[nsp] Calculating ATM Cell transfer rates

Alex Foster afoster at gammatelecom.com
Wed Feb 25 11:52:30 EST 2004

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?

-----Original Message-----
From: sthaug at nethelp.no [mailto:sthaug at nethelp.no]
Sent: 25 February 2004 16:36
To: afoster at gammatelecom.com
Cc: cisco-nsp at puck.nether.net
Subject: Re: [nsp] Calculating ATM Cell transfer rates

> I am trying to pin down the correct way of calculating ATM cell rates
> from a given speed in bps.  I have read various documents that contradict
> each other - my understanding of the correct method is as follows:

The Cisco specification (in kbps) is based on 53 byte cells. You can see
this by turning on suitable debugging info, and the router will print
the calculated number of cells per second.

(Rant of the day: The Cisco way of specifying this is also incredibly
stupid and confusing.  It would have been much easier if we could simply
specify the speed in cps directly.)

Steinar Haug, Nethelp consulting, sthaug at nethelp.no

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