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 ...
(a) ACR (b) Queue Lengths (c) Con guration Figure 9.13: Parking lot: best VS/VD option converges fast Figure 9.13(a) shows that the optimum value is reached at 40 ms. Part (b) of the gure shows that the transient queues are small. Further, the allocation is fair. 9.4.5 Link Congestion and Allocated Rate Update Frequency: Viable Options The allocated rate update has three options: a) update upon BRM receipt (in VS) and enter the value in a table to be used when an FRM is turned around, b) update upon FRM turnaround (at VD) and no action at VS, c) update both at FRM (VD) and at BRM (VS) without use of a table. The last option recomputes the allocated rate a larger number of times, but can potentially allocate rates better because we always use the latest information. 355
The allocated rate update and the e ects of link congestion actions interact as shown in Figure 9.14. The gure shows a tree where the rst level considers the link congestion (2 options), i.e., whether the next loop is also a ected or not. The second level lists the three options for the allocated rate update frequency. The viable options are those highlighted in bold at the leaf level. Figure 9.14: Link congestion and allocated rate update: viable options Other options are not viable because of the following reasons. In particular, if the link congestion does not a ect the next loop, the allocated rate update at the FRM turnaround is all that is required. The allocated rate at the BRM is redundant in this case. Further, if the link congestion a ects the next loop, then the allocated rate update has to be done on receiving a BRM, so that ACR canbechanged at the VS. This gives us two possibilities as shown in the gure (BRM only, and BRM+FRM). Hence, we have three viable combinations of link congestion and the allocated rate update frequency. A summary of all viable options (a total of 6) is listed in Table 9.2. The next section evaluates the performance of the viable VS/VD design options through simulation. 356
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(a) ACR<br />
(b) Queue Lengths<br />
(c) Con guration<br />
Figure 9.13: Park<strong>in</strong>g lot: best VS/VD option converges fast<br />
Figure 9.13(a) shows that <strong>the</strong> optimum value is reached at 40 ms. Part (b) of <strong>the</strong><br />
gure shows that <strong>the</strong> transient queues are small. Fur<strong>the</strong>r, <strong>the</strong> allocation is fair.<br />
9.4.5 L<strong>in</strong>k Congestion and Allocated <strong>Rate</strong> Update Frequency:<br />
Viable Options<br />
The allocated rate update has three options:<br />
a) update upon BRM receipt (<strong>in</strong> VS) and enter <strong>the</strong> value <strong>in</strong> a table to be used when<br />
an FRM is turned around,<br />
b) update upon FRM turnaround (at VD) and no action at VS,<br />
c) update both at FRM (VD) and at BRM (VS) without use of a table.<br />
The last option recomputes <strong>the</strong> allocated rate a larger number of times, but can<br />
potentially allocate rates better because we always use <strong>the</strong> latest <strong>in</strong><strong>for</strong>mation.<br />
355
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