Information Technology Reference
In-Depth Information
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
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 the two address fields, which identify the destination and source
stations. As with IEEE 802.3, addresses are 6 bytes in length.
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 frame check sequence (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.
Troubleshooting Token Ring
This section provides troubleshooting procedures for common Token Ring media problems. It describes
a specific Token Ring symptom, the problems that are likely to cause this symptom, and the solutions to
those problems.
