Allied Telesis AT-FS238b/2 - Manual (Page 26)

Overview
16
Note
MissingLink and Smart MissingLink are disabled when you perform a
link test. Consequently, to ensure that MissingLink or Smart
MissingLink is enabled on the bridging converter during normal
network operations, always set the Mode Selection button so that the
ML or SML LED is green.
Smart MissingLink
The Smart MissingLink feature performs exactly the same function as
MissingLink with one additional feature. When a link is lost on a port, the
LNK LED of the port which still has a valid connection to its end-node starts
to blink. This allows you to quickly determine which port still has a valid
connection (LNK LED blinking) and which port has lost its connection (LNK
LED off).
For example, if the network twisted pair cable to the 10Base-T/100Base-TX
port on the bridging converter were to fail, the LNK LED on the 100Base-FX
fiber optic port will blink, indicating a failed connection on the twisted pair
port. If the failure had started with the fiber optic cabling, the LNK LED on
the twisted pair port would blink.
The value to this type of network monitoring and fault notification is so that
you can quickly see which port has failed and troubleshoot your network
accordingly.