A video conference application wants to keep the end-to-end packet delay below 2

Question

A video conference application wants to keep the end-to-end packet delay below 200ms, and at a rate of at least 1Mb/s. The packets are all 10,000bits long and pass through 9 routers (and therefore across 10 links), and all the links are 200km long and run at 10Mb/s. At each router along the path, the video packets are placed into their own queue, which is served at a guaranteed minimum rate. Considering all the packetization, propagation and queueing delays in the system, how much buffering should be in each router, in terms of packets? You can assume each router has the same amount of buffering, and the speed of propagation is 2 x 10^8m/s.

Explanation / Answer

The packetization delay in the system = 10,000 bits / 107 i.e 1E4bits / 1E7 = 1ms

For 1 link it is 1ms. Hence for 10 links it is 10ms.

The propagation delay in the system = 2E6 / 2E8 = 10ms

The queueing delay = 200ms - 20ms = 180ms

Hence, the queueing delay per router is (180/9)ms = 20ms = 20E-3 second

Each queue is served at least at 1Mb/s = 1E6 byte/second

So, after considering all the delays in the system, the total buffering in each router = 20E-3s * 1E6 b/s = 20,000 bits

=2 packets

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