Ring Poll

Ring Polling

Ring polling or Neighbor notification is a process of "roll-calling" that occurs in a properly operating Token Ring network every seven seconds. The sole purpose of the ring polling process is to allow every station on the ring to learn its Nearest Active Upstream Neighbor (NAUN). Knowing its NAUN allows a station on a Token Ring network to identify a fault domain in which the cause of a problem must be located, thereby allowing the network administrator to more easily solve the problem.

Ring Polling Description

In this discussion, we will refer to an imaginary Token Ring Network consisting of stations A, B, C, and D. Station A is currently acting as the Active Monitor, and data flows downstream in alphabetical order with Station B downstream from Station A, Station C downstream from Station B, etc... and finally, Station A downstream from Station D. (Fig. 1)

The following events occur in ring polling:

1. A seven second timer in Station A expires. Station A sends out an Active Monitor Present (AMP) frame addressed to the broadcast address. At this stage, the Address Recognized (ARI) and Frame Copied (FCI) bits in the frame are set to 0. (Fig. 2)
2. Station B receives the AMP frame, and the following events occur:
   • Seeing that the ARI and FCI bits are set to 0, Station B sets both the ARI and FCI bit to 1.
   • Station B records the physical layer address of the sending station as its NAUN. It knows that the sending station is its NAUN because by definition, the first station to see either an AMP or SMP (Standby Monitor Present) frame with the ARI and FCI bits set to zero will set those bits to one. Since it received the AMP frame with the ARI and FCI bits set to zero, Station B knows it must be the first station to see the frame, and therefore must be the downstream neighbor of the sending station.
   • If Station B's NAUN has changed since the last time ring polling occurred, it queues for transmission a Report Stored Upstream Adapter Change frame. The Report SUA Change frame is addressed to the ring's Configuration Server.
   • Station B queues for transmission a Standby Monitor Present (SMP) frame to be sent out in 20 milliseconds. The reason for the 20ms delay is to allow a window in the ring polling process during which data can be transmitted onto the wire, preventing the ring from being clogged with neighbor notification frames.
   • Station B then regenerates the AMP frame out onto the wire, like it would any other frame. Other stations on the ring, seeing that the ARI and FCI bits are set to one ignore the frame and bit repeat it until it returns to the Active Monitor and is stripped from the wire. (Fig. 3)
3. When Station B's 20ms timer times out, it sends an SMP frame. Station C receives the SMP frame, and the process outlined in item 2 occurs in Station C. (Fig. 5) The fact that in this case a SMP frame is being received instead of an AMP frame makes no difference.
4. The process outlined in item 2 happens for each station. Eventually, Station D sends out an SMP frame with the ARI and FCI bits set to zero. (Fig. 7) When Station A, who is acting as the Active Monitor, receives an SMP frame with the ARI and FCI frames set to zero (i.e. from its NAUN), it recognizes the fact that neighbor notification has completed. If neighbor notification does not complete within seven seconds, the Active Monitor reports a Ring Poll Error to the ring's Error Monitor.

If a station on the ring does not see an AMP frame pass by at least once every 15 seconds, it initiates monitor contention.