Traffic Management for the Available Bit Rate (ABR) Service in ...
Traffic Management for the Available Bit Rate (ABR) Service in ... Traffic Management for the Available Bit Rate (ABR) Service in ...
An alternative way of averaging the quantities is by a xed time interval, T. This ensures that any source sending at a rate greater than (one cell/T) will be encountered in the averaging interval. This interval is independent of the number of sources, but is dependent upon the minimum rate of the source. In addition to this, if the aggregate input rate is low, the xed-time interval is smaller than the xed-cells interval. However, when there is an overload, the xed-cells interval provides faster response. One way of combining these two kinds of intervals is to use the minimum of the xed-cell interval and the xed-time interval. This combination ensures quick response for both overload and underload conditions. But it still su ers from the disadvantages of a xed-cell interval, where N and per-VC CCR cannot be measured accurately [62]. Another strategy for overcoming this limitation is to measure N and per-VC CCR over a xed-time interval, and the capacity and load factor over the minimum of the xed-cell and xed-time interval. The time intervals can be di erent as long as some correlation exists between the quantities measured over the di erent intervals. Typically, the intervals to measure CCR and N would be larger to get more stable estimates. 6.13 Selection of ERICA Parameters Most congestion control schemes provide the network administrator with a number of parameters that can be set to adapt the behavior of the schemes to their needs. A good scheme must provide a small number of parameters that o er the desired 169
level of control. These parameters should be relatively insensitive to minor changes in network characteristics. ERICA provides a few parameters which are easy to set because the tradeo s between their values are well understood. Our simulation results have shown that slight mistuning of parameters does not signi cantly degrade the performance of the scheme. Two parameters are provided: the target Utilization (U) and the switch measurement interval. 6.13.1 Target Utilization U The target utilization determines the link utilization during steady state condi- tions. If the input rate is greater than Target Utilization Link Capacity, then the switch asks sources to decrease their rates to bring the total input rate to the de- sired fraction. If queues are present in the switch due to transient overloads, then (1 ; U) Link Capacity is used to drain the queues. Excessively high values of target utilization are undesirable because they lead to long queues and packet loss, while low target utilization values lead to link underuti- lization. The e ectiveness of the value of target utilization depends on the feedback delay of the network. Transient overloads can potentially result in longer queues for networks with longer feedback delays. Due to this, smaller target utilization values are more desirable for networks with long propagation delays. Our simulation results have determined that ideal values of target utilization are 0.95 and 0.9 for LANs and WANs respectively. Smaller values improve the perfor- mance of the scheme when the tra c is expected to be highly bursty. 170
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- Page 169 and 170: eing divided into eight non-overlap
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- Page 173 and 174: have: and y 0 = y(1 ; ) z x + y 0 =
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- Page 185 and 186: efore competing sources can receive
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level of control. These parameters should be relatively <strong>in</strong>sensitive to m<strong>in</strong>or changes<br />
<strong>in</strong> network characteristics.<br />
ERICA provides a few parameters which are easy to set because <strong>the</strong> tradeo s<br />
between <strong>the</strong>ir values are well understood. Our simulation results have shown that<br />
slight mistun<strong>in</strong>g of parameters does not signi cantly degrade <strong>the</strong> per<strong>for</strong>mance of <strong>the</strong><br />
scheme. Two parameters are provided: <strong>the</strong> target Utilization (U) and <strong>the</strong> switch<br />
measurement <strong>in</strong>terval.<br />
6.13.1 Target Utilization U<br />
The target utilization determ<strong>in</strong>es <strong>the</strong> l<strong>in</strong>k utilization dur<strong>in</strong>g steady state condi-<br />
tions. If <strong>the</strong> <strong>in</strong>put rate is greater than Target Utilization L<strong>in</strong>k Capacity, <strong>the</strong>n <strong>the</strong><br />
switch asks sources to decrease <strong>the</strong>ir rates to br<strong>in</strong>g <strong>the</strong> total <strong>in</strong>put rate to <strong>the</strong> de-<br />
sired fraction. If queues are present <strong>in</strong> <strong>the</strong> switch due to transient overloads, <strong>the</strong>n<br />
(1 ; U) L<strong>in</strong>k Capacity is used to dra<strong>in</strong> <strong>the</strong> queues.<br />
Excessively high values of target utilization are undesirable because <strong>the</strong>y lead to<br />
long queues and packet loss, while low target utilization values lead to l<strong>in</strong>k underuti-<br />
lization. The e ectiveness of <strong>the</strong> value of target utilization depends on <strong>the</strong> feedback<br />
delay of <strong>the</strong> network. Transient overloads can potentially result <strong>in</strong> longer queues <strong>for</strong><br />
networks with longer feedback delays. Due to this, smaller target utilization values<br />
are more desirable <strong>for</strong> networks with long propagation delays.<br />
Our simulation results have determ<strong>in</strong>ed that ideal values of target utilization are<br />
0.95 and 0.9 <strong>for</strong> LANs and WANs respectively. Smaller values improve <strong>the</strong> per<strong>for</strong>-<br />
mance of <strong>the</strong> scheme when <strong>the</strong> tra c is expected to be highly bursty.<br />
170
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