ATM Wide-Area Networks (was: sell shell accounts?)

Tim Salo salo at
Tue Jul 23 16:57:04 UTC 1996

> From: Dave Siegel <dave at>
> Subject: Re: sell shell accounts?
> To: freedman at (Avi Freedman)
> Date: Fri, 19 Jul 1996 16:01:01 -0700 (MST)
> Cc: vansax at, richards at, agislist at,
>         nanog at
> > Acceptable arguments are:
> > o Switches can handle more throughput
> That's difficult to quantify in theory *or* practice.
> 	[...]

I may misunderstand your assertion, but it doesn't seem all that
difficult to quantify, at least to some coarse level.

We have been using wide-area ATM switches at OC-3c for some time.
It is pretty clear that the switches can handle OC-3c.  The
early switches had relatively small output buffers, so they tended
to loose cells before TCP could throttle back in congested circumstances.
We are now using switches with much larger output buffers, and TCP appears
able to throttle back fairly gracefully.

After we upgraded our switches to use much larger output buffers, most of
the problems we experienced were related to the hosts, (e.g., poor
TCP implementations, poor ATM interfaces, etc).

By the way, I believe that most people who are using ATM wide-area networks
in production or as part of the Internet are using static bandwidth
allocation to avoid a number of problems.

We also have a number of OC-12c interfaces for our switches.  They
appear to work, but we haven't really had a chance to stress them
yet.  (Part of the problem is that it is hard to find OC-12c data
sources and sinks.)

The current generation of routers appears unlikely to support OC-12c,
particularly since their backplanes are only about the same speed.

On a more theoretical note, switches, being circuit-switched, make
the complicated decisions when the connection is established, (or
configured for PVCs), and need to make relatively simple decisions
to switch each cell.  This probably scales very well is terms of
speed, although at some point might have some difficulty in scaling
to a very large number of simultaneous connections.

Routers, on the other hand, have to make a bit more complicated
decisions per packet.  This has some limitations in terms of speed
and number of simultaneous "connections."


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