How Does Bluetooth Work? Pairing, Hopping and BLE Explained

In everyday life

You listen to music on wireless earbuds. Your smartwatch shows notifications from your phone. You connect a keyboard and mouse to your computer without cables. Your car speakers play the voice on your phone call. Bluetooth is at work in most of these cases.

When you start using new earbuds, you pick their name in your phone's settings. The first time only, you go through a step called "pairing." After that, they connect as soon as you take them out of the case. In this article, we look step by step at what happens behind the scenes.

Bluetooth links nearby devices directly

With Wi-Fi, phones and computers reach the internet through the router at home, which acts as a relay. Bluetooth links nearby devices, such as a phone and earbuds, directly with no relay. It mostly carries data used on the spot, such as sound, button presses and sensor readings.

Earbuds from any company work with your phone because everyone follows the same rules. These rules cover how to send radio waves and how to send data. They are set by an industry group, the Bluetooth SIG. It was founded in 1998 by five companies: Ericsson, IBM, Intel, Nokia and Toshiba. In 1999, it released the first version of the standard (version 1.0).

The name comes from Harald Gormsson, a 10th-century king of Denmark nicknamed "Bluetooth." According to the Bluetooth SIG, he is known for uniting Denmark and Norway. The name expresses a hope: to unite the computer and mobile phone industries with one wireless link. It began as a temporary code name during development, but it stuck as the official name. The logo combines two old letters, called runes, for the king's initials H and B.

Inside a phone, a dedicated chip handles Bluetooth's radio signals. Sometimes it is combined with Wi-Fi on the same chip (see "What Chips Are Inside a Smartphone?").

Sharing the 2.4 GHz band with Wi-Fi and microwave ovens

Radio waves are told apart by how many times they swing per second. This number is called the frequency (for more, see "How Does Wi-Fi Work?"). Bluetooth uses the 2.4 GHz band, where waves swing about 2.4 billion times a second. The usable width is about 80 MHz (0.08 GHz).

This band was set aside internationally for radio use by industrial, scientific and medical equipment. So it is called the ISM band, from the first letters of Industrial, Scientific and Medical. A typical example is the microwave oven, which heats food with radio waves in this band.

Anyone can use the band without a license, as long as they follow set conditions. So Wi-Fi and Bluetooth sit in this same band too. In other words, the 2.4 GHz band is a busy place, where many devices send radio waves at the same time. To communicate reliably here, Bluetooth uses a clever trick.

Frequency hopping: switching channels again and again

Classic Bluetooth divides the 2.4 GHz band into 79 slices, each 1 MHz wide. Each slice is called a "channel." Two connected devices send and receive while switching channels in a fixed order, as often as 1,600 times a second.

This method is called "frequency hopping." The order of hops is calculated from the device number and clock of the device in charge, usually the phone. The other device does the same calculation. So both devices move to the same channel at the same moment.

Because the frequency keeps changing, interference on any one channel lasts only an instant. Data that doesn't get through can be sent again on the next channel. Since version 1.2 in 2003, devices can also spot busy channels and drop them from the hopping list. This is called "adaptive frequency hopping" (AFH).

Two types: classic and BLE

Bluetooth comes in two main types. The first has been there since the very first standard. Its official name is BR/EDR, but here we call it "classic." It suits uses that stream data nonstop, such as music and calls. Its top speed in the standard is 3 million bits a second (3 Mbps). A bit is a single digit that is either 0 or 1.

The other is Bluetooth Low Energy (BLE), added in version 4.0 in 2010. It was designed to use little power, for devices that send small amounts of data now and then. It divides the 2.4 GHz band into 40 channels, each 2 MHz wide. Three of them are reserved for "announcements" to whoever is around. An announcement is a signal that says "I'm here" or carries short information, without a set receiver.

A BLE device sends a signal for just a very short time, only when it has something to send. The rest of the time, it shuts down most of its circuits and rests. Much of a device's power goes into sending and receiving radio signals. So the shorter the sending time and the longer the rest, the longer the battery lasts. This made it possible to build products that run for years on a coin cell.

The tiny computer that runs the device, called a microcontroller, also has ways to use almost no power while resting. "What Is a Microcontroller?" shows how to estimate how many years a coin cell lasts.

The two types use the same 2.4 GHz band, but they use the radio differently. So they can't talk to each other as they are. Phones and computers support both, so they connect to classic earbuds and BLE sensors alike.

Pairing: making a shared "key" first

Bluetooth signals pass through walls, so anyone nearby can pick them up. So Bluetooth scrambles data into a form that can't be read without a certain key. This is called encryption (see "What Is Encryption?" for how it works).

Pairing is the step where two devices, connecting for the first time, create a shared key for encryption and both remember it. Today's method lets them swap the ingredients of the key by radio. Yet someone listening in still can't work out the key. This method is a type of what is called key exchange. From the second time on, the devices connect right away with the key they remember.

