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

The target utilization (U) and the TUB present afewtradeo s. During overload
(transients), U a ects queue drain rate. Lower U increases drain rate during tran-
sients, but reduces utilization in steady state. Further, higher U also constrains the
size of the TUB.
Anarrow TUB slows down the convergence to fairness (since the formula depends
on ) but has smaller oscillations in steady state. A wide TUB results in faster
progress towards fairness, but has more oscillations in steady state. We nd that a
TUB of 90%(1 0.1) used in our simulations is a good choice.
The switch averaging interval a ects the stability of z. Shorter intervals cause
more variation in the z and hence more oscillations. Larger intervals cause slow
feedback and hence slow progress towards steady state.
The OSU scheme parameters can be set relatively independent of the target work-
load and network extent. Variance in measurement isthekey error factor in the OSU
scheme, and a larger interval is desirable to smooth the e ect of such variance. Some
schemes, on the other hand, are very sensitive totheworkload and network diameter
in their choice of parameter values. An easy way to identify such schemes is that
they recommend di erent parameter values for di erent network con gurations. For
example, a switch parameter may be di erent for WAN con gurations than in a LAN
con guration. A switch generally has some VCs travelling short distances while oth-
ers travelling long distances. While it is ok to classify a VC as a local or wide area
VC, it is often not correct to classify a switch as a LAN switch oraWAN switch. In
a nationwide internet consisting of local networks, all switches could be classi ed as
WAN switches. Note that the problem becomes more di cult when the scheme uses
many parameters, and/or the parameters are not independent of each other.
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