Specifications

TOKEN RING OVERVIEW
2-12
when the AM loses power. The process to select a new AM is known as
Monitor Contention.
The process to select the new AM begins with one of several timers
expiring indicating that either the current AM has failed or that some
other network parameter has not been corrected by the current AM. The
Monitor Contention process can be started by any station on the ring and
begins as timers expire. As stations time out they immediately begin
broadcasting Claim Token frames. Stations receiving Claim Tokens will
compare the source address of the frame with their own address. Stations
with an address value less than the source address of the Claim Token will
repeat the received frame onto the ring and cease sending their own Claim
Token. If its own address is greater, it will then generate its own Claim
Token frame and delete the received token. Eventually, the station with the
highest address will receive its own Claim Token frame back, allowing it
to reissue three successive Claim Token frames to ensure the integrity of
the network. After receiving three consecutive Claim Tokens that station
then becomes the AM.
Latency
So that a token can circle the ring, the IEEE 802.5 standard specifies that
the ring must be big enough to accommodate a single token. The token is a
unique 24 bit pattern. The token bandwidth depends on the speed at
which it is being clocked, 4 or 16 Mbps. Thus the unit of time used to
measure the token transmission is the unit of a bit time, 250 ns or 62.5 ns. It
is impractical to physically build cabling on each end of every Token Ring
network to accommodate this token transmission time. Thus, the Active
Monitor is responsible for providing a latency buffer of a minimum 24 bit
times. This is described as the Assured Minimum Latency Buffer.
Signal Timing
As well as providing the token latency, the Active Monitor is responsible
for providing the master source timing for transmitted data using internal
clocking. As the data passes from the Active Monitor to the next
downstream station, the receiving station phase locks on the clock of the
received data and derives its transmit clock.
Accumulated Jitter
Due to variations in cable impedance and the accuracy of the locking clock
circuits, very small variations in speed may result. In worst-case scenarios
each station may add to the problem. The result is that the Active Monitor
receive clock could lock in at a data rate significantly faster or slower than