IP over ATM overhead

pkavi at pcmail.casc.com pkavi at pcmail.casc.com
Thu Mar 13 16:18:43 UTC 1997

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 
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 at 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

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