Specifications

TOKEN RING OVERVIEW
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PHYSICAL LAYER
The Token Ring topology is characterized as a logically circular,
unidirectional transmission path without defined ends. The physical
topology is a star wired system with each station having its own cable
running back to a central point. Although wired as a star, tracing of the
cable run will reveal a continuous ring.
The ring is most typically constituted by sets of MSAUs interconnected via
their Ring In/Ring Out (RI/RO) ports. Each MSAU typically has eight to
twenty-four Trunk Coupling Units (TCU), or lobe ports, into which
stations are connected via lobe cabling. In practice lobe cabling can consist
of various cable segments constituting the lobe-to-TCU connections.
Segments typically include patch cables from the TCUs to the main wiring
panel, the lengths of the main wiring to the station locations, and then
patch cables from floor/desk sockets to the stations. Token Ring has many
definitions that pertain only to this topology, the following definitions are
to clarify what is meant throughout this document.
Multi-Station Access Unit (MSAU)
An MSAU is a device that provides a centralized group of TCUs, each of
which offers station connection to the ring trunk. The MSAU is typically
called a concentrator or hub. MSAU can be active or passive and
intelligent or non-intelligent. If several MSAUs form a ring trunk, they
must be connected together through their Ring In/Out Ports. The MSAU
should not be confused with the Medium Attachment Unit (MAU), a term
that refers to media transceiver devices.
Trunk Coupling Unit (TCU)
The TCU is the physical device that couples the station lobe cable to the
trunk cable. The TCU provides the means to insert the station into the ring
or bypass it when the station is inactive or the lobe cable has failed.
Trunk Cable
The Trunk Cable is the cable that runs within and between MSAUs. It can
consist of either fiber optic or shielded or unshielded twisted-pair copper
cable. Twisted-pair copper cable uses two positive transmit wires in
normal mode, with no crossover, while fiber has one transmit fiber and
one receive fiber. The positive transmit circuit of the Ring Out port of an
MSAU connects to the positive receive circuit of the Ring In port on the
downstream MSAU. Thus, in normal mode, the second pair of wires is not
used and is known as the backup, or secondary, ring path. The backup
path is used to re-route the ring path at a port that is wrapped.