From: ksarkies@augean.eleceng.adelaide.edu.AU (Ken W. Sarkies)
Subject: Re: ATM "style" (was: Lockheed Switch)
Keywords: ATM switching traffic
Organization: Electrical and Electronic Eng., University of Adelaide
Date: Tue, 10 Nov 1992 06:50:59 GMT

In article <ru9l3ps@sgi.sgi.com> rpw3@rigden.wpd.sgi.com (Rob Warnock) writes:
>myoung@force.ssd.lmsc.lockheed.com writes:
>+---------------
>| I had a request to post info on the Lockheed switch... We have proposed
>| a bufferless control structure... A modified deflection routing scheme...
>| at each network switch, a data packet must bid for an output link... by
>| providing an ordered list of links for each packet... Conflicting bids
>| for the same output port are nondeterministically solved.... implemented
>| using technologies that are scalable both in speed and network configuration.
>+---------------
>
>In further conversations with one of the researchers referred to me by Matt,
>I found that this switching scheme is really quite clever. I wouldn't be at
>all surprised if it could be used to build inexpensive multi-gigabit LANs.
>
>However, the "deflection routing" architecture inherently re-orders packets
>(or frames, cells, whatever), very severely when put under a heavy load.
>(After all, it handles congestion by routing packets "somewhere else" for
>a while.) Using it in an ATM environment would be a real challenge. Using
>it directly with unmodified AAL1, AAL2, or AAL5 would be impossible.
>
>This points up one of the really fallouts that I predict is about to
>occur from our industry-wide rush towards ATM (and yes, I include myself
>in that rush). There are many kinds of potentially useful network and
>switch architectures which are going to be pushed aside because they are
>incompatible with the ATM "style" (small cells, ordered delivery, bandwidth
>declared at call setup, even the very requirement of "call setup").
>
..... other stuff deleted

>

In this department we have been working on ATM switch architectures for a
few years, and are now looking at performance aspects of various general
structures. A few general considerations which need to be kept in mind are

1. The BISDN is supposed to be a high speed network, so we do not want
to be encumbered by the protocols. They should be simple and efficient.
For that reason it is reasonable to look for switches which do not break
up cell ordering.

2. Switches must work at least as fast as the optical fibre links to which
they are connected. This is not likely to be possible electronically
without a great deal of hardware and parallelism. They should not be more
complex than is necessary.

3. Under normal conditions, switches will not be overloaded longer than
the buffers can handle. Rarely, brief overloads may occur on selected outputs.
A switch which recirculates blocked cells would be very useful, but not
necessarily essential. However cell reordering can be done at the switch
outputs, with little problem (probably in output buffers), to satisfy the
standards.

4. Under overload conditions (allowed by the traffic control in the network),
switches can be overloaded for longer periods. In such cases it is expected
that cells will be lost, in which case recirculation, deflection routing
etc. will not help. It is important only that in the switch, innocent traffic
is protected. Switches which have recirculation or other rerouting of overload
traffic may have the potential for the overload traffic to cause cell loss
on streams which are not passing to overloaded outputs. That in my opinion
is where the real problem with such architectures lies.

I believe that ultimately the simpler switches will turn out to be preferred
on the basis that they can be produced cheaper and faster and more flexible
than the more
complex architectures. Note that this is not a criticism of the Lockheed
architecture, but a general comment.

-- 
                                Ken Sarkies

No disclaimer. My boss takes all responsibility for my big mouth.
Newsgroups: comp.dcom.cell-relay