A good read for your QOS studies…

“Look at the diagram above. Both router links are clocked at 128Kbps, and the test packet flow has packet size of 1000 bytes each, being sent at a sustained rate of 16 packets per second, effectively saturating the 128Kbps link. Consider what happens when we shape the flow down to 64Kbps. Egress packets are also serialized at 128Kbps – therefore the shaper needs to buffer and delay packets to obtain the target average rate of 64Kbps. Shaper performs that by delaying every burst each Tc interval. For this example, the Bc value (shaper burst) equals to packet size, so effectively every 1/16s interval egress link is busy and the next 1/16s interval it is idle. The average bps rate is total volume of (4*1000) divided by 1/2s (time to send) and multiplied by 8 (to get bps) yielding the result of 64000bps.”

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