Workshop

Token-Ring does not have any hard-and-fast cable length limit on any given run. Instead, there’s a total
ring length limitation. If you think about it, this makes sense, because all the cables work together to
make one ring, right?
When using STP, the maximum distance around a ring can be 180 meters, or 590 feet. When using CAT-
V UTP, your ring distance can be 100 meters, or 330 feet. If you’re using CAT-III UTP, you might as
well set it all up in your garage—the longest roundtrip can only be 70 meters, or 230 feet. I’m throwing in
these numbers for reference only; you’re not going to need them unless you start designing Token-Rings.
To actually calculate whether your ring is within cable budget, you must use a complicated formula
(which gives me a headache when I look at it) that involves the longest individual cable run, the internal
wire distance of each MAU on the network, the inter-MAU cable runs, and the price of tea in China, ad
nauseam. People who design Token-Ring networks must invoke this mystical little formula. Fortunately
for you, though, this formula has little or nothing to do with troubleshooting Token-Ring.
Again, as is the case with Ethernet, if your network was working last week as well as the week before,
there’s dust all over the cables, and nobody has played with the cable plant, the last thing you would
suspect is that the cables have gotten longer all by themselves. If, on the other hand, someone has added a
couple of MAUs or a rather long cable to your Token-Ring and it’s starting to act flaky, you should
disconnect this stuff and see what happens. You don’t need to engage in higher mathematics or whip out a
tape measure to do this. This is mere change management, and it’s a whole lot easier than measuring
cables that are already in the wall and in use on a production network.
Packet Jacket
A Token-Ring can have several different packet sizes. They can be anything from 1KB to 16KB, with the
most popular packet size I’ve seen being 4KB. The packet size of a given Token-Ring is equal to the
smallest packet size specified on the network—that is, you can easily decrease the packet size on a Token-
Ring simply by adding a network card that’s configured to use a smaller packet size.
If you have a wide-area, routed network or a mixed Token-Ring and Ethernet environment, beware of large
packet sizes. Some (actually, more than you would think) TCP/IP applications don’t handle fragmentation
properly. Fragmentation is what happens when you need to put 10 pounds of stuff in 5-pound bags—the
packets get split into smaller, more manageable chunks. When a mixed environment is in use, packets will
almost definitely get chopped up in order to fit through the smaller interfaces on your router. You can
handle this by instructing your applications to use smaller packet sizes, or you can simply dial down the
packet size on your Token-Ring. Alternatively, you can ask your vendor to fix things; it depends on how
optimistic you feel that day.