From: Wolfgang Henke <wolfgang@whnet.com> Subject: Re: MCI [ATM overhead] To: johnc@msc.edu Date: Wed, 20 Mar 1996 08:38:08 -0800 (PST) Cc: nanog@merit.edu a [...] SONET (Synchronous Optical Network) speeds given in Mbps

nominal w/o Sonet ATM TCP/IP overhead

OC-3 STS-3c 155.520 149 122 137 future net backbone [...]

I think your 122 Mbps "ATM" number could be a bit confusing, even knowing the assumptions you described in earlier mail. (Also, more bandwidth seems to be available to "TCP/IP" than appears to be available from ATM...) If it helps, the following numbers are from John Cavanaugh's paper: Line Rate 155.520 Mbps Available to ATM 149.760 (SONET payload) Available to AAL 135.632 (ATM payload) John then computes the overhead for three MTUs, and yields rates available to IP and TCP: MTU 576 9180 65527 Available to IP 125.198 135.102 135.547 Available to TCP 116.504 134.513 135.464 These are the maximum available rates, namely they assume MTU-sized packets. The reader can apply their favorite packet size distributions to these numbers. Having said all that, I am not sure where that leaves us. One could theoretically remove the SONET overhead, but then one looses the ability to manage the SONET link. One could remove the ATM overhead, but then one has a point-to-point link, rather than a link over which data from many sources can be multiplexed. -tjs