Specifications

TOKEN RING OVERVIEW
2-6
Access Method
Communication between devices on a network requires that there be a
common method for transmitting and receiving messages, and every
device on the network must have the ability to talk at some point. Also, if
two devices attempt to talk at the same time the data will become
corrupted. Thus there are two conflicting requirements. The need to talk
on the LAN and exchange information and the need to make sure only
one device out of all the network devices talk at any one time. The
technique used to resolve these requirements is known as Media Access
Control (MAC), which controls the contention between users wishing to
gain access to the medium to transmit information. It should be noted that
on such a system, data transmitted from a device will be seen by all other
devices, only one of which is the receiving station. It is up to the receiving
device to decide what to do with the information. The Media Access
Control method known as token passing is implemented on a ring
topology.
The Token Passing Protocol works in the following way:
1. A unique series of bits, known as a token, is recognized by each station
on the network.
2. Only when holding the token is a station able to initiate data transfer.
A timer (the Token Holding Timer) controls the maximum time that a
station can use the network before passing the token.
3. The token is passed from one station to the next, providing each
station in turn with an “invitation to transmit.” Token passing on a
ring is implicit since each station on the ring accepts the token from the
station on its receive, or upstream, side and then passes to the station
on its transmit, or downstream, side. A station always passes it to the
next active station physically in line.
4. When transmitting the sending station sets the token to busy and
sends out a frame of data. The data is passed from station to station
around the ring, being copied by the station with the proper
destination address.
5. After being copied by the destination station, the data continues to
circulate, until it arrives back to the source, where it is deleted. When
transmission is complete, the token is released as a free token by the
source and passed to the next station.
6. Each station regenerates and repeats the information bit by bit.
7. The physical insertion or removal of a station on the ring does
temporarily interrupt the network operation, but the network
automatically recovers.