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

the steady state. The system operates at the knee of the throughput delay-curve as
shown in Figure 3.1.
The OSU scheme keeps the steady state bottleneck link utilization in the target
utilization band (TUB). The utilization is high and the oscillations are bounded by
the TUB. Hence, in spite of oscillations in the TUB, the load on the switch is always
less than one. So the switch queues are close to zero resulting in minimum delay to
sources.
The target utilization and target utilzation band per-link parameters are set by
the network manager based on the cost of the bandwidth, and the anticipated degree
of variance in the network demand and capacity. The target utilization a ects the rate
at which the queues are drained during overload. Ahighertarget utilization reduces
unused capacity but increase the time to reach the e cient region after a disturbance.
A lower target utilization may be necessary to cope with the e ects of variance in
capacity and demand due to the introduction of errors introduced in measurement
as a result of variance. A wide TUB results in a faster progress towards fairness. In
most cases, a TUB of 90%(1 0.1) is a good choice. This gives a utilization in the
range of 81% to 99%.
5.2.2 Parameters
The OSU scheme requires just three parameters: the switch averaging interval
(AI) , the target link utilization (U) , and the half-width of the target utilization
band ( ).
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