Optelecom 9522A Installation And Operation Manual page 22

(Refer to Figure 7)
F T R R
AULT OLERANT EDUNDANT ING
With the architecture of Figure 7, all modems are arranged around a dual counter rotating fiber ring (via one
fiber), starting with one optical port of the master modem (operating in Master Fault Tolerant mode), looping
through an indeterminate number of local terminal modems (operating in Local Fault Tolerant mode), and
terminating back into the second optical port of the master controller modem completing the ring. In this
architecture, the master modem transmits data in both directions around the ring and both signals arrive back
at the master (ECHO). The ring is terminated at the master modem. In other words, the data is not repeated
by the master modem, but instead it is terminated (ends) there. Each local modem in the ring transmits its
input data in both directions, receives from both directions, and retimes and retransmits the data received by
the optical receivers in both directions. Each local modem, receiving data from the master from both directions
around the ring, will select the data that arrives first and output that signal from both data ports. If the
transmission is interrupted in that direction, the modem will automatically select the data from the other
direction. The master modem also receives data from both directions from each of the local modems and
selects the first arriving data in the same manner. As described in the Daisy Chain Mode above, a fault
tolerant ring can be branched out using any port on the ring to a non-fault tolerant daisy chain. Unlimited
daisy chain branches may be connected to the fault tolerant ring.
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