[8154] in North American Network Operators' Group
Re: IP over ATM overhead
daemon@ATHENA.MIT.EDU (pkavi@pcmail.casc.com)
Thu Mar 13 11:46:30 1997
From: pkavi@pcmail.casc.com
Date: Thu, 13 Mar 97 11:18:43 EST
To: nanog@merit.edu, Stephen Balbach <stephen@clark.net>
The complete answer is that it depends upon packet size distributions.
Let's assume that typical Internet type traffic patterns, which tend to
have high percentages of packets at 40, 44, and 552 bytes.
If you have a DS3 backbone, and have PLCP turned on, the maximum cell rate
is 96,000 cells/sec. If you multiply this value by the number of bits in
the payload, you get:
96000 cells/sec * 48 bytes/cell * 8 = 36.864 Mbps
But this is unrealistic, as it assumes perfect payload packing. There are
two other things that eat into the overhead value, namely the wasted
padding, and the 8 byte SNAP header.
It's the SNAP header which causes most of the problems, forcing most small
packets to consume two cells, with the second cell to be mostly empty
padding. This causes a loss of about another 16% from the 36.8 Mbps ideal
throughput.
So the total throughput ends up being (for a DS3):
DS3 Line Rate: 44.736
PLCP Overhead - 4.032
Per Cell Header: - 3.840
SNAP Header & Padding: - 5.900
=========
30.960 Mbps
So, from a DS3 line rate of 44.736 Mbps, the total overhead is about 31%. Hope
this helps.
Prabhu Kavi
Manager, Network Architecture Group
Cascade Communications.
______________________________ Reply Separator _________________________________
Subject: IP over ATM overhead
Author: Stephen Balbach <stephen@clark.net> at SMTPLINK
Date: 3/13/97 10:47 AM
We are installing an ATM backbone connection and wondering what level
of overhead can be expected. Ive read from %10 to %50 - this will be a
LAN connection so we can assume almost no cell loss. Our provider has
said on average %12 bandwidth is overhead. It will be a Cisco->Cisco LAN
configuration. Thanks!
Stephen Balbach
VP ClarkNet
