Specifications

TOKEN RING OVERVIEW
2-16
The Standby Monitor
All other stations are Standby Monitors (SMs). The SMs verify that the AM
is on the ring and performing its duties. Each SM checks for good tokens
using the “Good Token Timer” (2.6s); the SMs also verify that the AM is
transmitting an Active Monitor Present MAC frame using the Receive Poll
Timer (15s). If an SMs’ Good Token Timer or Receive Poll Timer expires,
the station issues a Claim Token MAC frame. In the event there is a Claim
Token MAC frame issued, all stations enter into a monitor contention
dialogue, resulting in one station (the highest currently active address)
establishing itself as the AM.
Monitor Contention Process
All stations are capable of being AM but a ring must only have one. All
other stations go into SM mode.
The establishment of the AM is usually achieved at initialization time. It is
usually the first station to access the ring. However, there are obviously
situations when a new active monitor is required. This process of selecting
a new AM is known as Monitor Contention and can be started if any of the
following conditions occur:
• Good token not received every 2.6 seconds
• AMP (Active Monitor Present) frame not received every 15 seconds
• No AMP, SMP (Standby Monitor Present), or purge for 18 seconds
during insertion
• AM cannot successfully purge the ring
• Clock frequency moves out of specification
• Beacon transmitter receives Beacon with MC
Monitor contention uses a process known as Claim Token to determine
which station is going to be the AM.
Claim Token
The Claim Token process is the same regardless of the condition that
caused it. As timers expire each station starts to transmit a Claim Token
MAC frame onto the ring.
Thus, each station also receives Claim Token frames and compares the
source address in the frame to its own. A station whose address is
numerically less than the source of the Claim Token frame repeats the
frame and ceases issuing its own. If its address is numerically greater, it
generates its own Claim Token MAC frame and does not repeat the frame
it received.