Editing Bluetooth (section)

== Implementation ==
Bluetooth operates at frequencies between 2.402 and 2.480{{nbsp}}GHz, or 2.400 and 2.4835{{nbsp}}GHz, including [[guard band]]s 2{{nbsp}}MHz wide at the bottom end and 3.5{{nbsp}}MHz wide at the top.<ref name="Radio-Electronics.com">{{cite web |url=http://www.radio-electronics.com/info/wireless/bluetooth/radio-interface-modulation.php |title=Bluetooth Radio Interface, Modulation & Channels |publisher=Radio-Electronics.com |access-date=24 March 2012 |archive-date=2 January 2012 |archive-url=https://web.archive.org/web/20120102064023/http://www.radio-electronics.com/info/wireless/bluetooth/radio-interface-modulation.php |url-status=live }}</ref> This is in the globally unlicensed (but not unregulated) industrial, scientific and medical ([[ISM band|ISM]]) 2.4{{nbsp}}GHz short-range radio frequency band. Bluetooth uses a radio technology called [[frequency-hopping spread spectrum]]. Bluetooth divides transmitted data into packets, and transmits each packet on one of 79 designated Bluetooth channels. Each channel has a bandwidth of 1{{nbsp}}MHz. It usually performs 1600{{nbsp}}hops per second, with [[adaptive frequency-hopping spread spectrum|adaptive frequency-hopping]] (AFH) enabled.<ref name="Radio-Electronics.com"/> [[Bluetooth Low Energy]] uses 2{{nbsp}}MHz spacing, which accommodates 40 channels.<ref>{{cite web|url=https://www.bluetooth.org/DocMan/handlers/DownloadDoc.ashx?doc_id=421043|title=Bluetooth Specification Version 5.0|website=Bluetooth Special Interest Group|access-date=8 December 2016|archive-date=23 December 2018|archive-url=https://web.archive.org/web/20181223163509/https://www.bluetooth.org/DocMan/handlers/DownloadDoc.ashx?doc_id=421043|url-status=live}}</ref>

Originally, [[Gaussian frequency-shift keying]] (GFSK) modulation was the only modulation scheme available. Since the introduction of Bluetooth 2.0+EDR, π/4-[[DQPSK]] (differential quadrature phase-shift keying) and 8-DPSK modulation may also be used between compatible devices. Devices functioning with GFSK are said to be operating in basic rate (BR) mode, where an instantaneous [[bit rate]] of 1{{nbsp}}[[Mbit/s]] is possible. The term ''Enhanced Data Rate'' (''EDR'') is used to describe π/4-DPSK (EDR2) and 8-DPSK (EDR3) schemes, transferring 2 and 3{{nbsp}}Mbit/s respectively.

In 2019, Apple published an extension called HDR which supports data rates of 4 (HDR4) and 8 (HDR8)&nbsp;Mbit/s using π/4-[[DQPSK]] modulation on 4&nbsp;MHz channels with forward error correction (FEC).<ref>{{cite web|url=http://www.freepatentsonline.com/y2019/0104424.html |title=Ultra-Low Latency Audio Over Bluetooth – Apple Inc |publisher=Freepatentsonline.com |accessdate=21 July 2022}}</ref>

Bluetooth is a [[packet switching|packet-based protocol]] with a [[master/slave (technology)|master/slave architecture]]. One master may communicate with up to seven slaves in a [[piconet]]. All devices within a given piconet use the clock provided by the master as the base for packet exchange. The master clock ticks with a period of 312.5{{nbsp}}[[μs]], two clock ticks then make up a slot of 625{{nbsp}}μs, and two slots make up a slot pair of 1250{{nbsp}}μs. In the simple case of single-slot packets, the master transmits in even slots and receives in odd slots. The slave, conversely, receives in even slots and transmits in odd slots. Packets may be 1, 3, or 5 slots long, but in all cases, the master's transmission begins in even slots and the slave's in odd slots.

The above excludes Bluetooth Low Energy, introduced in the 4.0 specification,<ref>{{Cite journal|last1=Gomez|first1=Carles|last2=Oller|first2=Joaquim|last3=Paradells|first3=Josep|date=29 August 2012|title=Overview and Evaluation of Bluetooth Low Energy: An Emerging Low-Power Wireless Technology|journal=Sensors|volume=12|issue=9|pages=11734–11753|doi=10.3390/s120911734|issn=1424-8220|pmc=3478807|bibcode=2012Senso..1211734G|doi-access=free}}</ref> which [[Bluetooth Low Energy#Radio interface|uses the same spectrum but somewhat differently]].

=== Communication and connection ===
A master BR/EDR Bluetooth device can communicate with a maximum of seven devices in a piconet (an ad hoc computer network using Bluetooth technology), though not all devices reach this maximum. The devices can switch roles, by agreement, and the slave can become the master (for example, a headset initiating a connection to a phone necessarily begins as master—as an initiator of the connection—but may subsequently operate as the slave).

The Bluetooth Core Specification provides for the connection of two or more piconets to form a [[scatternet]], in which certain devices simultaneously play the master/leader role in one piconet and the slave role in another.

At any given time, data can be transferred between the master and one other device (except for the little-used broadcast mode). The master chooses which slave device to address; typically, it switches rapidly from one device to another in a [[round-robin scheduling|round-robin]] fashion. Since it is the master that chooses which slave to address, whereas a slave is (in theory) supposed to listen in each receive slot, being a master is a lighter burden than being a slave. Being a master of seven slaves is possible; being a slave of more than one master is possible. The specification is vague as to required behavior in scatternets.<ref>{{Cite journal |last3=Patil |last2=Koul |last1=Kurawar |first3=Viki Tukaram |first2=Ayushi |first1=Arwa |date=August 2014 |title=Survey of Bluetooth and Applications |journal=International Journal of Advanced Research in Computer Engineering & Technology |volume=3 |pages=2832–2837 |issn=2278-1323}}</ref>