MCI [ATM overhead]

Date: Thu, 21 Mar 96 08:48:40 GMT
From: "William Allen Simpson" <>
Subject: Re: MCI [ATM overhead]

> From: (Tim Salo)
> > From: Wolfgang Henke <>
> > 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:

      > > >
      > > >
     . .
    . .
   . ATM Wide-Area Service .
    . .
     . .

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.


[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.]