Specifications

TOKEN RING OVERVIEW
2-18
• Station B still has a frame to transmit but it cannot capture the present
token, which is set to priority 3; it only has a priority 1. As the token
passes through Station B it sets the Priority reservation field to 1 again.
Stacking Station
Station D captures the token and transmits its frame. Station D has raised
the service priority of the ring to 3, and is termed the Stacking Station. It
notes in its reservation register that R was set to 1. It uses this to lower the
ring priority later. A Stacking Station is said to be in a Priority Hold State.
Station D transmits its priority frames and when finished, generates a
token with priority and reservation set to that held in its register, in this
example, 1. Station B captures the token and transmits its frame.
Note that the Stacking Station is the only one that can lower the ring
priority. Thus, when Station B has finished its transmission, it generates a
new token at a higher priority of 1. Station D captures the token again and,
seeing no other higher reservation set generates a new token at a 0
priority. The ring is now back to normal operation.
Note that PDUs are assigned priorities according to application.
Ring Management
One station on each ring is designated the Active Monitor (AM). This
station is responsible for monitoring the token and providing other ring
management functions. All other ring stations are Standby Monitors that
are capable of becoming the AM if the need arises.
The AM protects the ring against error situations such as lost tokens,
frames, and priority tokens that circle the ring more than once. It also
provides the master clock for the ring and the 24-bit latency delay to
ensure correct physical operation of the ring.
Purging the Ring
The AM can generate a Purge Ring MAC frame (PRG) to clear the ring of
any errors.
Ring purges occur in response to the following conditions:
• A station has just become AM.
• A frame has circled more than once, indicated by monitor bit set to 1.
• The ring is quiet or with illegal traffic, so that the Timer Valid
Transmission (TVX) has expired.