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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.
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