Path: utzoo!attcan!uunet!lll-winken!csd4.milw.wisc.edu!bionet!agate!ucbvax!amdcad!rpw3
From: rpw3@amdcad.AMD.COM (Rob Warnock)
Newsgroups: comp.dcom.lans
Subject: Re: Request For Opinions: FDDI follow-up
Message-ID: <25286@amdcad.AMD.COM>
Date: 19 Apr 89 06:40:14 GMT
References: <4824@charon.unm.edu> <29505@bu-cs.BU.EDU> <1507@Portia.Stanford.EDU> <29650@bu-cs.BU.EDU> <30878@sgi.SGI.COM>
Reply-To: rpw3@amdcad.UUCP (Rob Warnock)
Organization: [Consultant] San Mateo, CA
Lines: 65

In article <30878@sgi.SGI.COM> vjs@rhyolite.SGI.COM (Vernon Schryver) writes:
+---------------
| No matter how deep the tree, there are always > 2 pairs of transceivers per
| station if you use concentrators, but exactly 2 pairs if you get on the
| ring.  MAC's are (or will be) cheap.  As I understand things, it is the
| transceivers that are not coming down in price.  If you don't believe in
| bypasses, putting everything on the dual ring looks cheaper.
| Vernon Schryver | Silicon Graphics | vjs@sgi.com
+---------------

This is quite true... if you believe that you *have* to use the
expensive transceivers (optics) for your Master/Slave connections.
The fact is, there are order-of-magnitude cheaper optics currently
available that will cheerfully do "FDDI" (in quotes 'cause if's not
"real" FDDI then they probably don't like you to call it FDDI), but
at 820 nanometers and "only" up to 700 meters or so (instead of 2 km).
So I venture to say that some workstation manufacturer might decide
to introduce "cheaper-FDDI", with concentrators that have "real" FDDI
on the backbone and "cheaper-FDDI" to the single-attach stations.

And you can even do cheaper still: Noticing that, as Vernon points out,
concentrators already have to have a *lot* of smarts in them, why not
make the concentrator really be a multi-port bridge instead, and run
something *really* cheap between the bridge and the workstations --
that is, a non-FDDI protocol [that still uses FDDI addressing, &c.].

I've been playing around lately with some designs for a super-simple
"external serial backplane" based on point-to-point links [a fiber
pair is a easier to handle than a SCSI cable, and a lot faster] using,
for example, AMD's "TAXI" chips with (say) an H-P plastic-package
125 Mbaud fiber link. Parts cost to get a full-duplex 100 Mbit/sec
"UART"-like interface is less than than $100 in volume ($40 + $43 + glue).
Some packet buffer SRAM, some counters, and some PALs would bring that
up to, say, a total of $150-200. [Yeah, cost, not retail. But that's
still about one-tenth the parts cost of FDDI.]

Now I was looking to make multi-master I/O "busses" ("Cheap-O-Bus"?)
out of this stuff, with simple star repeater hubs [and CSMA/CD, which
limits it to about 250 meters diameter, to reduce collisions], but
the same hardware and bus protocol could be used to make a link from
a workstation to an FDDI bridge. In fact, if you already had such
a fiber I/O bus, an FDDI bridge is just another "shared peripheral".

In that configuration, the economics shift considerably back towards
using a hub/bridge/concentrator, and to using FDDI for a "backbone",
where its extreme reliability features (SMT/CMT/etc.) and optical
bypasses have a clear need.

Besides, just like people sometimes use Ethernet transceiver multi-
plexers [DEC "DELNI", and equiv.] to make standalone "networks" that
have no yellow trunk cable, just so could a multi-master "external
serial backplane" be used as a limited-distance [250m diameter] LAN.

[Disclosure/disclaimer: While I am a sometime consultant to AMD, the
mention of specific AMD or H-P products above was just to show that
cheap 100Mb/s links *can* be made today (I've seen 'em run). There are
sure to be other vendors and other ways to do it -- it's "steamboat time".]


Rob Warnock
Systems Architecture Consultant

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