When both devices have screens, the same 6-digit number may appear on each, with a question like "Do these match?" This checks that no third device has slipped in between and is pretending to be each side. If one has, the two numbers will almost never match. If the numbers differ, or you get a connection request you don't recognize, don't allow it.

Devices with no screen and few buttons, such as earbuds, can't do this check. So they create the key without checking. According to a guide from the US National Institute of Standards and Technology (NIST), this method can't stop a device from slipping in between. It is safer to pair earbuds in a calm place with few people around.

Profiles: rules for each use

It's all Bluetooth, but earbuds and keyboards send different data. So for each use, there are rules about what data to send and how. These rules are called "profiles."

For example, there are rules for sending music, for exchanging voices in calls, and for sending keyboard and mouse input. BLE also has rules for sending readings such as those from heart-rate monitors. Two devices can only be used that way if both support the same profile.

Earbud sound: codecs and lag

Music on a CD records the height of the sound wave 44,100 times a second as a 16-bit number. There are two channels, left and right. So sending it as is takes about 1.41 million bits a second (about 1,411 kbps). Classic Bluetooth's top speed in the standard is 3 million bits a second. But the amount it can really send is lower, and it needs room for resending.

So the sound data is made smaller before sending, and the earbuds turn it back into sound. The method for this shrinking and restoring is called a "codec." Bluetooth's rules for sending music require every device to support a codec called "SBC." Others, such as "AAC," can be chosen if both sides support them.

In SBC's high-quality setting, the data rate is about 330,000 bits a second (328 kbps). That is roughly a quarter of the CD's raw rate. Both SBC and AAC shrink data by, for example, storing hard-to-hear parts more roughly. The parts they cut can't be brought back.

With wireless earbuds, game sound effects may reach you a little late. This is because sound is shrunk in fixed-length chunks. The earbuds also store a little before playing it, in case something needs resending. Restoring the sound takes time too. You notice this gap more in games where you act on what's on screen, or when playing an instrument.

LE Audio: a new way to send sound over BLE

In 2020, the Bluetooth SIG announced "LE Audio," a new way to send sound using BLE radio. In 2022, the full set of specifications was complete. At its center is a new codec, "LC3." According to the SIG's announcement, listening tests rated LC3 above SBC in sound quality, even at half SBC's data rate.

LE Audio also defines how to send sound straight to hearing aids. There is also "Auracast," which lets one sender reach any number of listeners at once. The SIG gives examples such as hearing airport announcements, or the sound of a TV in a shop, through your own earbuds or hearing aids. Both the sending and receiving sides must support it.

Range is not a fixed number of meters

The Bluetooth SIG says range depends on conditions, from less than 1 m to more than 1 km. It is set by signal strength, the receiver's sensitivity, the antennas and any obstacles in between.

Earbuds and smartwatches run on small batteries, so their signal strength is limited too. Besides walls and furniture, the human body is also an obstacle that weakens radio waves. That is why sound may cut out when your phone is in a pocket on the other side of your body.

Finding locations: beacons and "Find My" networks

BLE's announcement signals can also serve as location markers. A small device that keeps sending a fixed number at regular intervals is called a "beacon." A phone roughly estimates how close it is to a beacon from the strength of the signal it receives. Beacons are used for guidance in shops and public buildings.

Tags for finding lost items also use BLE announcements. Strangers' phones passing nearby pick up the tag's signal. They then encrypt their own location and send it to a server. The owner learns where the tag is from that information. According to Apple's documentation, in its "Find My" network, even Apple can't read the locations. The owner isn't told who found the tag either.

Using Bluetooth safely

Weak points can be found in a device's Bluetooth software too. These are called vulnerabilities. In 2017, for example, the security company Armis disclosed a group of vulnerabilities it named "BlueBorne." They affected some versions of Android, Windows, Linux and iOS. Someone nearby could take control of a device even without pairing. Each company fixed them with updates, and Japan's JPCERT/CC also urged users to install the updates.

BlueBorne was a flaw in each company's software, not in the standard itself. The fix is to keep your operating system and device software up to date. Remove devices you no longer use, and don't allow connection requests from devices you don't know.

Common myths

By the numbers

5.3 billion is about two-thirds of the world's population (8 billion in 2022, according to the United Nations). That many new devices are made every year.

Learn more

• See the differences in a table: Wi-Fi vs Bluetooth vs LPWA (range, speed and power use)
• Try the math: image, audio and video data size calculator (check the 1,411 kbps of CD audio), dBm converter (the unit for signal strength)
• Terms for the next step: sleep mode (shutting down circuits while idle), public-key cryptography (the idea behind key exchange), LPWA (low-power radio that reaches much farther)
• Related article: "What Is 5G?" (the case of mobile phone networks)