android noob here!
Is there a difference between when two android devices (smartphones) are 'paired' and when they're 'connected'? If so, what is the difference when transferring data from one to the other?
Please imagine yourself in such a situation:
You come to car with BT audio system first time.
You are not paired and not connected.
You pair the devices and because you are close to the car you immediately connecting. But when you go to home and you are away from car you are disconnected but paired. When you come close to the car again, you don't need pair, you don't need put pin again to pair the devices
Based on comment above: BT communication requires 'pairing' so as to confirm permission to communicate between two devices. I am interpreting this as analogous to creating a user account on a Linux system. If you have an account (devices are paired), then they are eligible to be connected. (Necessary but not sufficient condition)
It is unclear from above whether to be 'connected' means that both BT radios are on, in proximity of one another, and the amount of communication (polling?) that is occurring to be in connected state.
来源：https://stackoverflow.com/questions/32121893/android-bluetooth-paired-vs-connected-whats-the-difference