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Re: ATM vs. DS3
From: Stephen Balbach <stephen () clark net>
Date: Thu, 10 Jul 1997 16:06:54 -0400 (EDT)

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


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] 
        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
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