EMZETT.Bluetooth
In short: A radio standard for wireless data transfer over short to medium distances, classically for headphones, keyboards, mice and file transfer between devices.
In more detail: Unlike Wi-Fi, Bluetooth is designed for point-to-point connections between a few devices without a central access point, and uses considerably less energy in the process (especially the “Low Energy” variant, BLE) — ideal for wearables and IoT devices with small batteries.
In Depth
Origin of the name
Bluetooth is named after the Danish Viking king Harald “Bluetooth” Gormsson, who united parts of Denmark and Norway in the 10th century — as a metaphor for the standard “uniting” different devices and manufacturers under one common protocol. It was originally developed in 1994 by Ericsson; since 1998 it has been developed jointly by the Bluetooth Special Interest Group (SIG), a consortium of large technology companies, and released for royalty-free use.
Frequency hopping
Bluetooth works in the licence-free 2.4 GHz frequency band (the same band also used by Wi-Fi and microwave ovens) and uses frequency hopping (adaptive frequency-hopping spread spectrum) to minimise interference with other radio sources in the same band — the signal hops up to 1600 times per second between 79 different channels. “Adaptive” here means that Bluetooth devices continuously measure which channels are currently disturbed by other radio sources (e.g. a nearby Wi-Fi network) and actively avoid these channels — one reason why Bluetooth works relatively robustly in practice despite the shared frequency band.
Pairing and security
Two devices first have to be paired before they can communicate: in the process they exchange a shared security key (on more modern devices often via “Secure Simple Pairing” with a displayed number code for comparison, previously via a manually entered PIN), which is then stored for all further connections — no new pairing needed each time, as long as neither device deletes the stored key. Historically known forms of attack are “bluejacking” (sending unsolicited messages/contacts to other people’s Bluetooth devices within range, mostly harmless) and “bluesnarfing” (unauthorised access to a device’s data via security holes in the pairing process) — modern Bluetooth versions have largely closed these weaknesses, but older devices remain vulnerable.
Range and device classes
Bluetooth range is divided into three device classes: Class 3 (about 1 metre, hardly used any more), Class 2 (the usual class for headphones, keyboards and smartphones, about 10 metres) and Class 1 (up to 100 metres, mostly in industrial applications). Walls, other radio sources and the number of devices in the immediate vicinity can push the practical range well below the theoretical class figure.
Bluetooth Low Energy and profiles
Bluetooth Low Energy (BLE), part of the standard since Bluetooth 4.0, is optimised specifically for minimal power consumption rather than high data throughput — ideal for devices that are supposed to run for months on a coin cell (fitness trackers, sensors, smart home devices, tracking tags like Apple’s AirTag). Classic Bluetooth (“Bluetooth Classic”), on the other hand, remains the choice for audio streaming, where BLE’s lower bandwidth isn’t sufficient. Within the standard, so-called “profiles” define which functions two devices can actually use together — A2DP for stereo audio streaming, HFP for hands-free telephony, HID for input devices such as keyboards and mice. Newer Bluetooth versions (from 5.0) also support mesh networking, in which many devices use each other as relays to increase range and reliability in larger smart home installations, without every single device having to be within direct range of the controller.