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

section 9. Further, WAN switches would typically allocate some bandwidth
for aggregate ABR tra c to avoid zero capacity problems during congestion
(when the feedback loops are disrupted), and in general to reduce the variation
in available capacity for the service (which allows the switch algorithm to al-
locate rates more aggressively without worrying about e ects of errors due to
variation).
10.2.2 End-system issues
1. The end-system (whichisthenetwork interface card (NIC) for end-to-end ATM,
or an edge router in a backbone network) needs to implement the end-system
rules for ABR as speci ed in the standard. A mechanism needed for this imple-
mentation is one whichschedules cells of di erentVCs based upon a dynamically
changing set of ACRs. In practice, only a few ACR levels may be possible which
can lead to link underutilization. SES Rule 9 allows the source to reschedule
a cell on a VC based upon a new rate allocation. The implementation of this
mechanism per-VC can be di cult.
2. In the ABR service, the network allocated rate may not match the input rate of
aVC at a NIC. Mechanisms to control the actual sources of tra c are necessary
to avoid cell loss at the NIC. Further, the NIC needs to have a large number of
bu ers and typically needs to manage per-VC queues. Current implementations
are possible since the number of VCs per NIC is small. When ATM is deployed
in the backbone alone, then the cost increases in the edge routers/switches.
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