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

section, we outline some of the implementation problems for both switches and NICs,
and suggest solutions.
10.2.1 Switch issues
1. ABR requires the switch to process RM cells. The processing of RM cells takes
a longer time than processing data cells. As a result, the processing of RM cells
may disrupt the switch pipeline mechanism. Note that a pipeline mechanism
processes a job in several stages of a \pipeline" and assumes that the processing
time at each stage is simple and involves the same (small) amount of time.
Any task with disproportionate processing requirements disrupts the pipeline.
One solution is to extract such tasks from the stream before they enter the
pipeline, process them separately, and reinsert them into the stream. In this
case, we require a special hardware/software design to extract, process and
reinsert RM cells to/from the ABR VC. Note that this solution might extract
an RM cell from one point in the stream and reinsert it at a di erent point.
However, the tra c management 4.0 standard allows RM cells to be extracted,
processed separately, and reinserted, as long as the RM cell sequence within
each VC is maintained. Note that the correlation of the declared parameters
with the actual stream is lost under such conditions. For example, the CCR
eld may not be indicative of the rate of the VC (as measured) when the RM
cell is processed. Software processing of RM cells is possible if the Nrm (RM
cell frequeny parameter) is negotiated appropriately (eg: use a value like 192,
instead of the default value, 32).
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