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

Part b): When ABR load goes away, then the switches see a sudden underload
and allocate high rates to sources. Note that a switch whichseesno cells in an
interval or a small number of cells due to the fact that VBR load disappears is
dealing with transient, incomplete metric information. As a result, if it overal-
locates rates, then sudden queue spikes are seen. In the worst case, the queues
may grow unboundedly as a result of several such high priority VBR on/o
phases. The bu ering required under this condition is heavily dependent upon
the switch algorithm. We present the problem, simulation results, modi cations
we made to the ERICA algorithm, and results with the improved algorithm in
section 8.16 later in this chapter. We also model MPEG-2 tra c over VBR and
study its e ect on TCP tra c over ABR in section 8.17.
We have seen that the round trip time, feedback delay, the bandwidth variability
caused by VBR, and the switch algorithm are key factors which determine the bu er
requirements for TCP over ABR.
From items 4 (switch convergence time) and 5 (queue backlogs) above, we nd
that for le transfer over ABR, we require at least 3 RT T worth of bu er. Items
6 (two-way tra c) and 7 (VBR tra c) require a bu er of at least 5 RT T . Note
that the e ect of the averaging interval parameter dominates in LANs (because it
is much larger than RTT or feedback delay). Similarly, the e ect of the feedback
delay dominates in satellite networks because it can be much smaller than RTT. We
substantiate our claims with simulation results, where we will observe that the bu er
requirement is at most 3 RT T .
Though the maximum ABR network queues are small, the queues at the sources
are high. Speci cally, the maximum sum of the queues in the source and the switches
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