T1 Circuit actual throughput 1290Kbp

James Carlson carlson at ironbridgenetworks.com
Fri Jul 10 20:30:25 UTC 1998

> >(!)  The cell tax is about 10% -- the SAR expands user data from 48
> >bytes to 53 bytes, plus an additional amount of overhead for internal
> >fragmentation on the final cell (AAL-5), plus overhead for whatever
> >encapsulation mode is being used.
> The cell tax is actually about 20%, based on the traces of the backbone IP
> traffic. Reason for this is the additional overhead imposed of partially =
> filled cells.

That's what I meant about internal fragmentation.  Depending on frame
size, it can be a non-trivial part of the overhead.  If we assume
null-VC encapsulation and 256 byte packets, this is 1 cell partially
filled (20 bytes average ((48-8)/2)) for every 5 full cells, or an
additional 9.7% overhead.  (Which matches up nicely with your
measurements.  ;-))

James Carlson, Consulting S/W Engineer  <carlson at ironbridgenetworks.com>
IronBridge Networks / 55 Hayden Avenue  71.246W    Vox:  +1 781 402 8032
Lexington MA  02421-7996 / USA          42.423N    Fax:  +1 781 402 8092
"PPP Design and Debugging" --- http://people.ne.mediaone.net/carlson/ppp

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