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Tech Explained: Bluetooth


Our latest Tech Explained looks at how Bluetooth technology can connect wirelessly to devices, and what are the benefits and pitfalls of using it.

Most people use wireless communication every day – television, mobile telephones and WiFi all use radio waves to send information wirelessly. Bluetooth also uses radio waves, but in a different way from these other technologies. So how does Bluetooth work?

Bluetooth was named after Harald Bluetooth, the King of Denmark in the late 900s. King Harald Bluetooth united Denmark and Norway into a single kingdom. Bluetooth technology today is used to unite separate devices.

Whenever two devices need to talk to each other, they first have to agree on how they will speak to each other. To do this, they use a common set of commands and responses known as a protocol. Bluetooth is essentially a protocol that operates over short distances – usually those of around 10 metres (30 feet) or less. Common uses for Bluetooth include connecting a wireless mouse to a laptop, and linking a hands-free headset or wireless headphones to a mobile. At Springwise, we have also seen Bluetooth used for keeping track of users belongings and to operate hands-free AR glasses. All of these devices have built-in radio antennas which send and receive wireless signals to other Bluetooth-enabled devices.

Bluetooth communicates on a frequency band centred around 2.45 gigahertz, which has been set aside by international agreement. A wide number of devices, such as baby monitors and garage door openers, also use this frequency band. One way that Bluetooth avoids interfering with other devices is by sending out only very weak signals of around 1 milliwatts. Mobiles phones, in comparison, transmit a signal of around 3 watts. It is this low power which limits the range of Bluetooth devices.

Device manufacturers program each device with a range of addresses. When Bluetooth-enabled devices come within range of each other, they send out signals asking for a response from any units with an address in that range. When they find one, the devices form a tiny network. An electronic conversation takes place automatically to determine if they have any data to share, or if one device needs to control another. Any signal received from a device not in the network is ignored.

Bluetooth allows up to eight devices to connect at the same time to create a personal-area network, or piconet. Each piconet has one ‘master’ and up to seven ‘slave’ devices. The piconets use a technique called spread-spectrum frequency hopping to keep the networks separate. This involves cycling randomly through around 79 different frequencies within a designated range. The master device controls the synchronisation so that all the devices in the piconet are cycling at the same time.

Furthermore, Bluetooth transmitters change frequencies around 1,600 times every second, making it unlikely that any two transmitters will be on the same frequency at the same time. This allows piconets in close proximity to remain completely separated from one another. It also minimises the risk that other devices will disrupt the network, since any interference on a particular frequency will last only a tiny fraction of a second. It is possible for two or more separate piconets to join up, forming a scatternet.

As in any wireless network, security is a concern. Bluetooth connections are made automatically, based on proximity. It is possible for people to send data over the system without receiving permission first. Security problems unique to Bluetooth include Bluejacking, where contact information is sent to a nearby Bluetooth user. Once the information is in the users’ address book, the hacker can send messages that can be automatically opened because they’re coming from a known contact. In Bluebugging, hackers remotely access a user’s phone to place calls and sending text messages. The Car Whisperer is a piece of software that allows hackers to send and receive audio from a Bluetooth-enabled car stereo. To avoid these issues, users should set Bluetooth-enabled devices to only allow automatic contact from trusted devices. Bluetooth can also be set to a ‘non-discoverable’ mode while in public.

Bluetooth developers are continuously upgrading the system to allow connections between a wider range of simpler devices, using less power. Will Bluetooth continue to be used once the Internet of Things is more widespread, or will a new system be needed?