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

4.14.1 Common Drawbacks
Though the evolution of switch schemes has yielded increasingly e cient and fair
algorithms, with reduction in implementation complexity, and a broader scope, many
of these proposals su er from a common set of drawbacks as listed in this section.
In general schemes, with a few notable exceptions, have not been comprehen-
sive in design, i.e., they either do not address all the goals of a switchscheme and/or
make too many assumptions about measurement related aspects of the scheme.
For example, many schemes do not address the issue of how to measure
the ABR demand. The lack of information about the demand may lead to under-
allocation of rates. Several schemes (like those which use the concept of Mean ACR
(MACR)) approximate the average demand per connection. However, if the total
demand (aggregate input rate) is not measured, the scheme could be consistently
making estimation errors.
Other schemes do not monitor the activity of sources, and may overlook a
source becoming temporarily idle. If the idle source is considered while determining
allocations for all other sources, the allocations for the other sources may be reduced.
In brief, measurement is necessary to track the current network state used by the
scheme. Ideally, a scheme should measure every component of the network state it
uses for its calculations.
Another issue is how to measure the scheme metrics when there is high
variation in the tra c demand and available capacity. Several metrics need
to be observed over intervals of time and averaged over manysuchintervals to smooth
out the e ects of such variation. The length of the interval is a key factor in a
tradeo between quick response and accurate response. Implementation issues include
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