KTI Networks KCD-400 Series - Manual (Page 12)

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Far End Fault Function
The fiber port is facilitated with this function. When the fiber port detects
a link failure on its receiving circuitry, it will send out an FEFI (Far End
Fault Indication) signal to the remote connected device to indicate a
remote fault is detected.
It also is capable to receive FEFI signal sent from the remote link partner
if the link partner detected a receiving fault. Upon receiving an FEFI
signal, it indicates a link failure occurred on the transmitting path. This
function allows the converter to report a fiber link fault even when a link
failure occurred on transmitting fiber cable.