With the Token Ring, the identification signal, the token, is contained in the header of a complete message element. The Token Ring frame is created by converting the free token in the access control field into a busy token and by appending various control, monitoring and data fields.
The header passes through the individual stations in the Token Ring process. If it reaches a station that wants to transmit data and its identifier is identified as free, the adapter belonging to the station switches the identifier, adds the destination and sender addresses as well as the data, and thus generates the actual message element that then passes through the other stations.
The token in the token ring
The token consists of two special data fields which make the token or also the frame recognizable as such for a station, the starting delimiter field and the ending delimiter as well as the actual access control field. The data fields for the sender address and the destination address differ only slightly from each other. Like the token, the token ring frame has the starting and ending delimiter, the access control( FC) field, and additional control and information fields. The starting and ending delimiter fields contain the non-coded information bits J and K ( code violation) provided in the Differential Manchester Code, which only occur in these two data fields and thus enable unique identification of the token or frame. By means of the token bit in the access control field, the respective station can recognize whether it is a token (T = B '0') or a frame (T = B '1').
A total of six bytes are reserved for addressing. A distinction is made in the Token Ring between individual and group addresses and between addresses managed uniformly by the manufacturer and locally managed addresses. The sender address differs only slightly from the destination address.
Another data field in the Token Ring frame is the four- byte frame check sequence( FCR) field, in which the CRC procedure can be used to ensure that the message to be transmitted has not been modified during the actual transmission. The procedure corresponds to the CRC procedure for High Level Data Link Control( HDLC) or Synchronous Data Link Control( SDLC).
The MAC sublayer monitors the transmission of information between the physical layer and the LLC layer. Whether the individual case involves the transmission of a MAC frame or an LLC frame is determined in the frame to be transmitted in a control field provided for this purpose. In this way, each protocol layer recognizes whether the frame must be interpreted by it or not.
The maximum data packet length for Token Ring networks is 8000 bytes.