Alime.231
net.lan
utzoo!decvax!harpo!floyd!vax135!lime!martin
Fri Apr 23 09:25:34 1982
re: IBM local network

From: lime!vax135!houxi!ihnss!ihuxn!djmolny

The reliability questions Joe raised are mostly administrative in
nature.  I agree you can't get 99.999% reliability without duplex hardware
and related goodies, but good planning can get you 98%, which is by far
and away enough for most applications.

Here are some solutions for the problems Joe raised.  (Bear in mind that
the exact implementation is heavily processor- and medium-dependent.)

Master Down:    If the network master goes down, simply designate another
               node as master.  There doesn't have to be any special
               hardware associated with the master.  Assignment can be
               done manually or automatically, with only a brief pause in
               network traffic.

Two Machines Down: I'm not sure why this is more of a problem than one
               machine down.  To be sure, you need some way for other
               nodes to route around the downed system, but if implemented
               correctly, such a mechanism will work for any number of
               machines.  Typically, ring transmitters have a shunt
               to allow bits to go in one ear and out the other.

Sanity Timer Failure: Each machine should periodically check its own
               sanity timer by letting it fire.  Self-test, it's called.

Adding Machines: The answer to this question is dependent on the network
               hardware used, but again, can be solved by good hardare
               design.  All you need is an extra input and output path for
               each node, with a switch to select between paths.  Install
               the new machine on the alternate path, inform the other
               nodes of its existense, and flip the switch.


I'm not defending the IBM network (especially since I don't know anything
about it), but ring networks in general.  I'd also like to make it clear
that when I talk about ring networks, I mean logical as well as physical
rings.  A CSMA/CD transmission link can operate in ring fashion, if the
nodes grant transmit permission via tokens.  Simply a different protocol
on the same medium.

                                               -- DJ Molny
                                               ihnss!ihuxn!djmolny

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