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Network Design and Operations

On a 70 Mbps outgoing link of a router, three queues are served by weighted round robin with weights 4:2:1. All three queues always have packets waiting, and all packets are the same size. Which is closest to the bandwidth available to the queue with weight 2?

  1. AAbout 10 Mbps
  2. BAbout 20 Mbps
  3. CAbout 23 Mbps
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Answer: B. About 20 Mbps

In weighted round robin, each round sends as many packets from each queue as its weight. One round sends 4 + 2 + 1 = 7 packets, two of which come from the weight-2 queue, so its bandwidth is 70 × 2/7 = 20 Mbps. The weight-4 queue gets 40 Mbps and the weight-1 queue 10 Mbps, and when a queue is empty the others can use its share. With strict priority queuing, which always sends from the highest-priority queue first, the other queues cannot send at all when the priority queue is busy. A common combination is therefore to give strict priority only to small amounts of traffic such as voice and to divide the rest by weights.

  • A. This is the bandwidth of the weight-1 queue.
  • B. Correct. 70 × 2/7 = 20 Mbps.
  • C. This divides the link equally into three (70 ÷ 3), ignoring the weights.
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