Ring Maintenance

All token ring has a monitor that oversees the ring between the monitor's responsibilities are seeing that the token is not gone, taking action when the ring breaks, clean-up the ring when garbled frames appear and watching out for orphan frames or message frames. An orphan frame occurs when a station transmits a small frame in it is entirety onto a long ring and then crashes or is powered down before the frame can be removed. If nothing is done, the frame circulates for an indefinite period.

• Detection of orphan frames: The monitor find out orphan frames by setting the monitor bit in the Access Control byte whenever it passes through and if an incoming frame has this bit set, something is wrong given that the same frame has passed the monitor twice. Clearly it was not removed by the source, so the monitor empties it.
• Lost Tokens: The monitor has a timer that is set to the longest probable tokenless interval: when all node transmits for the full token holding time. If this timer goes off, the monitor empties the ring and issues a fresh token.
• Garbled frames: The monitor can finds such frames by their invalid format or

checksum, empty the ring and generates a fresh token. The token ring control frames for maintenance are:

The monitor periodically issues a message "Active Monitor Present" informing all nodes of its occurrence. When this message is not received for a specific time interval, the nodes find out a monitor failure. Every node that believes it can function as a monitor broadcasts a "Standby Monitor Present" message at  regular intervals, showing that  it  is ready to take on the monitor's job. Any node that find out failure of a monitor issues a "Claim" token. There are three possible outcomes:

1.  If the issuing node gets back its own claim token, then it becomes the monitor.

2.  If a packet different from a claim token is received, it seems that a wrong guess of monitor failure was made. In this case on receipt of our own claim token, we remove it. Note that our claim token may have been removed by some other node which has detected this error.

3.  If some other node has also issued a claim token, then the node with the bigger address becomes the monitor.

In order to resolve errors of duplicate addresses, Every time a node comes up it  has ot sends a "Duplicate Address Detection" message (with the destination = source) across the network. In a case when address recognizes bit has been set on receipt of the message, the issuing node realizes a carbon copy address and goes to standby mode. A node informs other available nodes of removal of a packet from the ring through a "Purge" message. One of maintenance function that the monitor cannot handle is locating breaks in the ring. If there is no movement detected in the ring (e.g. Failure of monitor to issue the Active Monitor Present token...), the usual events of sending a claim token are followed. If the declare token itself is not received moreover packets of any other kind, the  node then have to sends "Beacons" at  regular  intervals until a  message  is received indicating that the broken ring has been repaired.

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