3.2 Layer 2 Protocol Overview - LAN Protocols
3.2.6 Token Ring frame types and frame fields
Token Ring networks define two frame types: tokens and data/command frames. Both formats are shown in the figure to the left.

As you can see, each token is 3 bytes long and consists 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 includes signals that distinguish the byte from the rest of the frame by violating the encoding scheme used elsewhere in the frame.

The access control byte contains the Priority and Reservation fields, as well as a token bit (used to differentiate a token from a data/command frame) and a monitor bit (used by the active monitor to determine whether a frame is endlessly circling the ring).

Finally, the end delimiter signals the end of the token or data/command frame. It also contains bits to indicate a damaged frame and a frame that is the last in a logical sequence.

Data/command frames can 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 the two Address fields, which identify the destination and source stations. As with IEEE 802.3, addresses are 6 bytes long.

The Data field follows the Address fields. The length of this field is limited by the ring token holding time, which defines the maximum time a station may hold the token.

Following the Data field is the FCS field. This field is filled by the source station with a calculated value dependent on the frame contents. The destination station recalculates the value to determine whether the frame may have been damaged in transit. If damage did occur, the frame is discarded. As with the token, the end delimiter completes the data/command frame.