Fig. 11.24. Token ring topology.
Any station wants to send messages follow the procedures given below:
1. The station seize the token. The token is a three byte frame circulating around the network.
2. Once the station possesses the token, it inserts information into the token and transmits the frame on the ring.
3. The next station checks the destination address of the frame. If not matches, pass it to the next station. If the address matches, it performs the following function
(a) The destination station copies the message and sets the last two bits of the frame to inform the source that the frame was copied and the frame is retransmitted.
(b) The frame circulates on the ring until it reaches the source. Once it reaches the source, the source removes the frame from the ring.
(c) The source releases the token by changing the Token Bit (T bit) to one.
Token Frame format. The token is a three byte frame. Only one token is allowed on the network at anytime. Fig. 11.25 shows the token frame format.
Fig. 11.25. Token frame format.
Start Delimiter (SD). It is set to JK0JK000. J and K are non-data bits. It follows violated Differential Manchester coding. The purpose of this pattern is to keep the SD byte (or ED type) from repeating in the information field of token ring frame.
End of Delimiter (ED). It is set to JK1JK10E. E is always set to zero. If any station detects an error, it will set E to 1.
Access control byte (AC). It contains three priority bits (PPP or more), a token bit (T), monitor bit (M) and remaining reserved bits. The priority bits varies 000 through 111. If token bit T = 0, data is in token or if T = 1, no data in token. The Monitor Bit (M) is used to prevent frames from circulating onto the ring.
IEEE 802.5 Frame format. Fig. 11.26 shows the Token ring format.
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