Introduction to Data Communications
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45m. Token Ring Monitors and Servers

The Active Monitor (AM) is the active node with the highest address: it wins active monitor status by a token claiming process that takes place between all active nodes. Any node can become the AM, all other nodes become Standby Monitors in case the Active Monitor fails or turns off.

The duties of the Active Monitor are:

• Maintaining the Master Clock
• Ensuring Proper Ring Delay
• Initiating Neighbour Notification
• Monitoring Neighbour Notification
• Monitoring Token and Frame Transmission
• Detecting Lost Tokens and Frames
• Purging the Ring

Maintaining the Master Clock

The Active Monitor maintains the ring's master clock which controls timing and ensures that all other clocks on the ring are synchronized. The AM beats the drum for the other nodes to follow.

Ensuring Proper Ring Delay

It inserts a latency buffer (delay) to guarantee a minimum ring length. The delay is 24 bits long for 4 Mbps Token Ring and 32 bits long for 16 Mbps Token Ring.

Initiating Neighbour Notification

The Active Monitor periodically broadcasts the Active Monitor Present MAC frame to all ring stations on its ring, allowing each to acquire the address of its Nearest Active Upstream Neighbour (NAUN). The NAUN address is used during error isolation to determine if there is a failing component in a given ring station's fault domain (next node over).

The nodes on the ring are aware of the MAC addresses of their Nearest Active Upstream Neighbour. Notice the word "active", the neighbour must be connected to the ring.

Introduction to Data Communications
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