What is it?
Token Ring is a data link protocol developed by IBM. Another common term
used in connection with this protocol is IEEE 802.5 standards. Token Ring
networks typically run on shielded twisted pair cabling designated as IBM
Type 1, Type 6, or occasionally, other type designations. Token Ring can also
be run on unshielded twisted pair cabling, using media filters. The node
capacity of a ring varies with the cable type, and is significantly reduced when
using unshielded twisted pair cable. Token ring cabling contains two pairs of
wires, known as Ring In and Ring Out pairs. This is necessary because
Token Ring nodes are actually daisy-chained in a circle, although they may
appear physically to be star-connected to a hub. 
What does it do?
Token Ring arbitration makes use of a special packet called a Token. The
token circles the network, and any station that wants to transmit captures the
token and attaches a message. When the receiver gets the message, it sends
the token back to the sender, who releases it for use by another station. Token
Ring may be preferred to Ethernet in some cases, since it has the ability to set
priority levels for different nodes. The down-side of Token Ring is that if the
token is lost somehow, the network stops.
How is it used?
Token Ring is a common data link protocol. It is typically more expensive than
Ethernet. It has the advantage that it is easily integrated into IBM minicomputer
environments, though this distinction is fading. These networks are always
connected as logical rings. Physically, they make use of passive hubs called
media access units (MAUs) to connect in an apparent star topology. This is
deceptive, since each workstation cable creates a loop to and from the
workstation, and the MAUs are connected together in a ring as well. MAUs
are increasingly being replaced with active hubs, which offer greater
functionality and control.
Where do I get more information?
Contact your local CBV Office.
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