How Bluetooth Works: Hopping Channels and Pairing Devices
Bluetooth links nearby devices over a shared radio band by hopping between channels. Here is the difference between Classic and Low Energy, and where the name came from.
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Bluetooth is a short-range radio standard that lets nearby devices such as phones, headphones, watches and car systems exchange data without cables. It works in a crowded, licence-free radio band by hopping rapidly between channels, and it comes in two main forms: Bluetooth Classic, used for audio, and Bluetooth Low Energy, used for tiny low-power devices.
The radio band it uses
Both forms of Bluetooth use the 2.4 GHz ISM band, a range of radio frequencies set aside worldwide for unlicensed use. According to the Bluetooth Special Interest Group (SIG), the part used runs from 2.402 to 2.480 GHz. Wi-Fi, microwave ovens and many other gadgets share this band, so Bluetooth is built to cope with interference.
Frequency hopping
Bluetooth uses frequency-hopping spread spectrum. Instead of staying on one frequency, the two devices jump together between channels in a pattern both know. In Classic Bluetooth there are 79 channels, each 1 MHz wide, and Wikipedia says devices typically hop about 1,600 times per second. If one channel is noisy, only a tiny slice of the data is affected. An improvement called adaptive frequency hopping, added in version 1.2 in 2003, lets devices avoid channels that are known to be crowded.
Classic and Low Energy
The SIG's technology overview describes two radio options. The table of differences it gives is summarised here.
- Bluetooth Classic: 79 channels, data rates of 1, 2 or 3 Mb/s, suited to streaming audio, headsets, speakers and car systems.
- Bluetooth Low Energy (LE): 40 channels of 2 MHz (three used for advertising, 37 for data), data rates from 125 kb/s up to 2 Mb/s, suited to sensors, wearables and positioning.
- Topologies: Classic is point-to-point. LE supports point-to-point, broadcast and mesh, which allows large networks such as building controls.
Low Energy was introduced with version 4.0 in 2010 and is designed to sip power. Wikipedia lists a wake-up latency of about 6 ms for LE, against about 100 ms for Classic.
Connecting and pairing
Devices find each other by sending out and listening for short announcements. One device acts as the master and sets the hopping pattern, and the others follow. In a Classic network, Wikipedia says one master can talk to up to seven active devices in a group called a piconet.
Pairing creates a trusted link. Secure Simple Pairing arrived in version 2.1 + EDR in 2007 to make the process easier and safer. Wikipedia lists the encryption as 56- or 128-bit for Classic and 128-bit AES for Low Energy.
How far does it reach?
Range depends on transmit power. Wikipedia describes power classes from Class 3, at under 1 mW, up to Class 1, at 10 to 100 mW. Typical range is about 10 metres, and can reach about 100 metres with higher-power equipment. Walls, bodies and antenna design all change the real-world distance, and the weaker device in a pair sets the limit.
Why is it called Bluetooth?
Development began in 1989 at Ericsson in Lund, Sweden, with the aim of wireless headsets, and Wikipedia credits Jaap Haartsen and Sven Mattisson among the engineers. The name was suggested in 1997 by Jim Kardach of Intel. It refers to Harald Bluetooth, a 10th-century Danish king said to have united Danish tribes, as a nod to uniting different communication protocols. The logo combines the runes for his initials. The Bluetooth SIG was announced on 20 May 1998 by Ericsson, IBM, Intel, Nokia and Toshiba.
Versions and compatibility
Version 2.0 + EDR in 2004 introduced faster data rates. Version 4.0 in 2010 added Low Energy. Version 5.0 in 2016 added longer-range and faster LE options, and version 5.2 added LE Audio. Wikipedia lists version 6.0 in 2024 as adding Channel Sounding, which measures distance, with later 6.x updates after that. Wikipedia states that all versions are backward compatible with earlier ones, so newer devices can still talk to older ones, though only with the features both support.
The SIG says more than five billion Bluetooth products ship each year. Feature support varies by product, so check a device's specification for the version it uses.
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