Token Passing and Frame Timing Parameters

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.