Bluetooth technology and its system principles

Bluetooth technology is a new wireless communication technology jointly announced by five world-renowned companies, Ericsson, Nokia, Toshiba, IBM, and Intel, in May 1998. It is proposed for the current connection between portable devices that are very close to each other, which is mainly carried out by infrared link (IrDA for short). Although the use of infrared transceiver connection can avoid the connection of wires or cables, it is inconvenient to use. Not only is the distance limited to 1~2m, but it must be directly aligned in the line of sight without any obstruction in the middle. At the same time, it is limited to linking between two devices, and more devices cannot be linked at the same time. On the other hand, people are very familiar with radio communication technology, which can realize long-distance communication, satellite communication and even communication in space. But people may not notice that radio can also play a huge role in the same room or in a place close to each other. The purpose of Bluetooth technology is to enable seamless resource sharing between specific mobile phones, portable computers and hosts of various portable communication devices within a short distance.
1. Bluetooth technology
"Bluetooth" was originally the name of Harald II, the Danish king in the Middle Ages in Europe. He made immortal contributions to the unification of the fragmented Sweden, Finland and Denmark. Ericsson of Sweden named this wireless technology, which is about to become a universal wireless technology, perhaps with the implication of unifying the world.
Bluetooth technology is an open global specification for wireless data and voice communications. It is based on low-cost short-range wireless connections and establishes a special short-range radio technology for fixed and mobile device communication environments. Its essence is to establish a common radio air interface and its open standard for control software, so that communications and computers can be further combined, so that portable devices produced by different manufacturers can have interoperability in a short range without wires or cables connecting them. Its program is written in a 9mm×9mm microchip.
The role of "Bluetooth" technology is to simplify the communication between small network devices (such as mobile PCs, PDAs, mobile phones) and between these devices and the Internet, eliminating the need to install wires, cables and connectors between cordless phones or mobile phones, modems, headsets, PDAs, computers, printers, slide projectors, local area networks, etc. Moreover, this technology can be extended to completely different new devices and new applications. For example, if Bluetooth technology is introduced into mobile phones and laptops, the annoying connection cables between mobile phones and laptops can be removed and communication can be established wirelessly. Printers, PDAs, desktop computers, fax machines, keyboards, game joysticks and all other digital devices can become part of the Bluetooth system. In addition, Bluetooth wireless technology also provides a universal interface for existing digital networks and peripherals to form a group of personal special connection devices away from fixed networks.
The connection distance of the radio transceiver of "Bluetooth" technology can reach 30 feet, not limited to the straight line range, and even devices not in the same room can be linked to each other; and multiple devices can be linked, up to 7, which can link all the devices around the user to form a "personal area network".
2. Bluetooth system
Before understanding the structure of the Bluetooth system, you should first be familiar with several common terms in the Bluetooth system.
Piconet: All devices connected together through Bluetooth technology are considered to be a piconet. A piconet can be just two connected devices, such as a portable computer and a mobile phone, or it can be 8 connected devices. In a piconet, all devices are units of the same level and have the same permissions. However, when the piconet network is first established, one unit is defined as a master and the other units are defined as slaves.
Master unit: Other units, that is, in a piconet, its clock and frequency hopping sequence are used to synchronize the devices of other units.
Slave units: Slave units, that is, all devices in a piconet that are not masters.
Scatternet: Several independent and unsynchronized piconet form a scatternet.
Mac address: A 3-bit address used to distinguish each unit in a piconet.
Parked units: Parked units, i.e., devices in a piconet that are synchronized with the network but do not have a Mac address.
Sniff and hold mode: A mode of operation that is synchronized with the network but enters a sleep state to save energy.
A Bluetooth system generally consists of the following four functional units: antenna unit, link control (firmware) unit, link management (software) unit, and Bluetooth software (protocol) unit.
