Cell Relay Archive

[James Carlson <carlson@ironbridgenetworks.com>]

Walt Weber <waltweber@sprintmail.com> writes:
> How does a network using routers and IP over Sonet compare with a network
> using IP over ATM on a throughput as well as cost per bit basis?

The additional overhead is approximately 12%, if you're comparing
IP-over-SONET versus IP-over-ATM-over-SONET.

When running IP directly over SONET, the per-packet overhead is a
fixed 9 bytes of HDLC address/control, PPP protocol number, 32 bit
CRC, and flag character plus 2/256 of the data (on average) for
escaping.  If you assume 256 byte user packets, this means that 267
bytes are transmitted for 256 bytes of data, giving about 96%
utilization.

When running IP over ATM, there are at least two penalties.  There's a
5/53 cell tax plus a per-packet internal fragmentation loss in the
final AAL-5 frame of, on average, (48-8)/2 bytes.  (If you're trying
to run anything but IP in that VC, or if you have to do FRF.8
internetworking, then you also have a penalty of 6 to 8 bytes of
LLC/SNAP cruft, and ATM is a little worse.)

If we again assume average 256 byte packets and null-encapsulation
(the best case for ATM overhead), this is an average of 20 bytes of
overhead for fragmentation plus 5 bytes for each cell, giving about
84% utilization.

If you want to do it as cost-per-bit, then ATM is about 15% more
expensive than raw IP (96/83).

-- 
James Carlson, Consulting S/W Engineer  <carlson@ironbridgenetworks.com>
IronBridge Networks / 55 Hayden Avenue  71.246W    Vox:  +1 781 372 8132
Lexington MA  02421-7996 / USA          42.423N    Fax:  +1 781 372 8090
"PPP Design and Debugging" --- http://people.ne.mediaone.net/carlson/ppp