MCI [ATM overhead]

Tim Salo salo at
Thu Mar 21 19:07:57 UTC 1996

> Date: Thu, 21 Mar 96 08:48:40 GMT
> From: "William Allen Simpson" <wsimpson at>
> To: nanog at
> Subject: Re: MCI [ATM overhead]
> > From: salo at (Tim Salo)
> > > From: Wolfgang Henke <wolfgang at>
> > > 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     .
    .                             .
     .                           .

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.]

More information about the NANOG mailing list