How Bluetooth devices connect

Bluetooth networks (commonly referred to as piconets) use a master/slave model to control when and where devices can send data. In this model, a single master device can be connected to up to seven different slave devices. Any slave device in the piconet can only be connected to a singlemaster.

Bluetooth connection between two devices is a multi-step process involving three progressive states:
Inquiry — One Bluetooth device run an inquiry to try to discover the other. One device sends out the inquiry request, and other device respond with its address, name and other information.
Connecting — Each device understands the address of others and creates a connection
Connected — After a device has completed the connecting process, it enters the connection state. While connected, a device can either be actively participating or it can be put into a low power sleep mode.

There is a difference between pairing and bonding. Pairing is the exchange of security features between devices. Master send their features and slave confirms having those features. This is actually a compatibility test among Bluetooth devices. Once the pairing feature exchange is complete, a temporary security key is exchanged and the connection is encrypted, but only using the temporary key. So The devices are paired. Simultaneously the devices STORE and USE those keys for next time they connect. Keys can be exchanged using the bonding procedure. Thus common key creates bonding. Whenever such two devices are close together, they read the bonding key and automatically pair up.

In the case of our car, the smartphone gets connects to the car’s Bluetooth system beacause of these stored keys. Headphone gets connected to the smartphone in the same way.

Drain on battery power has improved with the advent of new technologies. Now this problem is less significant than it used to be.

The range is limited, usually up to 33 feet (10 meters), and obstacles such as walls, floors, or ceilings reduce this range further.The range has considerably increased in Bluetooth5

The pairing process may also be difficult, often depending on the devices involved, the manufacturers, and other factors that all can result in frustration when attempting to connect.

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