Information Technology Reference
In-Depth Information
around the ring. All stations test each passing frame for messages addressed to them,
process the message if it is theirs, and pass a marked token back around the ring. The
original transmitting station “releases the token” when it returns around the loop.
The Token-Ring signaling structure is called a Differential Manchester Code,
and has no DC voltage component. It can thus be directly, inductively, or capacitively
coupled to networking components. This lends itself to the “self-healing” aspect of
the physical topology. The MSAU is the key to this architecture. (The MSAU is also
commonly called an MAU and pronounced as a word by Token-Ring users, although
it is quite different from the Ethernet component by that same name.)
Each port of the MSAU contains a small relay that connects the ring signal to
the next port, in turn, when no cable is plugged in. If no cables are plugged in, the
MSAU is a small ring within itself. As each cable is plugged into a port on the
MSAU, a phantom voltage from the associated workstation opens the relay and the
ring signal is diverted down that cable to the workstation. The workstation moni-
tors the ring signal and repeats it back to the MSAU, where it may be diverted to
the next active workstation. If a workstation is not active (powered up or inserted),
the relay remains closed and bypasses the ring signal to the next port, ignoring the
attached cable.
The data rate of Token-Ring is either 4 Mbps or 16 Mbps. A particular ring
must operate at just one speed. A ring may be extended to additional MSAUs
through the ring-in/ring-out ports of each MSAU. Copper repeaters and fiber-optic
repeaters (or converters) may be used to extend the distances of a ring. Rings are
joined together by means of bridges or routers, as with the other network topologies.
Token-Ring Cabling
Token-Rring uses two pairs of wires to connect each workstation to the MSAU. The
pairs may be incorporated in a shielded or unshielded cable that typically comprises
four pairs. The popular IBM cabling system uses STP cabling in conjunction with
special data connector jacks and cables to support the traditional Token-Ring instal-
lations. The IBM cabling system is described in Chapter 6. UTP wiring normally
uses an 8-pin, modular (RJ-45 style) plug and jack. This is called the
medium-
dependent interface connection
in the 802.5 terminology.
The wiring patterns of the data connector and the 8-pin modular jack are
shown in Fig. 2.9.
Station wire for the Token-Ring requires two pairs and the cable may be either
STP or UTP. Cables are always run in a star pattern from each workstation outlet
to a central wiring closet. The TIA/EIA-568-C wiring standard will support Token-
Ring networking because it contains four pairs in a compatible pattern. Category 3,
