Cell Relay Archive

[Koen Peeters <koen@ciminko.be>]

James Carlson wrote:
> 
> 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).
> 

I'm not sure this is really an issue !
ATM gives you QOS with a overhead of 15% it seems.
QOS in IP make the assumption of abundance in bandwidth ( = 50% extra
bandwidth overhead ??).
Newer ATM encapsulation techniques with QOS in mind use TCP over ATM,
thereby getting rid of the IP overhead of TCP/IP. Is bandwidth really
the issue ?
> --
> 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

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