A station gains the right to transmit frames onto the medium when it detects a token passing on the medium. Any station with traffic to transmit, on detection of the appropriate token, may capture the token by modifying it to a start-of-frame sequence and append the proper fields to transmit the first frame. After completion of it, information will transfer; and after appropriate checking for proper operation, the station initiates a new token, which provides other stations with the opportunity to gain access to the ring. MAC Frame Structure. As shown in Figure 11.12, there are eight fields in the MAC frame: preamble, start frame delimiter (SFD), the addresses of the frame's source destination(s), a length field to indicate the length of the following field containing the FCS COVERAGE WHERE: SFS - START-OF-FRAME SEQUENCE INFO - INFORMATION (0 OR MORE OCTETS)* SD - STARTING DELIMITER (1 OCTET) FCS - FRAME CHECK SEQUENCE (4 OCTETS) AC - ACCESS CONTROL (1 OCTET) EFS - END-OF-FRAME SEQUENCE FC - FRAME CONTROL (1 OCTET) ED - ENDING DELIMITER (1 OCTET) DA - DESTINATION ADDRESS (2 OR 6 OCTETS) - FRAME STATUS (1 OCTET) SA - SOURCE ADDRESS (2 OR 6 OCTETS) * ALTHOUGH THERE IS NO MAXIMUM LENGTH FOR THE INFORMATION FIELD, THE TIME REQUIRED TO TRANSMIT A FRAME MAY BE NO GREATER THAN THE TOKEN HOLDING PERIOD ESTABLISHED FOR THE STATION. NOTE: TOKEN CONSISTS OF SD, AC AND ED FIELDS Figure 11.12 Frame format for token ring. ENTERPRISE NETWORKS I: LOCAL AREA NETWORKS SD = Starting Delimiter (1 octet) AC = Access Control (1 octet) ED = Ending Delimiter (1 octet) Figure 11.13 Token format for a token-passing ring network. LLC data, a field that contains padding6 if required, and the frame check sequence (FCS) for error detection. All eight fields are of fixed size except the LLC data and PAD fields, which may contain any integer number of octets (bytes) between the minimum and maximum values determined by a specific implementation. The format of the token for token ring operation is illustrated in Figure 11.13. 11.7 LAN INTERWORKIN