Specifications

B.8 Ring Operation
B–16 FDDI Overview
B.8 Ring Operation
B.8.1 Overview
FDDI rings operate according to the rules of the Timed Token Protocol.
The operation of the ring can be divided into two states:
• Ring Initialization State
• Steady State Operation
B.8.2 Ring Initialization State
The FDDI ring is initialized by a process known as the claim token process, that is
invoked whenever a station enters or exits the ring, or if a failure of the normal ring
operation is detected. The purpose of the claim token process is to set the operational
timers for the ring, and to choose a station that is responsible for creating the token.
B.8.3 The Claim Token Process
All stations send a special frame, known as a claim frame, containing a bid for the
length of time that the station is willing to wait between receipt of tokens. The station
that wins the bidding is the station whose bid indicates that it needs the token the
most often. If the outcome of the bidding indicates a tie, the station with the highest
MAC address wins.
The outcome of the claim process is that all stations agree to abide by this time
period, which is known as T_Negotiated (T_Neg). Typical values for T_Neg are in
the range of 5-10 ms, with 8 ms being a common choice.
Varying the value of T_Neg can dramatically change the utilization and latency of a
very busy FDDI ring, but it has minimal affect on the utilization or latency of a
lightly loaded ring. Digital recommends not changing the value of T_Neg from the
device manufacturer’s default setting.
B.8.4 The Beacon Process
If a ring fails to complete the claim process within a certain time, the ring enters the
beacon process. The beacon process causes special frames known as beacon frames
to traverse the ring in an attempt to isolate the location of the fault.
Understandably, when beacon frames are detected traversing the ring, it usually indi-
cates a serious problem with the operation of the ring.