Explanation about Bc / Be

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Jul 9th, 2008
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How can we know what Bc is OK when configuring rate limit in an interface? We've seen that you can divide the CIR / 4 and that could be OK but, is there any document or something explaining when to use that formula? Is the same for VoIP networks?

Thanks in advance,

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Marwan ALshawi Tue, 07/15/2008 - 16:26
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A shaper uses the CIR, Bc, and Be to smooth a traffic stream to a specified rate. It achieves the given CIR by dividing the line speed of the interface into equal-length time slots (intervals, or Tc). Then it sends a smaller portion (Bc) of the traffic during each time slot.

The time slot size is governed by the Bc parameter (Tc = Bc/CIR). In Figure 4-9, a line rate of 128 kbps is shaped to 64 kbps. For Frame Relay traffic shaping, the Bc, by default, is one eighth of the CIR (or 8 kbps). Each second is divided into eight time slots of 125 ms each, and the shaped rate of 64,000 bps is divided into eight bursts of 8000 bps each (Bc). Each burst takes 62.5 ms to transmit (at a 128-kbps line rate), so the shaper transmits information for the first 62.5 ms of each 125-ms time slot and is silent for the remaining 62.5 ms of the time slot. Over the span of a second, this achieves the average rate of CIR.

The Be value is used to determine the peak rate of sending and is calculated as follows:

Peak Rate = CIR (1+ Be / Bc)

Peak-rate shaping allows the router to burst higher than average-rate shaping. However, when peak-rate shaping is enabled, any traffic that exceeds the CIR could be dropped if the network becomes congested.

The design goal for one-way latency across a real-time network is 150 ms (per the G.114 specification for voice end-to-end delay), and the jitter target is less than 10 ms per hop. Introducing up to 125 ms of shaping delays at a single hop destroys VoIP quality. Therefore, it is recommended that shaped interfaces carrying real-time traffic be shaped to 10-ms intervals (Tc). Because the Tc cannot be administered directly by Cisco IOS commands, Bc tuning is required to affect Tc indirectly. This interval value can be achieved (remember, Tc = Bc / CIR) by setting the Bc equal to CIR/100, which results in a Tc value of 10 ms. The recommended value for Bc on an 64-kbps interface/PVC carrying real-time traffic is, therefore, 64,000 / 100, which equals 640 bits and represents 10 ms of transmission on a 64-kbps line

source Cisco press

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