Source Loss Behavior

The sACN Receiver module uses a custom algorithm, designed by ETC's network team, to react predictably in the event that multiple sACN Sources are lost in quick succession.

Consider the following scenario:

+-----------------+  +-----------------+
| Source 1        |  | Source 2        |         +----------+
| Priority: 90    |  | Priority: 100   |         | Receiver |
| All levels 0xff |  | All levels 0x00 |         +----------+
+-----------------+  +-----------------+              ||
        ||                   ||                       ||
      ====================================================
      |                  Network switch                  |
      ====================================================

Sources 1 and 2 are sending sACN consistently with the parameters shown on the same universe, to which the receiver is listening. Then, the receiver is disconnected from the switch. The receiver starts a source loss (network data loss) timer for both sources.

The timer for Source 2 could easily expire first, in which case the receiver will for a brief time consider Source 1 to have taken over. This will result in a short flash of all of the DMX levels on that universe to full, before the source loss timer for Source 1 also expires.

To avoid this unwanted behavior, the algorithm is specified as follows:

• Whenever a source is considered to be lost, either due to the Stream_Terminated bit being set or a source loss timeout, the library shall compile a list of all other sources currently being tracked for that universe and wait until their online/offline status has been verified before sending a sources_lost() notification.
• If multiple sources are determined to be offline during this verification, they shall all be included in the same sources_lost() notification so that the application can react to them simultaneously.

This results in a time period, hereafter referred to as a settling time, after a source has been determined to be lost, during which the online/offline status of all other sources is verified. This settling time runs concurrently with the expired notification wait time, which is a global option settable through sacn_receiver_set_expired_wait(). The settling time could range from almost instantaneous to 2.5 seconds, and thus could be longer or shorter than the expired notification wait time. The sources_lost() notification is not sent until both time periods expire.

In this example, a source is lost due to timeout, and the settling time is shorter than the expired notification wait time:

Source stops                      Source                               Application
sending data                     times out                               notified
     +                               +                                      +
     |                               |                                      |
     |------------------------------>|                                      |
     |    sACN Source Loss Timeout   |---------------------------->         |
     |          2.5 seconds          |       Settle Period                  |
     |                               |      0 to 2.5 seconds                |
     |                               |                                      |
     |                               |------------------------------------->|
     |                               |       Notification Wait Period       |
     |                               |    User-settable, default 1 second   |

Here is another example where a source is lost due to termination, and the settling time is longer than the expired notification wait time:

   Source sets                                     Application
Stream_Terminated                                   notified
       +                                               +
       |                                               |
       |                                               |
       |---------------------------------------------->|
       |                Settle Period                  |
       |               0 to 2.5 seconds                |
       |                                               |
       |------------------------------------->         |
       |       Notification Wait Period                |
       |    User-settable, default 1 second            |