Specifications

2-11
TOKEN RING OVERVIEW
With Differential Manchester Encoding the sequence of signal polarities is
completely dependent on the polarity of the second half of the previous
bit cell time. If a bit to be transmitted during any given bit cell time is a 0
(zero), then the polarity of the first half of that bit cell time is opposite to
that of the previous bit cell time, consequently as polarity changes occur at
the start of the bit cell time as well as the forced mid-bit time. If the bit to
be transmitted is a 1 (one), the polarity of the first half of that bit cell time
is the same as that of the previous bit cell time and as such there is no
polarity change at the start of the bit cell time.
Figure 2-3. Token Ring Manchester Differential Encoding
Active Monitor
Each ring must have a master station known as the Active Monitor (AM).
This station plays a critical role in both the MAC/Data Link layer and the
Physical layer functions. All stations have the ability to be the Active
Monitor but only one Active Monitor is allowed per ring at any one time.
All other stations become Standby Monitors. The AM is usually selected at
initialization time and is usually the first station to access the ring.
However, there are situations when a new AM is required, for example,
101100
1J00K
One Bit
Cell Time
Bit = 0 if there are two polarity changes in a bit cell time
Bit = 1 if there is one polarity change in a bit cell time
Bit = J if there is no polarity change at start of bit time
Bit = K if there is a polarity change at start of bit time