These frames are used as a keep alive signal to verify the
operation of the RPR layer. An SF is declared upon the detection of a major physical
layer outage (e.g., signal loss, loss of frame), if fairness frames are not detected within
a period of time (loss of keep alive), or a miscabling defect has been declared. A SD is
declared upon the detection of degradation in the signal received by the physical layer
(e.g., low bit error ratio in SONET), some physical layers may not be able to generate
the SD indication.
Passthrough Mode The optional passthrough mode enables stations to enter or exit
the ring without disconnection of fibers (e.g., upon detection of internal failure conditions),
and without triggering a signal fail event. Passthrough allows a station to leave
the ring while maintaining a closed-ring topology, avoiding a protection switch in the
case that the transit path of the station is operating normally, but another part of the
station failed.
Periodic transmission of TP frames allows detection of a station entering passthrough.
When another station appears to have changed its location, TP frame transmission is triggered
to facilitate fast rediscovery of the topology and restoration of strict mode traffic.
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