In BLE, does the master device keeps on performing channel sensing to identify which are used and which are unused?

[Subhanshu Sahu]

I know that master sents "LL_CHANNEL_MAP_REQ" to slave to indicate that there is a new channel map. I want to know does master keeps on doing channel sensing to know which channels are currently free and which are not. 

Or does master perform channel sensing after some defined period? In this case, what is the duration of channel sensing period

[haakonsh]

"I want to know does master keeps on doing channel sensing to know which channels are currently free and which are not."

A master does not need to do this according to BT spec, it is an optional feature. None of the Softdevices for the nRF51 series have this functionality.