[nsp] Calculating ATM Cell transfer rates

Brian Turnbow b.turnbow at twt.it
Wed Feb 25 13:01:41 EST 2004

Cisco routers using ATM cards give you the kps rate of aal5, because it
"thinks" in IP.
Wheras an ATM switch uses cps because it "thinks" in ATM.
you can see cell counters on cisco atm cards using the sh conntrollers atm
X/X command.
Unfortunately it won't give you a cps rate only cells rx and cells tx.
It is also not possible to get any decent atm values from snmp... only aal5.
take a look at this

-----Original Message-----
From: cisco-nsp-bounces at puck.nether.net
[mailto:cisco-nsp-bounces at puck.nether.net]On Behalf Of Alex Foster
Sent: mercoledi 25 febbraio 2004 18.46
To: sthaug at nethelp.no
Cc: cisco-nsp at puck.nether.net
Subject: RE: [nsp] Calculating ATM Cell transfer rates

That now makes sense - will look at the debug to get
a true feel.  To your knowledge do most vendors base
their kbps on 53 byte cells - I mainly use Marconi
ATM products that require rates in cps not kbps -
makes life much easier.



-----Original Message-----
From: sthaug at nethelp.no [mailto:sthaug at nethelp.no]
Sent: 25 February 2004 17:06
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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