The decision on which device should be the slave and master is application specific, and depends on the desired topology, power constraints and even user interface considerations on both sides of the link.
Channels are bidirectional, so the master does not need to be the data source. The slave will incur higher power consumption during channel acquisition, so generally it is desirable to set the most power constrained device as the master. In some cases, the topology will restrict this decision (e.g. in a shared channel the hub is always the master).
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