From: vjs@rhyolite.wpd.sgi.com (Vernon Schryver)
Newsgroups: comp.dcom.cell-relay
Subject: Re: Comment made at Next Generation Networks conference
Date: 23 Nov 92 16:42:12 GMT
Organization: Silicon Graphics, Inc.  Mountain View, CA

In article <By5E8o.G0K@usenet.ucs.indiana.edu>, robelr@bronze.ucs.indiana.edu (Allen Robel) writes:
> In <smji130@rhyolite.wpd.sgi.com> vjs@rhyolite.wpd.sgi.com (Vernon Schryver) writes:
> 
> >> Nope.  On page II-27 of the spiralbound (November 16th) handout, top
> >> slide, second bullet item he states:
> >>
> >> "100 Mbs ATM delivers Less Bandwidth Than 100 Mbs FDDI"
> >
> > Where is the conflict?  Rob wrote "this is slightly lower (6.8%) than
> >FDDI's 97.3 Mb/s", refering to the number of bits a single TCP/IP
> >virtual circuit might get out of either FDDI or ATM.
>  
> You just mentioned where the conflict is when you say "a SINGLE
> TCP/IP virtual circuit." [emphasis mine].  What would be the 
> throughput for that SINGLE circuit when you've got 20 other
> fast machines using the same bandwidth?  Now compare that figure
> to the same twenty machines connected with an ATM switch...


hmmph!  Let's not chop nits.  I was only trying to point out that there
exists an interpretation of the quote that is entirely true, and that
Rob Warnock's numbers were consistent with that interpretation, as well
as suggesting ways in which an ATM-LAN can deliver more bandwidth.

Obviously, if all 22 machines are bashing the same FDDI ring, then
an ATM switch might do better.  I'd generally expect 11 pairs of machines
each wanting 100Mb to do better on an ATM switch than on one FDDI ring.
However, that is not necessarily the case:
    -it might be that the FDDI links go to a hub containing a crossbar
	switch, similar to the ethernet switches, instead of to a
	concenterator.

    -what is the nature of the ATM switch?  Aren't there some 100 Mb
	switches that are not the equivalent of a full cross bar,
	so that there is some potential for conflict?

I think this part of this thread is uninteresting.  The basis seems to
be a comparison of FDDI and ATM-as-a-LAN.  Bandwidth matters, and since
155 is more than 100, ATM switches should provide more bits/sec than
FDDI rings.  However, that is not now an major consideration when
comparing FDDI and ATM-LAN's.  More interesting are:

    -current ATM adapters tend to (or all do?) require host programed I/O.
	In other words, current ATM products are sloooooooooow!  One
	can reasonably hope that will soon change.

    -I understand that all current ATM-LAN's use fixed and permanent
	curcuits among all hosts.  That seems unlikely to work in a
	real LAN of hundreds of TCP/IP hosts, all coming and going.
	One obvious answer, to treat ATM virtual circuits as is done
	with IP over x.25, does not sound inviting to me.  
	Having ARP imply creating a virtual circuit seems strained.

    -multicast needs work.  The various mechanisms that are currently
	used to implement broadcasts are not sufficient for multicast.

    -The Standards Process must be navigated, and as shown by FDDI
	and ISDN, that can take literally decades.


ATM switches as LANs offer a lot of hope.  If you must now pick
something that will replace FDDI, then point at ATM.  However, many
years must pass before anyone can know if that hope is reasonable.  My
favorite example of a technology that everyone claimed would solve
everyone's problems was "plated wire."  If you don't recognize it, then
think of "bubble memory", which had a similar story about 15 years
later.


Vernon Schryver,  vjs@sgi.coml
Newsgroups: comp.dcom.cell-relay