At 05:12 PM 4/5/96 -0800, Kent W. England wrote:

I have been doing some more arithmetic on the "ATM cell tax" to see if I can find out how folks get from 10% (which is the 5/53 overhead) to 35% overhead. I would really really like to see some backup for those sort of numbers -- I can't make the arithmetic work out to anything near 35% for what I think are useful assumptions. Of course, if you want to posit 100% of IP traffic with 49 bytes, but that doesn't seem realistic and you shouldn't forget that 40 of those are TCP/IP overhead. :-)

What traffic mix do you use to calculate ATM overhead and what results do you get?

Trying to avoid reinventing the arithmetic wheel here, I've taken the liberty of cutting-and-pasting a couple of snippets from earlier discussions on this topic. [snip] Date: Tue, 19 Mar 1996 19:51:33 -0800 From: Jim Forster <forster@cisco.com> Sender: owner-nanog@merit.edu He's talking about the overhead due to carrying variable length IP packets in fixed length ATM cells. Consequently the last cell of an AAL5 frame will contain 0 - 39(?) bytes of padding, which is wasted bandwidth. Assuming random length distributions (which they're not), the average waste is about 20 bytes per packet. My rough estimate, based on an average packet size of 200 bytes (used to be correct, not sure anymore), is that the waste due to cell padding is about 10%. Due to the highly skewed packet size distribution the actual overhead might vary substantially. Note that this 10% is on top of the ~10% overhead due to the 5 byte ATM cell headers (5/53 ~= 10%), and various other overheads (some of which are also present in frame over Sonet schemes). There's beginning to be some expectation that there will be a transmission capacity crunch in the carrier's Sonet nets, and this ~25% ATM cell tax may be looked at carefully as packet over Sonet solutions emerge. [snip] From: Wolfgang Henke <wolfgang@whnet.com> Date: Tue, 19 Mar 1996 20:34:05 -0800 (PST) Sender: owner-nanog@merit.edu Great. Let's estimate the ATM bandwidth available on an OC-3. With STS-3c SONET at 155.520 Mbps it gets reduced to 149.760 Mbps due to section, line and path SONET overhead. Next we reduce 10% due to 20 bytes lost on average 200 byte long packets due to incomplete ATM cell fills resulting in 134.784 Mbps. Then we need to subtract 9.43% due to ATM headers of 5 bytes in 53 byte packets. Thus we end up with a 122.069 Mbps true information rate which is about 78.5% of the nominal OC-3 capacity. And we still have not raised the issues of wasted reserved bandwidth and routing ease which were recently addressed very eloquently by Vadim Antonov when comparing connection based with packet based transport. [snip] Me again. Finally, these are the various overheads mentioned in Jim's message above: ATM AAL5 8 bytes LLC snap encap 8 bytes unused part of cell 0-40 bytes (avg. 20) --- 36 bytes (avg.) - paul