EXFO FTB-8510B Guia Del Usuario página 937

DSn Tabs
FDL RX
Link Activity
Indicates the activity of the following parameters during the last second of
measurement A link activity is indicated by an LED.
Idle indicates that only idle codes have been detected in the last second.
Priority indicates that at least one valid priority message has been
detected in the last second.
Command/Response indicates that a least one valid command and
response has been detected in the last second.
Unassigned indicates that at least one unassigned message has been
detected in the last second. Therefore, since an unassigned message is
part of a Command/Response codewords, the Command/Response LED
will also be red.
PRM indicates that at least one PRM has been detected in the last second.
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SONET/SDH Application