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

c) Variation in load: As described in section 8.3, even an in nite tra c source
running on top of TCP looks like a bursty source at the ATM layer. When
a number of such sources aggregate, the load experienced at the switch
can be highly variant.
d) Variation in capacity: The ABR capacity depends upon the link band-
width, and the bandwidth usage of the higher priority classes like CBR
and VBR, and can exhibit variation accordingly.
Due to these e ects, switches may make errors in measuring quantities which
they use to calculate feedback. Due to the out-of-phase e ect, the input rate
and overload measured in ERICA over the last interval is zero, because no cells
are seen in the forward direction. Due to the clustering e ect, the number of
active sources may be underestimated in any interval (for example, N di erent
sources may be seen in each interval when the total number of sources is 2N),
leading to overallocation of rates in ERICA.
The third problem is variation in load. Due to the variation in load, it is
possible to have a long period of underload, followed by a sudden burst which
builds queues. As a result the maximum queue may be large even though the
utilization/throughput is low. Schemes like ERICA can track the variation in
load and lter it, because we use the average load as a metric. However, several
schemes use the queue length metric exclusively. Queue length has a higher
variation than the average load, and it also varies depending upon the available
capacity. Further, a queue length of zero yields little information about the
utilization of the link. It has been argued that schemes which look at only the
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