# QoS for VoIP

May 28th, 2007

From this doc:

http://www.cisco.com/en/US/products/hw/switches/ps700/products_tech_note09186a00801c8c4b.shtml#supported

Burst?defines the maximum number of tokens that the bucket can hold at any one time. To sustain the specified traffic rate, burst should be no less than the rate times the interval. Another consideration is that the maximum-size packet must fit into the bucket.

To determine the burst parameter, use this equation:

Burst = (Rate [bps]) * 0.00025 [sec/interval]) or (maximum packet size [bits]), whichever is greater.

For example, if you want to calculate the minimum burst value needed to sustain a rate of 1 Mbps on an Ethernet network, the rate is defined as 1 Mbps and the maximum Ethernet packet size is 1518 bytes. The equation is:

Burst = (1,000,000 bps * 0.00025) or (1518 bytes * 8 bits/byte) = 250 or 12144.

The larger result is 12144, which you round to 13 kbps.

Note: In Cisco IOS? Software, the policing rate is defined in bits per second (bps), as opposed to kbps in Catalyst OS (CatOS). Also in Cisco IOS Software, the burst rate is defined in bytes, as opposed to kilobits in CatOS

From my CAT6500

CAT6500> (enable) set qos policer aggregate COMMUNICATOR-VOICE rate 128 burst ?

<burst> Burst 1..32000 (kb)

From SRND and QoS enterprise guide:

http://www.cisco.com/en/US/products/sw/voicesw/ps556/products_implementation_design_guide_chapter09186a00806e8c24.html#wp1107648

set qos policer aggregate COMMUNICATOR-VOICE rate 128 burst 8000 drop

Why the burst is specfied to 8000 ?

How can i calculate it?

Thanks

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## Replies

Anonymous (not verified) Mon, 06/04/2007 - 08:14

TCP window size is calculated to be 8000

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