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

The gain factor (K), a parameter of the smith predictor determines the rate of con-
vergence to a steady state. There is also a tradeo between the bu er space needed,
maximum achievable throughput, and the maximum RTD estimation error supported.
The queue implementation (FIFO or per-VC queuing) also has a signi cant impact
on convergence. In the default case, the scheme requires a separate Smith Predictor
for each VC. The conversion to the single predictor, and the implementation of the
FIFO service at the switches requires additional complexity attheswitches.
The challenge faced by the scheme designers was to estimate the network delays
accurately. Errors in delay would cause the system to be of a higher order. Due to
these constraints, the default implementation of the scheme requires per-VC queuing
at the switches, and the rate computation to be done at the source end system.
Another reason for this was that the round trip times of VCs (required for the smith
predictors) can be estimated better at the sources rather than at all switches. Since
the ATM Forum standard [32] does not specify rate computation at the source end
system or provide hooks for measuring the round trip time at the source end system,
the scheme is incompatible with the standards. Note also that the ATM Forum
standard expects the switch to compute rates and feedback the rates and not the
queue length.
One contribution of the scheme is in its mechanisms for estimating the round trip
delays. The scheme uses two mechanisms for dealing with delays, acting in di erent
time scales: a) a long time scale delay, keeping track ofthevariation of the round trip
delay due to queuing at intermediate switches and b) a short time scale delay, which
is called \virtual feedback." The latter mechanisms measures the variability of the
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