Introduction to Data Communications
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The AM monitors the ring to make sure that Tokens and Frames only circle the ring once. There is a Monitor bit in the MAC frame and whenever a MAC frame is repeated by the AM, the AM sets the Monitor bit to "1". All frames that are received with the Monitor bit set to 1 are not repeated - this means that the frame has already circulated the ring once.

Detecting Lost Tokens and Frames

The AM has a timer to check that there are Tokens and Token Frames circulating the ring. The timer is set for the absolute longest time that it would take for a Token or Frame to circulate the ring. If the timer times out before a new Token or frame is received. The ring is purged and a new Token is released.

Purging the Ring

The AM broadcasts the Ring Purge MAC frame to all ring stations on its ring before originating a new Token. Receipt of the returned frame indicates to the AM that a frame can circulate the ring without incident. The Ring Purge Frame resets the ring stations to Normal Repeat mode.

Introduction to Data Communications
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