EXFO RTU 310-310G IP Services Test Head - Manual (Page 370)

Advanced Tab
358 FTB-8500 Series and FTB-8120NGE/8130NGE
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 red if there is an SDT, and lasts until the next No
Defect Time period for Defect Mode, and until the next frame for the
No Traffic Mode. For No Traffic Mode, the LED will only turn red when
the No Traffic Mode period has been reached or exceeded. The C LED
is green when there is no SDT. After an SDT with Defect Mode, the C
LED will only turn green after the No Defect Time has been met.
The H (History) LED indicates if any SDT occurred in the past (LED is
red) or not (LED is green).