If the slave advertises when it is available for receiving, how do the master and slave sync when data should be sent?

Answer

A slave will advertise a packet (up to 31 bytes) consisting of data seen as a broadcast. The packet contains header bits that effectively say "you can now connect to this radio within 150 microseconds and at this address". The central end (i.e. master), then requests a connection with parameters saying "I will poll you every X milliseconds" (ranging from a few to 4000). Once that connection is up, the peripheral end (i.e. slave) wakes up every X ms (as negotiated in the connection request) to see if data is to be exchanged over that pipe. The protocol used over that pipe to exchange data is called GATT protocol.

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