IP over ATM overhead

Darren Kerr dkerr at cisco.com
Thu Mar 13 20:04:16 UTC 1997

> 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.


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