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

depth in chapter 8. Averaging the number of sources and averaging the load factor
as explained earlier in this chapter can also alleviate this problem.
6.22.9 ACR Retention
ACR retention is the problem which occurs when sources are not able to fully use
their rate allocations. For example, the input to the ATM end-system can be steady,
but have a rate lower than its ABR allocation (allowed cell rate). Another example
is an end-system which supports multiple VCs (to possibly di erent destinations) on
a single outgoing link. A VC may notbe able to use its ACR allocation because the
outgoing link is running at capacity. In such situations, the switches reallocate the
unused capacity to the other sources which are unconstrained. However, if the ACR
retaining sources suddenly use their allocations, a potential overload situation exists.
Figure 6.33 illustrates the performance of ERICA when there are ten VCs sharing
a link. This larger number of connections has been selected to demonstrate the
scalability of ERICA to more VCs, as well as to aggravate the problem of ACR
retention. Initially, the ten sources are retaining their ACRs, and each cannot send
at a rate of more than 10 Mbps. After 100 ms, all the sources suddenly start sending
at their full allocations. ERICA rapidly detects the overload and gives the appropriate
feedback asking sources to decrease their rates. All the ten sources stabilize at their
optimal rates after that.
Figure 6.34 shows how the per-VC CCR measurement option can mitigate the
overload situation arising when all the ACR retaining sources start transmission at
their full capacities. The per-VC CCR measurement results in more conservative
initial allocations, and hence smaller queues in this case.
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