Information Technology Reference
In-Depth Information
wants to transmit data, it seizes the token, produces a data frame by appending data, and sends
it to the destination. The receiving station reads the frame and forwards it along the ring back
to the source station. The receiving station also sets the address-recognized and framed-copied
bits on the forwarded frame. The source station verifies that the data frame was read and
releases a token back onto the network. The default operation permits the use of the Token
in round-robin fashion.
To k en Ring Priority
Token Ring includes an optional priority system that permits stations configured with a higher
priority value to use the network more frequently than permitted by the default round-robin
operation. Eight levels of priority are provided using a 3-bit reservation field and a 3-bit priority
field. As an information frame passes, a station sets a higher priority in the reservation field,
which reserves the token. The transmitting station then sends a token out with the higher
priority set. After the high priority station completes sending its frame, it releases a token
with the normal or previous priority.
Active Monitor (AM)
One station on the Token Ring is selected to be the AM. This station performs a variety of ring-
maintenance functions. The AM removes continuously circulating frames that are not removed
by a failed transmitting station. As a frame passes the AM, the monitor count bit is set. If a frame
passes with the monitor count bit set, the AM assumes that the original sender of the frame was
unable to remove the frame from the ring. The AM purges this frame, sends a Token Soft Error
message to the Ring Error Monitor, and generates a new token.
The AM provides timing information to ring stations. The AM inserts a 24-bit propagation
delay to prevent the end of a frame from wrapping onto the beginning of the frame, and also
confirms that a data frame or token is received every 10 milliseconds.
Standby and Ring Error Monitors are also on Token Ring networks. Standby Monitors take over
as AM if the primary AM is removed from the ring or no longer performs its functions. Ring
Error Monitors can also be present on the ring to collect ring status and error information.
Beaconing
A beacon frame is sent by a station that does not receive any more frames—either a data frame
or a token—from its upstream neighbor. An adapter keeps beaconing until it begins to receive
frames again. A beacon MAC frame includes the beaconing station's MAC address and the
address of the station's nearest active upstream neighbor (NAUN), indicating that the problem
lies between the two stations.
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