Receive Signal Strength Indication (RSSI)
The RSSI line outputs an analog voltage that is proportional to the signal
strength present on the channel at the time. In normal operation, the
module is hopping rapidly from channel to channel. In this case, the
RSSI value varies greatly and does not provide much useful information.
However, it can be used to keep a module awake by sampling the RSSI
line to determine if the module is processing a packet before putting it to
sleep.