Path: utzoo!dptcdc!jarvis.csri.toronto.edu!mailrus!ncar!ames!sgi!vjs@rhyolite.SGI.COM
From: vjs@rhyolite.SGI.COM (Vernon Schryver)
Newsgroups: comp.dcom.lans
Subject: Re: Request For Opinions: FDDI follow-up
Summary: is single attach legal?
Message-ID: <30878@sgi.SGI.COM>
Date: 18 Apr 89 19:45:45 GMT
References: <4824@charon.unm.edu> <29505@bu-cs.BU.EDU> <1507@Portia.Stanford.EDU> <29650@bu-cs.BU.EDU>
Sender: daemon@sgi.SGI.COM
Organization: Silicon Graphics, Inc., Mountain View, CA
Lines: 49

In article <29650@bu-cs.BU.EDU>, kwe@bu-cs.BU.EDU (kwe@bu-it.bu.edu (Kent W. England)) writes:
> 
> 	FDDI contains specs for ring concentrators, dual and single
> ring modes, and the SMT (soon) to make all these options and modes work
> together....

I thought single attach rings were declared illegal last year.  My
reading of section 6 of X3T9.5/84-49, Update 2-1-89, finds only
dual-attach A/B and and Master/Slave connections.

Notice that the new CMT diagrams have boxes labeled "optical bypass",
and that PC-Reset has words about when you should or should not
fiddle with your bypass.

If you have only a few stations on the ring, and they are well maintained
exemplars of mature products, then one clearly can avoid bypasses (e.g.
Proteon).  If you like single attach rings, how can you not like optical
bypasses?  With single attach, aren't bypasses absolutely required?

A lot of people talk about FDDI concentrators.  I can't make the $'s come
out in a way that makes concentrators plausible, assuming everything
is dual attach or M/S.  What am I missing?  Consider:

a concentrator on the dual ring and 5 slave stations:  I count a total of
 (12*big $)	12 transceivers (the concentrator has 2 on the ring, and
		    5 masters, while the 5 stations have 5 slaves)
  (cheap)	<= 6 MAC's
  (?)		one dual-attach optical bypass  (or two single-attach
		    bypasses), unless you don't by bypasses.
  (big $)	1 concentrator, with at least enough smarts, power supply,
			etc. to do things like PC-Reset
  --		5 workstations, each with SMT, OSI, NFS, and maybe a real
		    application if it is small

5 stations on the ring:  I count 
 (10*big $)	10 transceivers 
 (cheap)	5 to 10 MAC's.
 (5*?)		5 dual-attach optical bypass  (or 10 single-attach bypasses)
  --		5 workstations, each with SMT, ...

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