Workshop

Down Town
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Most Token-Rings chug away pretty well. The most problematic Token-Rings are usually the largest
ones, where error recovery doesn’t work as well. (Although you can theoretically put more than 100
workstations on a single Token-Ring, it’s suicidal to go above 60, and in this day of cheap switching, you
might as well put one or two dozen per segment.) Nonetheless, Token-Rings are known to go down.
A downed ring usually results from one of these two causes:
• Beacons
• Excessive errors
A beacon, most often, is when a signal loss occurs on the wire. That is, a workstation has not exited the
ring gracefully, and the hub thinks the station is still in use. Because the workstation doesn’t respond
anymore (caused by a broken NIC), its downstream neighbor detects a signal loss and tells everyone. (It
can still talk downstream, even though its upstream data has been cut off.)
The active monitor initiates a ring purge (or do over). If this doesn’t work, the downstream neighbor
thinks to itself, “What if it’s me?” and hurries to check this out. It removes itself from the ring, performs
self-diagnostics, and reinserts itself. This cycle keeps happening until the signal loss is corrected. It’s a
pretty bad scene until you come along.
Fortunately, you have your trusty Token-Ring monitor program, and the defective workstation has been
reported to the active monitor as the reporting station’s NAUN. You walk the NAUN list, find the bad
workstation, remove it from the hub, and everything is okay. You can then figure out why the signal loss
was occurring at your leisure (it’s usually a bad card or cable). Most times, this is how a beacon
troubleshooting session goes.
Excessive errors are troubleshot in pretty much the same way. You can look at the NAUN of the
offending machine and remove it from the network. Even though I’ve seen all sorts of errors caused by
bad drivers, your trouble will most often be with a single card, cable, or hub. If you don’t have a Token-
Ring monitor (and again, some vendors provide this for free-it’s part of what Token-Ring offers over
Ethernet) or network analyzer (or if your analyzer provides no clues), the divide-and-conquer method
will usually win over any problem.
Summary