MCI [ATM overhead]
salo at msc.edu
Thu Mar 21 19:07:57 UTC 1996
> Date: Thu, 21 Mar 96 08:48:40 GMT
> From: "William Allen Simpson" <wsimpson at greendragon.com>
> To: nanog at merit.edu
> Subject: Re: MCI [ATM overhead]
> > From: salo at msc.edu (Tim Salo)
> > > From: Wolfgang Henke <wolfgang at 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 .
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
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
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