FDDI uses token passing to control which station has access to the
network at any given time. When a station that is ready to transmit data receives the
token, the station removes the token from the ring and transmits its data. When data
transmission is complete, the station places the token back on the ring.
A token can, occasionally, become lost or corrupted. FDDI stations use a
configurable setting called the Maximum Token Rotation Time (tmax-lower-bound) to
regenerate a token if this situation occurs. The Maximum Token Rotation Time is the length
of time a station waits to receive the token, since the last time it had possession. If
the Maximum Token Rotation Time is exceeded, the station then uses a second value, the
Requested Token Rotation Time (t_req) to negotiate for token possession. The node with the
lowest Requested Token Rotation Time wins the bid for the token. If more that one station
has the same Requested Token Rotation Time, the station with the highest MAC address wins.
Similarly, FDDI stations use a configurable setting called the Valid
Transmission Time (tvx-timer) to determine the maximum amount of time the interface waits
to receive a valid data frame. The absence of valid data frames for an extended
period of time might be caused by conditions such as babble errors, repeated partial
frames, or loss of the token. If the Valid Transmission Time is exceeded, the token claim
process is reinitiated. |