What is the Difference Between Automatic Protection Switching (APS) Time and Service Disruption Time (SDT) Measurements?

APS Time was meant to measure the time it took for an impaired link to switch to a backup path. The newer SDT concept focuses on the end-to-end impact caused by APS events as well as other network impairments.

Automatic Protection Switching (APS) is a network feature that automatically switches traffic to a backup link or path, when a primary link fails. It is meant to minimize the service downtime by quickly rerouting data.

The APS Time measurement, popular during the early days of digital communications and fiber optics, measured the link's switching time. For example, how long did it take for the signal to recover, in the event of a fiber cut or severe impairment. APS Time measurements were originally based on physical layer events and measured the time between the fault detection and the signal recovery. The expected APS time values were between 20 ms and 50 ms.

APS time measurements may no longer be considered adequate for modern communication systems. It has been replaced by Service Disruption Time (SDT).

SDT is meant to provide a better measurement of the impact a network failure has on the actual duration of a service interruption. The SDT measurement includes the time it takes to detect the fault, initiate the APS (if available or required), and the time it takes the endpoints to resync and pass traffic on the backup path, providing a better insight into the overall impact of a failure on the user experience.

Certain VeEX test equipment still offer the legacy APS time measurement, since some customers still require it (or still refer to it as APS). However, we recommend the use of SDT instead, since it offer better resolution, multiple triggers and offer details of what happened during the event. SDT measurements can still be used to measure pure APS, by selecting the right triggers.

How is Service Disruption Time Measured?

A Service Disruption event starts at the onset (detection) of the first trigger (sensor) and ends once the link or tributary remains error free for the X amount of time defined by the Gate Time. The SDT is declared (measured) between the first and last events detected. Users can select multiple error, alarm, defect, anomaly types to be considered as the Triggers for service disruption.

Note: To strictly measure traditional Automatic Protection Switching (APS) Times with the SDT method, (a) select the Loss Of Signal (LOS) as the trigger/sensor when testing at the physical link switching characteristics level, or (b) select the AIS sensors when monitoring channels or tributaries (at the payload levels).

Here is a simplified graphical example of the SDT measurement, in relationship with different Gate Times, applied to the analysis of the same event:

Basic diagram depicting how service disruption event times are measured and the effect of the Gate Time.

Note that if the events are measured using a 300 ms error-free Gate Time, the instrument will report two SDT events. However, if the same event is measurement with a 100 ms Gate Time, the instrument will measure three SDT events, since there is enough error-free time after the first hit, to close the event and start looking for a new one.

If enabled, the SDT measurements may run concurrently with the BER test (BERT). The sensing and disruption measurement starts when the BERT is started.

The simplified diagram above shows different errors (E) and alarms (A) occurring during the service disruption time, with some of them being part of the recovery process. Users can check the SDT tab on the test results and (if available) click on Events Details, to get a list of all the errors and alarms that occurred during the service disruption event. This information can help with understanding what may have caused the event and in the troubleshooting process.

This BER test results page shows the main 102.5 ms Service Disruption event and a detailed list of all the alarms and errors that occurred as part of the fault and during the recovery part.

What is the Difference Between Automatic Protection Switching (APS) Time and Service Disruption Time (SDT) Measurements?

APS Time was meant to measure the time it took for an impaired link to switch to a backup path. The newer SDT concept focuses on the end-to-end impact caused by APS events as well as other network impairments.

How is Service Disruption Time Measured?

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