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Error Terror: When a NAUN Is Not a Person, Place, or Thing
Token-Ring’s pecking order is wonderful when it works. It’s also pretty good when it doesn’t, because
Token-Ring provides a lot of rules for error handling. In the way that a parliamentarian can deal with
certain rule infractions without adjourning the meeting, the active monitor of the ring also can deal with
certain situations. However, just like any democratic society, each citizen is also left to do his or her part.
For a packet to make it all the way around the ring, each station must transmit the packet to its
neighboring station; in the process, each station gleans the location of its neighbors. That is, each station
knows the address of its nearest address upstream neighbor (its NAUN, pronounced noun). Is this
important? You bet!
First of all, what’s upstream? Well, because a Token-Ring passes packets around the ring in sequence,
upstream refers to the direction in which packets are not flowing.
Let’s talk briefly about what constitutes downstream. Toss out theory, for the moment, and consider this
from a purely pragmatic standpoint. You plug a bunch of workstations into a Token-Ring hub, plug in a
server, and you’re ready to rock and roll, right? Downstream, in this case, is the direction from first port
to the last port.
Plugging in another hub means that you have some means in which to extend the ring. Each Token-Ring
hub (MAU) has a ring in and a ring out, which are the means for extending the ring. It’s simple: Packets
flow downstream through the out and then flow into the next hub through the in, which, by definition, is
upstream of its ports. This may sound crazy, but when you look at Figure 10.2, it’ll make more sense.
Figure 10.2 Downstream is always toward the out and upstream is always toward the in of a Token-
Ring MAU. The direction of upstream and downstream changes as you move on the ring.
Back to NAUNs. The reason why it’s so cool that a station knows its NAUN is because when errors are
detected by a particular station, it reports them two seconds later to the active monitor—the Chairman of
the Ring. If you’re running a network analyzer, you’ll see the error report, plus the address of the NAUN.
If you see the NAUN reported in the error log, you have a good idea where the error originated.