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

of arithmetic averaging used above. Averages such as (x 1 + x 2 + :::xn)=n can be
replaced by the exponential average of the variable xi.
The ow chart of gure C.7 describes this averaging method.
Observe that the load factor thus calculated is never zero or in nity unless the
input rate or ABR capacity are always zero. If the input rate or the ABR capacity
is measured to be zero in any particular interval, the boundary cases for overload are
not invoked. The load level increases or decreases to nite values.
6.12 Time and Count Based Averaging
The load factor, available ABR capacity andthenumberofactive sources need to
be measured periodically. There is a need for an interval at the end of which the switch
renews these quantities for each output port. The length of this interval determines
the accuracy and the variation of the measured quantities. As mentioned before,
longer intervals provide lower variation but result in slower updating of information.
Alternatively, shorter intervals allow fastresponse but introduce greater variation in
the response. This section proposes alternative intervals for averaging the quantities.
The averaging interval can be set as the time required to receive a xed number
of ABR cells (M) at the switch in the forward direction. While this de nition is
su cient to correctly measure the load factor and the ABR capacity at the switch,
it is not su cient to measure the number of active VCs (N) or the CCR per VC
accurately. This is because the quantities N and CCR depend upon the fact that at
least one cell from the VC is encountered in the averaging interval. Moreover, when
the rates are low, the time to receive M cells may belarge.Hence the feedback inthe
reverse direction may be delayed.
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