LE50-868 LBT not working properly

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  1. We are using a LE50-868 star network with 27 clients. Clients send 10 byte messages periodically every 20 minutes and also 25 byte messages sporadically several times a day. The clients are operated with LBT active S226=2, Repetitions S223=10, Random Waiting S227=1, Software Version GC.S00.01.04-B008.

    In our application we observed some loss of data frames which we suspect to be caused by data collision. 

    In order to further investigate this anomaly, we have set up a test network of only three clients. Sending simultaneously a 25 bytes data frame from any two clients leads to 90% loss of one of the two frames. When sending the message frame to the serial ports of the two clients, it can be seen that as expected one of them starts sending and the other one is quiet. After the first one is finished and the radio channel is free, the second one, however, does not start transmission but instead reports ERROR on the serial port.

    When we switch off LBT (S226=0) and keep all other settings as before, then the loss of any one frames is less than 5%.

    Is there anything we overlooked in the setting for LBT operation?

    When LBT is ON, what is the maximum time which a module waits until the radio channel is free?