[18370] in North American Network Operators' Group
Re: T1 Circuit actual throughput 1290Kbp
daemon@ATHENA.MIT.EDU (James Carlson)
Mon Jul 13 13:16:34 1998
Date: Fri, 10 Jul 1998 16:30:25 -0400
From: James Carlson <carlson@ironbridgenetworks.com>
To: vaananen@NASBPD01BS.ntc.nokia.com
CC: nanog@merit.edu
In-reply-to: <1998Jul10.161500.1991.265457@rhino.ntc.nokia.com>
(vaananen@NASBPD01BS.ntc.nokia.com)
> >(!) 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@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
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