Workshop

Follow the Rules: Share and Enjoy!
Token-Ring, regardless of its dubious market presence, makes one heck of a great shared network.
Unlike Ethernet’s free-for-all rules, where everybody arbitrarily and randomly talks and then retries if
they get an error, Token-Ring’s party line has strict rules. Whereas Ethernet is a “be home by 11:00”
parent, Token-Ring is the kind of parent who wants to know who you’re going with, where you’re going,
who’s driving, and so on. Token-Ring has a lot more rules and therefore has a lot more recovery and
manageability.
Just for a moment, think of everybody in a Token-Ring network as being in one big conference room.
Suppose there are a lot of people (say, more than 50). How does anybody get a word in edgewise? Think
about what you might do in such a situation. You’d probably elect a chair, and this chair might give
everyone a turn to speak. This is pretty much what Token-Ring is all about. In large groups, Token-Ring
is much more efficient than Creepernet, uh, I mean Ethernet. It’s sort of like using parliamentary
procedure rather than family discussions: It’s a wasted effort for small groups, but a lifesaver for large
groups.
First, how does Token-Ring work physically? Each workstation that enters the ring can be thought of as
filling a seat at the conference table. This is actually how Token-Ring is wired (that is, in a ring rather
than in the “star” topology that Ethernet uses). Even though the wiring looks star-like, the path of the
wiring actually forms a loop from station to hub to station, as in Figure 10.1. You might be wondering
why doesn’t this act the way Christmas lights do? The answer is because all cables come to a hub; if a
break occurs, the hub (called a MAU or multiple access unit) can route signals around the break (unless
something else is wrong).
Figure 10.1 Even though a hub is present, Token-Ring’s wiring actually forms a ring, which makes it
look like a star.
Think of each station that connects to the MAU as sitting down at the conference table (the ring). The
first station on the network is called the active monitor (the elected chair), and it acts as a point of contact
for others to report errors, maintains the ring’s timing, detects certain kinds of ring errors on its own, and
initiates a “do over” if the ring gets certain serious errors. Each station on the network that subsequently
joins the ring becomes a standby monitor and can take over if the active monitor leaves the ring.
The active monitor is also responsible for issuing a special packet called the token. Each packet on the
wire is handed from Token-Ring card to Token-Ring card, all the way around the ring. Everyone takes