Token Ring technology was developed in the 1970s by IBM. Token-passing networks move a small frame, called a token, around the network. Possession of the token grants the right to transmit. If a node receiving the token has no information to send, it passes the token to the next end station. Each station can hold the token for a maximum period of time.
If a station possessing the token does have information to transmit, it seizes the token, alters 1 bit of the token (which turns the token into a start-of-frame sequence), appends the information that it wants to transmit, and sends this information to the next station on the ring.
Token Ring Frame Format
Token Ring and IEEE 802.5 support two basic frame types: tokens and data/command frames. Tokens are 3 bytes in length and consist of a start delimiter, an access control byte, and an end delimiter. Data/command frames vary in size, depending on the size of the Information field. Data frames carry information for upper-layer protocols, while command frames contain control information and have no data for upper-layer protocols. Both formats are shown in Figure .
Token Frame Fields
The three token frame fields illustrated in Figure are summarized in the descriptions that follow:
•Start delimiter—Alerts each station of 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.
•Access-control byte—Contains the Priority field (the most significant 3 bits) and
the Reservation field (the least significant 3 bits), 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 circling the ring endlessly).
•End delimiter—Signals the end of the token or data/command frame. This field also contains bits to indicate a damaged frame and identify the frame that is the last in a logical sequence.
Data/Command Frame Fields
Data/command frames have the same three fields as Token Frames, plus several others.The Data/command frame fields illustrated in Figure are described in the following summaries:
•Start delimiter—Alerts each station of 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.
•Access-control byte—Contains the Priority field (the most significant 3 bits) and
the Reservation field (the least significant 3 bits), 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 circling the ring endlessly).
•Frame-control bytes—Indicates whether the frame contains data or control information. In control frames, this byte specifies the type of control information.
•Destination and source addresses—Consists of two 6-byte address fields that identify the destination and source station addresses.
•Data—Indicates that the length of field is limited by the ring token holding time, which defines the maximum time a station can hold the token.
•Frame-check sequence (FCS)—Is filed by the source station with a calculated value dependent on the frame contents. The destination station recalculates the value to determine whether the frame was damaged in transit. If so, the frame is discarded.
•End Delimiter—Signals the end of the token or data/command frame. The end delimiter also contains bits to indicate a damaged frame and identify the frame that is the last in a logical sequence.
•Frame Status—Is a 1-byte field terminating a command/data frame. The Frame Status field includes the address-recognized indicator and frame-copied indicator.
Summary
If a station possessing the token does have information to transmit, it seizes the token, alters 1 bit of the token (which turns the token into a start-of-frame sequence), appends the information that it wants to transmit, and sends this information to the next station on the ring.
Token Ring Frame Format
Token Ring and IEEE 802.5 support two basic frame types: tokens and data/command frames. Tokens are 3 bytes in length and consist of a start delimiter, an access control byte, and an end delimiter. Data/command frames vary in size, depending on the size of the Information field. Data frames carry information for upper-layer protocols, while command frames contain control information and have no data for upper-layer protocols. Both formats are shown in Figure .
Token Frame Fields
The three token frame fields illustrated in Figure are summarized in the descriptions that follow:
•Start delimiter—Alerts each station of 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.
•Access-control byte—Contains the Priority field (the most significant 3 bits) and
the Reservation field (the least significant 3 bits), 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 circling the ring endlessly).
•End delimiter—Signals the end of the token or data/command frame. This field also contains bits to indicate a damaged frame and identify the frame that is the last in a logical sequence.
Data/Command Frame Fields
Data/command frames have the same three fields as Token Frames, plus several others.The Data/command frame fields illustrated in Figure are described in the following summaries:
•Start delimiter—Alerts each station of 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.
•Access-control byte—Contains the Priority field (the most significant 3 bits) and
the Reservation field (the least significant 3 bits), 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 circling the ring endlessly).
•Frame-control bytes—Indicates whether the frame contains data or control information. In control frames, this byte specifies the type of control information.
•Destination and source addresses—Consists of two 6-byte address fields that identify the destination and source station addresses.
•Data—Indicates that the length of field is limited by the ring token holding time, which defines the maximum time a station can hold the token.
•Frame-check sequence (FCS)—Is filed by the source station with a calculated value dependent on the frame contents. The destination station recalculates the value to determine whether the frame was damaged in transit. If so, the frame is discarded.
•End Delimiter—Signals the end of the token or data/command frame. The end delimiter also contains bits to indicate a damaged frame and identify the frame that is the last in a logical sequence.
•Frame Status—Is a 1-byte field terminating a command/data frame. The Frame Status field includes the address-recognized indicator and frame-copied indicator.
Summary