Bluetooth pairing: everything you've always wanted to know
Thomas Rutter

Pairing and using Bluetooth devices isn't as complicated as some make it out to be. This article will attempt to explain in simple terms how Bluetooth pairing works and what some of the biggest misconceptions about it are.

What is Bluetooth?

Bluetooth is a type of wireless communication for use over short distances between devices. This "short distance" is key to understanding where Bluetooth fits in the scheme of wireless protocols - it's designed for covering smaller smaller areas that Wi-fi does, such as "in the same room", "on the same desk" or even "worn on the same person". While Bluetooth can in cases communicate over longer distances, reliability of communication typically drops off after distances of over 10m / 30ft in open space, and more quickly if it has to travel through objects such as walls, furniture or even the human body.

Bluetooth also uses less power than Wifi. It was designed with small battery-powered devices in mind such as computer mice and keyboards. Since Bluetooth version 4.0, a new feature called Bluetooth LE (Low Energy) enables some types of Bluetooth devices to run on even smaller amounts of power or last for a much longer time on a single set of batteries.

As a result, Bluetooth is not intended for communicating large amounts of data, like Wi-fi is. Devices which require high-speed network access can now include their own Wi-fi functionality instead should that be desired.


What is Bluetooth pairing?

Bluetooth pairing is a two-way, permanent association between two Bluetooth devices. When two Bluetooth devices are "paired" it means both devices "remember" each other, storing the other's unique codes in its own table of pairings so that the other device can be uniquely identified as being a device it is "safe" to connect to. Pairing information is stored on both devices, and remembered even after turning them off and on again, so that next time they are in range of each other they can find each other and know that it's safe to connect.

Pairing helps keep Bluetooth secure. Two devices are only supposed to be able to pair if the owner of the devices has both devices present at once and confirms the pairing. In practice, this usually means going into the Bluetooth devices menu on devices with a screen, or entering a "pairing mode" on devices without a screen, then initiating pairing on one device and accepting it on the other. The pairing process means that a stranger should not be able to connect to your Bluetooth device simply by being within signal range, because their device won't have been paired by you.

How many devices can be paired to one device?

Bluetooth devices store pairing information in a pairing table and the number of devices it can simulteously store pairing informaiton for is limited by the number of entries in this table. For high powered devices like smartphones this table is essentially unlimited, while smaller Bluetooth devices may only allow around 6, 8, or some other small number of pairings.

It should be noted that even though almost all devices can store several pairings, they can't necessarily connect to more than one of these at once.

What is connecting and how is it different to pairing?

To enable two Bluetooth devices to communicate with each other, the devices need to be connected. To connect, the Bluetooth devices must be in signal range with each other and one must initiate a connection with the other. In practice, many Bluetooth devices will automatically connect with any paired device as soon as it is switched on and in range, removing the need for manual connection. Nonetheless, devices such as phones or laptops include a Bluetooth menu allowing you to manually disconnect from, or attempt a connection to, a given Bluetooth device that's in range. For the simple case of a peripheral device which is paired and will always be used only with the one particular host device, there should never normally be a need to manually connect, because the pairing information will ensure they automatically connect.

How many devices may be connected to one device?

This depends a lot on what type of devices it is. Devices that are designed to be "hosts" like smartphones or laptops usually have the ability to connect to many Bluetooth devices at the same time, though limitations in the device may prevent two simultaneous connections of certain types, such as audio output devices.

Devices that are intended as peripherals, such as keyboards, mice and headsets, are usually limited to only being able to connect to one "host" device at a time. Once connected, it will stay connected while that device is in range even if other paired devices come into range. This creates a phenomena whereby your Bluetooth headset or other device will only connect to the first host device it sees, and ignores connection opportunities or requests from other devices as long as this first device is in range.

Do some devices like Bluetooth headsets support connections to multiple hosts at a time?

Yes, this is a feature on some devices of this kind. Sometimes labeled "multipoint", this is a feature whereby a single Bluetooth headset or headphones can connect to more than one host device, and either of those may send audio to it or conduct phone calls over it.

This creates challenges due to the way Bluetooth audio or Bluetooth headsets work. Devices which support multipoint are essentially working as if they are two devices, each one connected to a different host. But, they can only play audio from one at a time, or conduct a phone call from one at a time. The device itself has to be able to respond to an incoming call or playback request from one device while still doing something with the other device, overriding the other device - and perhaps even priority must be taken into account: one device shouldn't be able to interrupt a phone call from the other device, but may be able to interrupt music playback with a phone call.