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 ...
or active. The Phantom algorithm [3] is an example of an O(1) space complexity algorithm. If the algorithm is not O(1) in space, it should utilize no more than a constant amount of space per connection (extra space in the VC table) to store per-VC parameters. The next requirement is for the algorithm to execute a constant number of steps to calculate feedback, especially when an RM cell is being processed. Such an algorithm is said to have an O(1) time complexity. Some algorithms like those developed in this dissertation may do some simple O(N) operations in the background (like theaveraging of measured quantities, clearing of bits etc). Such operations can also be e ciently compiled on parallel architectures to have alower execution complexity like O(log N). However, the algorithm should not require complex implementations like per-VC queuing and scheduling. 3.6 ABR Switch Scheme Limitations ABR switch algorithms are limited in the following respects: Initial Cell Rate of Sources: Sources negotiate the Initial Cell Rate (ICR) parameter from the switches along the path during connection setup. Switch algorithms attempt to allocate the available capacity among the currently active sources. In practice, though a large number of VCs are setup, only a few are active at any time. If a number of inactive VCs suddenly become active, and they send data at the negotiated ICR. This may cause a bu er over ow at the switch before the feedback reaches the sources, and the zero loss goal is not met. It is possible that the negotiated Cell Loss Ratio (CLR) may be violated under such conditions. This is an inherent tradeo in ABR, but the probability 49
of such an occurrence is expected to be low. For high latency paths, the cell loss can be controlled using source end system parameters (open loop control) during the rst round trip. ICR negotiation for ABR is a connection admission control (CAC) problem. Time lag for feedback to be e ective: Observe that a switch doesnotgive rate feedback with every cell. In particular, a feedback may be given in an RM cell traversing in the forward or reverse direction. Sometimes the switch experiences a load, but may not nd RM cells to give the feedback. Again, the result is that the feedback is delayed. It is also possible that the load disappears when the RM cell actually arrives. In summary, there is a time lag between the switch experiencing a load, giving feedback, and experiencing the new load due to the feedback. There are three components which a ect this time lag: Round trip time and Feedback delay: There is a non-zero delay between the switch giving feedback, the sources receiving the feedback, and the switch experiencing the new load. The round trip time (RTT) is the time taken by a cell to traverse the path from the source to the destination and back. It includes the transmission, propagation and the queuing delays. The sum of the transmission and propagation delays is called xed round trip time (FRTT). The time between the switch giving the feedback and it experiencing the load due to the feedback is called feedback delay (FD). In general, the feedback delay is less than or equal to the round trip time. The shortest feedback delay (SFD) is twice the propagation and transmission delay from 50
- Page 25 and 26: 7.11 E ect of UILI on Medium Bursts
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of such an occurrence is expected to be low. For high latency paths, <strong>the</strong> cell<br />
loss can be controlled us<strong>in</strong>g source end system parameters (open loop control)<br />
dur<strong>in</strong>g <strong>the</strong> rst round trip. ICR negotiation <strong>for</strong> <strong>ABR</strong> is a connection admission<br />
control (CAC) problem.<br />
Time lag <strong>for</strong> feedback to be e ective: Observe that a switch doesnotgive<br />
rate feedback with every cell. In particular, a feedback may be given <strong>in</strong> an<br />
RM cell travers<strong>in</strong>g <strong>in</strong> <strong>the</strong> <strong>for</strong>ward or reverse direction. Sometimes <strong>the</strong> switch<br />
experiences a load, but may not nd RM cells to give <strong>the</strong> feedback. Aga<strong>in</strong>, <strong>the</strong><br />
result is that <strong>the</strong> feedback is delayed. It is also possible that <strong>the</strong> load disappears<br />
when <strong>the</strong> RM cell actually arrives. In summary, <strong>the</strong>re is a time lag between <strong>the</strong><br />
switch experienc<strong>in</strong>g a load, giv<strong>in</strong>g feedback, and experienc<strong>in</strong>g <strong>the</strong> new load due<br />
to <strong>the</strong> feedback. There are three components which a ect this time lag:<br />
Round trip time and Feedback delay: There is a non-zero delay between<br />
<strong>the</strong> switch giv<strong>in</strong>g feedback, <strong>the</strong> sources receiv<strong>in</strong>g <strong>the</strong> feedback, and <strong>the</strong><br />
switch experienc<strong>in</strong>g <strong>the</strong> new load.<br />
The round trip time (RTT) is <strong>the</strong> time taken by a cell to traverse <strong>the</strong> path<br />
from <strong>the</strong> source to <strong>the</strong> dest<strong>in</strong>ation and back. It <strong>in</strong>cludes <strong>the</strong> transmission,<br />
propagation and <strong>the</strong> queu<strong>in</strong>g delays. The sum of <strong>the</strong> transmission and<br />
propagation delays is called xed round trip time (FRTT).<br />
The time between <strong>the</strong> switch giv<strong>in</strong>g <strong>the</strong> feedback and it experienc<strong>in</strong>g <strong>the</strong><br />
load due to <strong>the</strong> feedback is called feedback delay (FD). In general, <strong>the</strong><br />
feedback delay is less than or equal to <strong>the</strong> round trip time. The shortest<br />
feedback delay (SFD) is twice <strong>the</strong> propagation and transmission delay from<br />
50
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