How do I correctly calculate ATM Cell Tax overhead ?

[mark.edwards]

I'm looking to use CBFWQ on a CPE device. On the following link there are details on how ATM cell Tax Overhead is calculated. The cisco explanation is not clear.

Here is a link to show cisco's calculation; link:http://www.cisco.com/en/US/products/sw/iosswrel/ps1835/products_configuration_guide_chapter09186a00800b75a9.html#1001225

If you goto the section on CBWFQ allocation you'll see what I mean. Where does the 62 figure come from and what does it mean ?

So my question is, how do I correctly calculate (for example) ATM cell tax overhead for a 100kbps link with an average packet size of 256k ?

[donewald]

62 in this link refers to the overage of the 256 byte packet calculation. Here's how they came to that number.

256 byte packets (would be split into six cells to be sent on a VC, giving a total of 6 * 53) = 318 bytes (this would be total data size after it had been encapsulated into ATM cells. The difference of 256 to 318 is 62 bytes. 62 bytes would refer to the overhead that was associated with transmitting 256bytes of data.

Hopefully the above explaination will help you with the (below) answer that is for your 100kbps question.

Suppose the average packet size for the class is 256 bytes and the class needs 100 kbps (which translates to 49 packets per second) of guaranteed bandwidth. Each 256-byte packet would be split into six cells to be sent on a VC, giving a total of 6 * 53 =318 bytes. In this case, the ATM cell tax overhead would be 62 bytes or 49 * 62 * 8 = 24.34 kbps. When configuring CBWFQ in this example, ensure that the sum of all the configured class bandwidths is less than the VC bandwidth by at least 24.34 kbps to ensure desired payload guarantee for the configured classes (in this example, there is only one class)

Hope this helps you,

Don

[jwmiller]

Something you might find of interest is at

http://cco.cisco.com/en/US/customer/tech/tk648/tk362/technologies_tech_note09186a0080093c9a.shtml