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 ...
6.4 Forward CCR Used for Reverse Direction Feedback Earlier schemes [43] provided their feedback to the RM cells going in the forward direction. This ensured that the CCR in the RM cell was correlated to the load level measured by the switch during that interval. However, the time taken by the forward going RM cell to travel back to the source was long and this slowed down the response of the system. Figure 6.1: Reverse direction feedback Switches can indicate their feedback to the sources in the reverse path of the RM cell. The backward going RM (BRM) cell takes less time to reach the source than the forward going RM (FRM) cell which has to reach the destination rst. Thus, the system responds faster to changes in the load level. However, the CCR carried by the BRM cell no longer re ects the load level in the system. To maintain the most current CCR value, the switch copies the CCR eld from FRM cells, and uses this information to compute the ER value to be inserted in the BRM cells. This ensures 159
that the latest CCR information is used in the ER calculation and that the feedback path is as short as possible. Figure 6.1 shows that the rst RM cell carries (in its backward path), the feedback calculated from the information in the most recent FRM cell. The CCR table update and read operations still preserve the O(1) time complexity ofthealgorithm. 6.5 Single Feedback in a Switch Interval The switch measures the overload, the number of active sources and the ABR capacity periodically (at the end of every switch averaging interval). The source also sends RM cells periodically. These RM cells may contain di erent rates in their CCR elds. If the switch encounters more than one RM cell from the same VC during the same switch interval, then it uses thesamevalue of overload for computing feedback in both cases. For example, if two RM cells from the same VC carried di erent CCR values, then the feedback in one of them will not accurately re ect the overload. As a result, the switch feedback will be erroneous and may result in unwanted rate oscillations. The switch thus needs to give only one feedback value per VC in a single switch interval. The above example illustrates a fundamental principle in control theory, which says that the system is unstable when the control is faster than feedback. But the system is unresponsive if the control is slower than feedback. Ideally, thecontrol rate should be matched to the feedback rate. In our system, the delay between successive feedbacks should not be greater than the delay between successive measurements (controls). 160
- Page 135 and 136: 5.2.3 Use Measured Rather Than Decl
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6.4 Forward CCR Used <strong>for</strong> Reverse Direction Feedback<br />
Earlier schemes [43] provided <strong>the</strong>ir feedback to <strong>the</strong> RM cells go<strong>in</strong>g <strong>in</strong> <strong>the</strong> <strong>for</strong>ward<br />
direction. This ensured that <strong>the</strong> CCR <strong>in</strong> <strong>the</strong> RM cell was correlated to <strong>the</strong> load level<br />
measured by <strong>the</strong> switch dur<strong>in</strong>g that <strong>in</strong>terval. However, <strong>the</strong> time taken by <strong>the</strong> <strong>for</strong>ward<br />
go<strong>in</strong>g RM cell to travel back to <strong>the</strong> source was long and this slowed down <strong>the</strong> response<br />
of <strong>the</strong> system.<br />
Figure 6.1: Reverse direction feedback<br />
Switches can <strong>in</strong>dicate <strong>the</strong>ir feedback to <strong>the</strong> sources <strong>in</strong> <strong>the</strong> reverse path of <strong>the</strong> RM<br />
cell. The backward go<strong>in</strong>g RM (BRM) cell takes less time to reach <strong>the</strong> source than<br />
<strong>the</strong> <strong>for</strong>ward go<strong>in</strong>g RM (FRM) cell which has to reach <strong>the</strong> dest<strong>in</strong>ation rst. Thus, <strong>the</strong><br />
system responds faster to changes <strong>in</strong> <strong>the</strong> load level. However, <strong>the</strong> CCR carried by<br />
<strong>the</strong> BRM cell no longer re ects <strong>the</strong> load level <strong>in</strong> <strong>the</strong> system. To ma<strong>in</strong>ta<strong>in</strong> <strong>the</strong> most<br />
current CCR value, <strong>the</strong> switch copies <strong>the</strong> CCR eld from FRM cells, and uses this<br />
<strong>in</strong><strong>for</strong>mation to compute <strong>the</strong> ER value to be <strong>in</strong>serted <strong>in</strong> <strong>the</strong> BRM cells. This ensures<br />
159
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