Re: T1 Circuit actual throughput 1290Kbp
(!) 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. Fixwest 960926: total packets seen = 15315121, total payload bytes seen = 4276560002 ATM framing bytes = 5155881287 ATM efficiency = 82.95% ATM w/snap framing bytes = 5387212772 ATM w/snap efficiency = 79.38% Funet 970608: total packets seen = 6699991, total payload bytes seen = 2336213700 ATM framing bytes = 2783454053 ATM efficiency = 83.93% ATM w/snap framing bytes = 2919019891 ATM w/snap efficiency = 80.03% Pasi
(!) 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 "PPP Design and Debugging" --- http://people.ne.mediaone.net/carlson/ppp
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vaananen＠NASBPD01BS.ntc.nokia.com