token passing
The token method is a media access method based on the fact that the person who has just finished broadcasting passes on the right to broadcast to a successor (physically or logically determined). This successor is then allowed to transmit for a certain time if he wants to transmit something and must then pass on the right to transmit.
If the token receiver does not want to use the send right, he passes the token directly to his successor. Assuming that the transmission time is limited and that one does not skip any station when passing on the token, the procedure is fair, since everyone gets a turn after a finite waiting time, the maximum of which is predictable.
The token is passed from node to node in order of falling address. After a node has transported some LLC data transmission frames and possibly performed some maintenance functions, the node passes the token to its successor by sending out a token frame. After that, the node listens to the transmission medium to make sure that the successor has received the token and is active. If it then receives a correct frame, it can assume that the successor has the token and is transmitting. If it does not receive a correct frame, it assumes (within a slot time) that the successor did not hear the token frame and retransmits it. If the successor also does not respond to the second token, the node assumes that the successor is faulty.
The node now sends out a "Who follows?" frame with the successor's address in the data field. All nodes compare the data field of the "Who is following?" frame with the address of their predecessors. The node whose predecessor is the successor of the sending node responds to the "Who is following?" frame by specifying its address. The node that has the token now establishes a new successor using that address by bypassing the actual, erroneous successor.