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Basics of Bluetooth Technology

Bluetooth is a wireless technology for short range communication. It's application ranges from wireless computer mouse, keyboard, headset, and printer, to home security and wearable health and sports monitoring devices. It uses the Uses 2.4 GHz (2400 2483.5 MHz) ISM (Industrial, Scientific and Medical) spread spectrum radio band.

The ISM radio band is used by microwave ovens, cordless phones, garage door openers, and other common devices, so it can be noisy. Bluetooth provides functionality to detect nearby Bluetooth devices, and in order to mitigate interference, implements frequency hopping.

To detect nearby Bluetooth devices a master device Sends out an inquire, which is a request for nearby devices (within 10 meters) to issue a response that allows them to be discoverable. A master device can also work in slave mode and issue a response that allows them to accept connection to another master device. A slave device can not initiate a connection, only accept connections. Note that nearby devices where the signal is blocked by something like a metal file cabinet can be undiscoverable.

Bluetooth Scatternet
Bluetooth Scatternet

Up to 8 Bluetooth devices can be connected in master/slave configuration called a Piconet There can be up to 7 active slaves for each master. A three bit address (AM_ADDR) is given to each active slave. Piconets can be combined to form scatternets providing unlimited device connectivity.

Inquiry Procedure

To be found by other Bluetooth devices, the first device must be set to discoverable mode, this allows other Bluetooth devices within the discoverable distance to detect its inquire and issue a response in attempt to establish a connection.

When two devices find each other they detect what type of device they are, cellphone, headset, etc., along with its Bluetooth device name. The Bluetooth device name is assigned by the manufacturer, or it can be assigned by the user.

The two devices agree on a code they will use for passkeys and the master device sends a passkey.

The slave device sends the passkey and the passkeys are compared and if they are both the same, a trusted pair is formed and a Bluetooth connection is established.

Once the Bluetooth pairing has occurred, communication can be exchanged between the devices.

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