[8160] in North American Network Operators' Group
Re: IP over ATM overhead
daemon@ATHENA.MIT.EDU (Darren Kerr)
Thu Mar 13 15:20:02 1997
Date: Thu, 13 Mar 1997 12:04:16 -0800
From: Darren Kerr <dkerr@cisco.com>
To: stephen@clark.net
CC: nanog@merit.edu
In-reply-to: <Pine.LNX.3.91.970313100137.21788F-100000@bisco.clark.net>
(message from Stephen Balbach on Thu, 13 Mar 1997 10:05:34 -0500
(EST))
> 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
It probably depends on what you define as overhead. You might define
it as that 16 bytes of AAL5/SNAP header & trailer per IP datagram,
plus 5 bytes per cell, plus whatever trailing bytes are wasted
in the last cell.
Compare this with packet over sonet (1 byte of IFG, 4 bytes of
ppp encapsulation, and 4 bytes of CRC).
Peter Lothberg gathered some packet size distributions at various
internet routers in January, and found that using the above definitions,
atm overhead consumed 22% of bandwidth, vs. 3.1% for POS overhead.
Looking at it another way, POS can move 24% more payload than ATM
using the same packet size distribution. I've taken other snapshots
at other routers since then, and the results come very close.
/Darren
