be tested using transient
sources which start after other sources have started and/or stop before the other
sources have stopped. Good schemes must be able to respond rapidly to these load
transients and achieve optimal performance in the steady state. The di erence be-
tween steady state and transient performance is shown in gure 3.5.
The transient performance implies that the scheme should converge quickly from
overloaded or underloaded states to the steady state, given that the sources can
respond to the feedback. Typically, in linear control systems, the transient can either
be short and sharp (i.e., resulting in large transient queues), or slow and smooth
(underutilization and near-zero queues). On the other hand, if the scheme is optimized
just for transients, the steady state may exhibit oscillations. Wedesireascheme which
optimizes both the transient and the steady state performance, i.e., quickly converges
to a solid steady state from any initial conditions, and drains the queues produced
in the transient phase rapidly. The \congestion avoidance" steady state is nally
reached when both the rates and the queues stabilize.
3.5 Miscellaneous Goals
Robustness and Handling High Variation Workloads: Another complexity is-
sue for the ABR service (and in general for data service classes in high speed
integrated service networks) is the fact the available capacity for data tra c
is variable. Older networks typically had constant capacity links dedicated for
data transmission and the capacity was also lower. As a result, the congestion
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