From: macomber@thoreau.nsc.com ( Robert Macomber)
Newsgroups: comp.unix.aix
Subject: Re: FDDI Trouble...
Keywords: FDDI fiber
Date: 12 Oct 92 17:11:49 GMT
Reply-To: macomber@thoreau.nsc.com( Robert Macomber)
Organization: National Semiconductor Corporation




In article <1992Oct5.221427.2293@fcom.cc.utah.edu>,
ben@osiris.usi.utah.edu (Ben Pratt) writes:
> 
> 
> When I add a third machine it no longer communicates, i.e. ping to any
> ring neighbor doesn't work.
> 
>     +-------+
>     | xmit  |---+
>     | recv  |-+ |
>     +-------+ | |
>               O O <-- barrel connector
>     +-------+ | |
>     | xmit  |-+ |
>     | recv  |-+ |
>     +-------+ | |
>               O O <-- barrel connector
>     +-------+ | |
>     | xmit  |-+ |
>     | recv  |---+
>     +-------+
>  This doesn't work, why?
> 
> This configuration doesn't work.  I'm mystified.  Yes I know everyone else
> is using concentrators, but is that a requirement?  I DO have good reasons
> for this implementation, money (98 versus 13000) and the machines aren't
> subject to user or office worker whims.  They are in a locked machine room
> and serve only as compute servers.  In smit you can make some configuration
> changes on the device but they don't make it any different (i.e. they
> don't make it work).
>
> Should this physical single ring work?  The fddi spec. seems to indicate
> that it's not a problem.  All hardware and cables and software do work in
> a 2 node environment.
> 

        This sort of FDDI ring will only work without FDDI Station
        Management (SMT).  There is nothing in FDDI hardware that
        forbids this kind of topology, but the software/firmware that
        establishes connections and manages the network (SMT) assumes
        that *all* connections are full-duplex with both the transmit
        and receive fibers connected between just two PHYs.

        Why?  When SMT brings up a link it exchanges information with
        the peer via "bit signalling".  This is how SMT can tell the
        port type of the other PHY, if the Link Confidence Test passes
        and other stuff.  The bit signalling protocol is constructed so
        that each station doesn't progress to the next stage until the
        peer has caught up. When the PHYs are daisy-chained SMT gets
        confused because it is sending signals to one PHY and receiving
        signals from a different PHY.

        If you have an option to add a card to turn one of your
        stations into a Dual Attach Station (two PHYs), you should be
        able to connect the three stations together without a
        concentrator.  Before you shell out the bucks for an upgrade
        ask the vendor how ConnectionPolicy is configured and whether
        or not you can change it in the field.  ConnectionPolicy is a
        SMT MIB attribute that defines the type of connections that are
        allowed.  If all of the PHYs are configured to disallow A->S
        and B->S connections then the stations still won't connect
	[basically, if either PHY signals that it likes the
	combination of Port types then both stations should accept it].

	At one point I think that there was talk about including 
	provisions for simplex rings (called lobes) in the FDDI-II
	standard.  I don't know the current state of this work, though.	


Robert L. Macomber MS 11-21   Advanced Communications
National Semiconductor Corp.  Internet: macomber@thoreau.NSC.COM
333 Western Avenue            UUCP: {sun|hplabs|decwrl}!nsc!thoreau!macomber
South Portland, Maine 04106   My opinions are my mine.
Newsgroups: comp.unix.aix