Bluetooth network connection & pairing

- summary of Bluetooth network configurations - how Bluetooth networks operate, how Bluetooth connections are set up and Bluetooth pairing.

Bluetooth networks often operate as a single connection, or a Bluetooth network may involve many devices. Bluetooth also allows for a scheme known as Bluetooth pairing where devices can quickly associate.

The Bluetooth specification defines a variety of forms of Bluetooth network connection that may be set up. In this way Bluetooth networking is a particularly flexible form of wireless system for use in a variety of short range applications.

Bluetooth network connection basics

There are a variety of ways in which Bluetooth networks can be set up. In essence Bluetooth networks adopt what is termed a piconet topology. In this form of network, one device acts as the master and it is able to talk to a maximum of seven slave nodes or devices.

The limit of seven slave nodes in a Bluetooth network arises from the three bit address that is used. This number relates to the number of active nodes in the Bluetooth network at any given time.

Bluetooth scatternets

Bluetooth network connections are also able to support scatternets, although because of timing and memory constraints this form of Bluetooth network has rarely been implemented. For a Bluetooth scatternet, a slave node or slave device is able to share its time between two different piconets. This enables large star networks to be built up.

Bluetooth connection basics

The way in which Bluetooth devices make connections is more complicated than that associated with many other types of wireless device. The reason for this is the frequency hopping nature of the devices. While the frequency hopping reduces the effects of interference, it makes connecting devices a little more complicated.

Bluetooth is a system in which connections are made between a master and a slave. These connections are maintained until they are broken, either by deliberately disconnecting the two, or by the link radio link becoming so poor that communications cannot be maintained - typically this occurs as the devices go out of range of each other.

Within the connection process, there are four types of Bluetooth connection channel:

  • Basic piconet channel:   This Bluetooth connection channel is used only when all 79 channels are used within the hop-set - it is now rarely used as the Adaptive piconet channel is more often used as it provides greater flexibility.
  • Adapted piconet channel:   This Bluetooth connection channel is used more widely and allows the system to use a reduced hop-set, i.e. between 20 and 79 channels. Piconet channels are the only channels that can be used to transfer user data.
  • Inquiry channel:   Theis Bluetooth connection channel is sued when a master device finds a slave device or devices within range.
  • Paging channel:   This Bluetooth connection channel is sued where a master and a slave device make a physical connection.

Bluetooth pairing

In order that devices can connect easily and quickly, a scheme known as Bluetooth pairing may be used. Once Bluetooth pairing has occurred two devices may communicate with each other.

Bluetooth pairing is generally initiated manually by a device user. The Bluetooth link for the device is made visible to other devices. They may then be paired.

The Bluetooth pairing process is typically triggered automatically the first time a device receives a connection request from a device with which it is not yet paired. In order that Bluetooth pairing may occur, a password has to be exchanged between the two devices. This password or "Passkey" as it is more correctly termed is a code shared by both Bluetooth devices. It is used to ensure that both users have agreed to pair with each other.

The process of Bluetooth pairing is summarised below:

  • Bluetooth device looks for other Bluetooth devices in range:   To be found by other Bluetooth devices, the first device, Device 1 must be set to discoverable mode - this will allow other Bluetooth devices in the vicinity to detect its presence and attempt to establish a connection.
  • Two Bluetooth devices find each other:   When the two devices: Device 1 and device 2 find each other it is possible to detect what they are. Normally the discoverable device will indicate what type of device it is - cellphone, headset, etc., along with its Bluetooth device name. The Bluetooth device name is the can be allocated by the user, or it will be the one allocated during manufacture.
  • Prompt for Passkey:   Often the default passkey is set to "0000", but it is advisable to use something else as hackers will assume most people will not change this.

    However many more sophisticated devices - smartphones and computers - both users must agree on a code which must obviously be the same for both.
  • Device 1 sends passkey:   The initiating device, Device 1 sends the passkey that has been entered to Device 2.
  • Device 2 sends passkey:   The passkeys are compared and if they are both the same, a trusted pair is formed, Bluetooth pairing is established.
  • Communication is established:   Once the Bluetooth pairing has occurred, data can be exchanged between the devices.

Once the Bluetooth pairing has been established it is remembered by the devices, which can then connect to each without user intervention.

If necessary, the Bluetooth pairing relationship may be removed by the user at a later time if required.

By Ian Poole

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