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## RE: ATM vs. DS3

• From: Chris A. Icide
• Date: Thu Jul 10 16:30:01 1997

```We monitor both the octet output of the cisco, and the received cells
on our ATM switch.  One then would divide the cisco octet value by the
cell value * 53.  This would result in a percentage value that would

Efficiency (%) = octet rate / (cell rate * 53)

This assumes that you can measure both parameters.

Chris A. Icide
Sr. Engineer
Nap.Net, L.L.C.

-----Original Message-----
From:	Stephen Balbach [SMTP:stephen@clark.net]
Sent:	Thursday, July 10, 1997 3:07 PM
To:	Ben Black
Cc:	Karl Denninger; Chris A. Icide; 'Josh Beck'; nanog@merit.edu
Subject:	Re: ATM vs. DS3

Question: On a Cisco with an AIP card, how do you determain what the
overhead is? We connect to our upstream provider via ATM.

.stb

On Thu, 10 Jul 1997, Ben Black wrote:

> i've never heard anything *less* than 20% loss in ATM overhead.
>
> On Wed, 9 Jul 1997, Karl Denninger wrote:
>
> > On Tue, Jul 09, 1996 at 10:38:57PM -0500, Chris A. Icide wrote:
> > > On Wednesday, July 09, 1997 9:34 PM, Josh Beck [SMTP:jbeck@connectnet.com]
> > > wrote:
> > > > Hello,
> > > > 	I just thought of something. We are in the process of purchasing a
> > > > 4 Mb CIR from another backbone. Now, we have the choice of ATM or standard
> > > > T3 delivery (over a DS3 either way). Now, if we get ATM, that 4 Mb CIR
> > > > turns into:
> > > >
> > > > [ (53-5)/53 ] * 4 Mb/s = 48/53 * 4 Mb/s = 3.62 Mb/s
> > >
> > > Emperical data shows that we are currently losing about 20.5% of capacity
> > > to IP over ATM overhead on fairly aggregated traffic.  This means that *IF*
> > > your new connection is being measured as 4Mbps of cell bandwisth, you
> > > will only be getting 3.18Mbps.  You may want to verify from the company
> > > providing this link what exactly are they limiting you to?
> > >
> > > btw, the extra overhead is lost in things like the last cell of a packet not
> > > being full, etc.
> > >
> > > Chris A. Icide
> > > Sr. Engineer
> > > Nap.Net, L.L.C.
> >
> > My God, someone admits it?
> >
> > I've used 20% as the general ATM overhead now for almost two years, and have
> > been poo-pooed by lots of people claiming that it wasn't anywhere near that
> >
> > Funny how it all comes out in the end. :-)
> >
> > --
> > --
> > Karl Denninger (karl@MCS.Net)| MCSNet - The Finest Internet Connectivity
> > http://www.mcs.net/~karl     | T1's from \$600 monthly to FULL DS-3 Service
> > 			     | 99 Analog numbers, 77 ISDN, http://www.mcs.net/
> > Voice: [+1 312 803-MCS1 x219]| NOW Serving 56kbps DIGITAL on our analog lines!
> > Fax:   [+1 312 803-4929]     | 2 FULL DS-3 Internet links; 400Mbps B/W Internal
> >
>

```