EXFO FTB-8510B Guia Del Usuario página 1064

Advanced Tabs
Service Disruption Time (SDT)
Service Disruption: Indicates the time (in seconds) during which
there is a disruption of service due to the absence of traffic or to the
detection of defects. The H and C LEDs indicate respectively the
current (C) and history (H) SDT measurement states.
The C (Current) LED is green when there is no SDT. The C LED is red if
there is an SDT, and last until the next No Defect Time has been met or
the test period is elapsed.
The H (History) LED indicates if any SDT occurred in the past (LED is
red) or not (LED is green).
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FTB-8100 Series Transport Blazer