[nsp] Calculating ATM Cell transfer rates

Shawn Solomon ssolomon at ind.net
Wed Feb 25 12:37:51 EST 2004

This is a similar calculation as I've used in the past, however, it
assumes perfect stuffing of the 48 byte payload.  Using the traffic
mixes that we see on our network, I used the 256*53/48 as a guide, but
ended up more around 256*53/45 to get a real-world traffic rate at the
desired level.

Shawn Solomon
Senior Network Engineer / Systems Design
317.263.8875   ssolomon at ind.net   fx317.263.8831

-----Original Message-----
From: cisco-nsp-bounces at puck.nether.net
[mailto:cisco-nsp-bounces at puck.nether.net] On Behalf Of
sthaug at nethelp.no
Sent: Wednesday, February 25, 2004 12:06 PM
To: afoster at gammatelecom.com
Cc: cisco-nsp at puck.nether.net
Subject: RE: [nsp] Calculating ATM Cell transfer rates

> surely this would give you an inaccurate Cell rate - if
> we wanted to shape a PVC for 256Kbps - using Ciscos
> interpretation this would give you a cell rate of 603 cps
> based on 53 byte cells.  But 603 cps is only 231Kbps of
> actual payload data based on 48 bytes of payload per cell.
> Or am I mis-understanding the whole calculation process?

I am saying that the Cisco specification in kbps is based on 53 byte
cells. Thus if you want 256 kbps of *payload*, you have to specify
256*53/48 = 282 (or 283 if you round up) kbps in your Cisco router

As I mentioned, you can check this yourself by using suitable debug

Steinar Haug, Nethelp consulting, sthaug at nethelp.no
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