|  | 7.1 | Basics of Token Ring
| 7.1.2 | Token Ring frame format
Tokens Tokens are 3 bytes in length and consist of a start delimiter, an access control byte, and an end delimiter. The start delimiter alerts each station to the arrival of a token, or data/command frame. This field also includes signals that distinguish the byte from the rest of the frame by violating the encoding scheme used elsewhere in the frame. Access Control Byte The access control byte contains the priority and reservation field, and a token and monitor bit. The token bit distinguishes a token from a data/command frame, and a monitor bit determines whether a frame is continuously circling the ring. The end delimiter signals the end of the token or data/command frame. It contains bits that indicate a damaged frame, and a frame that is the last of a logical sequence. Data/Command Frames Data/command frames vary in size depending on the size of the information field. Data frames carry information for upper-layer protocols; command frames contain control information and have no data for upper-layer protocols. In data/command frames, a frame control byte follows the access control byte. The frame control byte indicates whether the frame contains data or control information. In control frames, this byte specifies the type of control information. Following the frame control byte are two address fields that identify destination and source stations. As with IEEE 802.5, their addresses are 6 bytes in length. The data field follows the address field. The length of this field is limited by the ring token that holds the time, thus defining the maximum time a station may hold the token. Following the data field is the frame check sequence (FCS) field. The source station fills this field with a calculated value dependent on the frame contents. The destination station recalculates the value to determine whether the frame has been damaged in transit. The frame is discarded if it has been damaged. As with the token, the end delimiter completes the data/command frame.