How Does Bluetooth Work? Short-Range Radio That Hops Between Channels
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The name on the side of every wireless headphone box belongs to a tenth-century Danish king, Harald Bluetooth, who united the tribes of Denmark and Norway, and the engineers who picked it in 1997 meant it as a joke about uniting the tribes of computing and telephony. It stuck. The technology beneath it is a low-power radio designed to link devices a few metres apart without wires or a network, and it has done that for some five billion devices a year while staying mostly invisible.
A radio in the crowded band
Bluetooth transmits in the 2.4 gigahertz band, a slice of the radio spectrum that regulators around the world left free for anyone to use without a licence, which is why it is also home to Wi-Fi, microwave ovens, baby monitors and garage door openers. The band is crowded and noisy, and Bluetooth's founding trick is to cope with that by never staying still. It divides the band into 79 channels one megahertz wide and hops between them 1,600 times a second in a sequence the two connected devices have agreed, so that interference on any one channel spoils only a tiny fraction of the data, which is then resent.
The power is deliberately low, about a milliwatt for most devices, a thousandth of a phone's cellular transmitter, which limits the range to around ten metres and the drain on a small battery. Higher-power classes reach a hundred metres, and the latest versions can reach further with better coding, but the design philosophy is short range and long battery life rather than the reverse.
Pairing
Two devices that have never met need to find each other and agree a secret. Pairing begins with one device advertising itself on a few dedicated channels and the other listening; the phone shows a list of names, the user picks one, and the two exchange keys. Modern pairing uses public-key cryptography so that an eavesdropper watching the exchange cannot recover the secret, and the six-digit code that some devices ask the user to compare exists to defeat an attacker sitting between them and impersonating each to the other. Once paired, the two remember each other, and subsequent connections happen without the user doing anything.
A connection has a master and slaves. The master, usually the phone, sets the clock and the hopping sequence, and up to seven active devices can follow it in a small network called a piconet. The word, and the whole standard, comes from a project at Ericsson in Sweden in 1994 to replace the cables between a phone and its accessories, later opened to Intel, Nokia, IBM and Toshiba, who formed the group that still controls the specification.
Two Bluetooths
Since 2010 there have been two largely separate technologies under the one name:
- •Classic Bluetooth: a continuous connection built for streaming, used by headphones, speakers and car audio, where the audio is compressed by a codec such as SBC, AAC or aptX and sent as a steady stream
- •Bluetooth Low Energy: a different radio design that sleeps most of the time and wakes to send small packets, used by fitness bands, smart watches, tracking tags, heart rate monitors and the contact-tracing apps of 2020, and able to run for a year on a coin cell
Why the sound sometimes cuts out
The band's crowding is the usual reason. A busy office full of Wi-Fi, a body between phone and earpiece, which absorbs 2.4 gigahertz well, since the frequency is close to the one microwave ovens use to heat water, or a microwave running in the next room can each drop enough packets for the audio to stutter. Latency is the other complaint: compressing, sending and decoding audio takes between about forty and two hundred milliseconds depending on the codec, which is invisible for music and noticeable for games and for video, where the sound arrives late unless the player delays the picture to match. The newest audio standard, LE Audio, cuts the delay, lets one phone broadcast to any number of earpieces, and is the basis of a scheme for hearing aids to pick up audio in public places.
Security
Early Bluetooth was famously loose, and the 2000s produced a run of attacks with names like bluejacking and bluesnarfing that read contacts from phones left discoverable. The modern protocol is far stronger, but the attack surface remains: a device that is discoverable can be seen, pairing requests can be spoofed, and flaws in individual implementations are found every year. The practical advice has not changed: keep devices updated, do not leave them discoverable when not pairing, and refuse pairing requests you did not initiate.
The takeaway
Bluetooth is a low-power radio standard for linking devices over a few metres in the unlicensed 2.4 gigahertz band, hopping between 79 channels 1,600 times a second to dodge interference. Devices pair by exchanging cryptographic keys and then remember each other; classic Bluetooth streams audio while Bluetooth Low Energy sends small packets and sleeps; and the drop-outs and delays come from a crowded band, the human body, and the time it takes to compress and decode sound.