Date: Thu, 21 Mar 96 08:48:40 GMT From: "William Allen Simpson" <wsimpson@greendragon.com> To: nanog@merit.edu Subject: Re: MCI [ATM overhead]

...

From: salo@msc.edu (Tim Salo)

...

From: Wolfgang Henke <wolfgang@whnet.com> 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...)

I believe that the number is for TCP/IP carrying capacity _without_ ATM.

I don't know. It doesn't look right.

...

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.

Rather, that leaves us with the excellent (very desirable) option of a link where data from many sources are multiplexed by TCP/IP....

You are correct in observing that IP traffic can be multiplexed across a point-to-point link. As shown below, ATM provides link-layer multiplexing of data from multiple [link-layer] geographic locations. Of course, IP traffic can be multiplexed over ATM virtual channels, just as it can across point-to-point links.

I do not see what ATM buys in this situation.

I believe that we have at least one mid-level using ATM to connect to multiple NAPs in roughly the following configuration: NAP NAP NAP | | | | | | ........................... . . . . . ATM Wide-Area Service . . . . . ........................... | | Mid-Level There are also several production wide-area IP networks which are using a similar configuration, including parts of ESNet and NASA NREN. While I have not been privy to the economic analysis which justified these networks, I suspect that ATM wide-area networks provided a useful price/performance point. I also believe that a number NSPs are using ATM as a multiplexing technology, or are using carrier services which use ATM as a multiplexing technology. -tjs [These arguments sound a bit like the Cray [the supercomputer person/company] approach to memory: "real computers have real memory." I guess those who couldn't afford or didn't need gigabytes of real memory made do with virtual memory.]