Conference 7.286::fddi

    Digital's position is that the ONLY Dual Attached Station on the FDDI
    ring should be the Wire Concentrator (DECconcentrator 500).  There can
    be multiple concentrators on the ring.  ALL other stations should be
    SAS.
    
    The reasons are simple.  The FDDI ring will "heal itself" if a DAS
    device goes down, meaning you will still have a ring.  If you lose 2
    Dual Attachment Stations however, you will end up with 2 rings, which 
    can not communicate with each other.   The DECconcentrator 500's
    reliability is supposed to be so good, we don't have severe worries
    about losing one, much less two, on the same ring.  
    
    By using SAS devices everywhere else, if one goes down, the ring stays
    up, regardless.
    
    The FDDI engineers in this notesfile can explain this is far better
    terms than I can.
    
    -Patrick 

	Re .0:  Digital is only supporting FDDI-1 in this first phase
  	    of products.
    
        Re -1:  Wiring concentrators are not Dual Attachment Stations - they
            are DACs - Dual Attachment Concentrators.  

    	    A DAS has a MAC and A,B phy ports.  A DAC also has a MAC and
    	    A,B phy ports but it also contains one or more M type ports.  
    	    DAS's and DAC's are able to connect directly to the trunk
	    ring, ie., the A port of one station is connected to the B
    	    port of another. Single Attachment Stations, such as our
    	    bridges and adapters, only attach to the ring by connecting to 
    	    Concentrator M ports.
    

    One further note: the DECconcentrator 500 can also operate as a Single
    Attachment Concentrator.  It can be arranged in a branching tree
    topology, with up to 7 LEVELS of concentrators.

Re .4: that's not literally correct.  Our concentrator is definitely a DAC.
However, a DAC can be connected in a tree configuration in much the same
manner as a SAC would be, using its B port where the SAC would use the S port.

That doesn't make it a SAC, though: that port is still a B port, not an S port.

	paul

I thought that you could configure the concentrator without the dual cable
module; does this mean you can't use that version for the `branching tree'
configuration?

If you leave out the A/B port module, you get what is sometimes called
a "NAC" (No attachment concentrator)  That is not a widely recognized
term, however.

In any case, such a beast has only M ports and consequently can serve
as the root of the tree hierarchy, but not in any other place.  Furthermore,
the MAC chip lives on the A/B port board, so a concentrator without that
board is not accessible to remote management.  For this reason, the M port
only concentrator is supported as the only concentrator in an FDDI, i.e.,
when you have at most 12 nodes and want just one concentrator.  For more
complex networks, we want to have the management/A/B port board to be
present to ensure remote management is available.  

Does that address your question?

	paul

re:.7

Yes, nicely.  The points you raised where not made in the FDDI phase review
to the best of my knowledge in the overviews of the concentrator packaging.

Interesting...
As being one, that just had the FDDI PID training, where these specifics
like m, s a/B etc where not mentioned, could someone post a description of
the acronyms used in .1:.8 or point to a document which explains this in
somewhat more deatil without having to read thru the standard...

thanks in advance

Erik

    Quickly, the A and B ports the the ports that the dual ring attaches to
    on the concentrator (the DAS connections), the M (master) port is the 
    connection on the port cards which you would plug a bridge or a node into 
    (a SAS connection) and the S (slave) port are the ports on the other side of
    that connection, i.e. a system adapter or a bridge. For more
    information, look thru this notes file for the note which describes the
    on-line papers that are available or get the handout from Raj Jain's
    Intro to FDDI seminar.
    
    Debbie