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how to calculate end-to-end latency in case of congestion?


I am trying to calculate or estimate the time to transfer a 1500 bytes packet over multilink with and without congestion. When there is no congestion the 1500 bytes packet will be fragmented and will utilize the 3 links total bandwidth of 4.5 mbps. It will take 2.6 ms according to my calculation.

1500 * 8/ 4.5 M = 2.6 ms

Is there any other delay I need to add to this calculation?

What will happen when there is congestion in network, say the link is 100% utlized. WHat will happen if this packet is marked EF and what if it is marked default? How we going to estimate the end-to-end latency in this case?



Re: how to calculate end-to-end latency in case of congestion?


Another thing that can increase your latency is queue length. If you have the line fully utilized and the interface cannot process all necessary packets, they will wait in a queue. If this queue is filling in, and you have no policy apply to it, it will start to tail drop packets, so a packet that is dropped has to be retransmitted, adding latency.

If you have packets with EF and there is congestion, then this packets will have priority in the queue and they will be send before other packets (regular priority). Usually Voice packets are marked with EF bit.