HP 200 Series Services And Applications page 240

Shining a Light on FDDI
How FDDI Works
Concentrators connect directly to the backbone of the ring, and provide
indirect access to the ring for other devices. Concentrators support multiple
DAS and SAS connections, and add a degree of fault tolerance to the
network by isolating end nodes from the ring.
Ring Access
When a station is first attached to the FDDI ring, it undergoes an initializa-
tion process. During this initialization process, a newly inserted station
initiates a connection management process (CMT) with its neighbor. CMT
tests port types, performs physical port tests or link confidence tests, and
transmits link-state characters upon completion of the tests. Stations initiate
CMTs between each upstream and downstream neighbor.
Upon successful completion of the initialization phase, the ring begins the
claim process to generate a token. The claim process begins with all stations
negotiating a target token rotational timer (known as TTRT). Each station
makes a bid for the timer, and the one with the lowest value wins the right to
initialize the ring by inserting the token. If two stations happen to make the
same bid, the station with the highest address wins the right to generate the
token. Once the claim process has finished, the ring enters a steady state, to
be disrupted only by the insertion of another station, or by a break in the
ring.
3-46