2.1 Antenna unit
Bluetooth requires that its antenna part be very small and light, so the Bluetooth antenna is a microstrip antenna. The Bluetooth air interface is based on an antenna level of 0dBm. The air interface follows the Federal Communications Commission (FCC) standard for the ISM band with a level of 0dBm. If the global level reaches above 100mW, the extended spectrum function can be used to add some supplementary services. The spectrum extension function is achieved by 79 frequency hopping points with a starting frequency of 2.402GHz, an end frequency of 2.480GHz and an interval of 1MHz. Due to certain local regulations, Japan, France and Spain have reduced the bandwidth. The maximum hopping rate is 1660 hops/s. The ideal connection range is 100mm~10m. However, the distance can be extended to 100m by increasing the transmission level.
Bluetooth operates in the 2.4GHz ISM (industrial, scientific and medical) frequency band that is used worldwide. The data rate of Bluetooth is 1Mb/s. The ISM band is a band open to all radio systems, so using one of the bands will encounter unpredictable interference sources. For example, some home appliances, cordless phones, garage door openers, microwave ovens, etc. may be interference. For this reason, Bluetooth has specially designed fast confirmation and frequency hopping schemes to ensure link stability. Frequency hopping technology is to divide the frequency band into several hop channels. In one connection, the radio transceiver continuously "hops" from one channel to another according to a certain code sequence (i.e. a certain rule, technically called "pseudo-random code", which is a "fake" random code). Only the sender and receiver communicate according to this rule, and other interference cannot interfere according to the same rule; the instantaneous bandwidth of frequency hopping is very narrow, but through the spread spectrum technology, this narrow band is expanded hundreds of times into a wide band, making the impact of interference very small. The time division duplex (TDD) scheme is used to achieve full-duplex transmission.
Compared with other systems working in the same frequency band, Bluetooth hops faster and has shorter data packets, which makes Bluetooth more stable than other systems. The use of FEC (Forward Error Correction) suppresses random noise in long-distance links; frequency hopping transceivers using binary frequency modulation (FM) technology are used to resist interference and prevent fading.
2.2 Link Control (Firmware) Unit
In current Bluetooth products, people use three ICs as connection controllers, baseband processors, and RF transmission/receivers, and also use 30 to 50 separate tuning elements.
The baseband link controller is responsible for processing baseband protocols and some other low-level conventional protocols.
The baseband controller has three error correction schemes: 1/3 forward error correction (FEC) code; 2/3 forward error correction code; automatic request retransmission scheme for data.
The purpose of using the FEC (forward error correction) scheme is to reduce the number of data retransmissions and reduce the data transmission load. However, to achieve error-free data transmission, FEC will inevitably generate some unnecessary overhead bits and reduce data transmission efficiency. This is because the data packet is flexibly defined for whether to use FEC. The header always has a 1/3 ratio of FEC code for protection, which contains useful link information.
In the unnumbered ARQ scheme, the data transmitted in a time slot must be confirmed in the next time slot. Only when the data is considered error-free after passing the header error detection and cyclic redundancy detection at the receiving end, will the confirmation message be sent back to the sending end, otherwise an error message will be returned. For example, the voice channel of Bluetooth uses the Continuous Variable Slope Delta Modulation (CVSD) voice coding scheme to obtain high-quality audio coding for transmission. CVSD coding is good at handling lost and damaged voice samples. Even if the bit error rate reaches 4%, the CVSD-coded voice is still audible.
Cambridge Silicon Radio, a subsidiary of Cambridge Consultants, has put forward their views. The company's entry-level product is a single-chip transmitter and link controller. The company calls it BlueCore and BlueStack. This is a complete Bluetooth that does not require external SAW filters, ceramic capacitors or sensors. The product is highly integrated and uses 0.18µm or 0.15µm technology. The baseband circuit can be added to the chip with almost no increase in cost.
2.3 Link Management (Software) Unit
The link management (LM) software module carries the link data settings, authentication, link hardware configuration and some other protocols. LM can discover other remote LMs and communicate with them through LMP (Link Management Protocol). The LM module provides the following services:
(1) Send and receive data.
(2) Request a name.
(3) Link address query.
(4) Establish connection.
(5) Authentication.
(6) Link mode negotiation and establishment.
(7) Determine the frame type.
(8) Set the device to sniff mode. The master can only send data regularly in specific time slots.
(9) Set the device to hold mode. In order to save energy, the device working in hold mode stops receiving data for a longer period and activates the link once every 4 seconds on average. This is defined by LM and the specific operation of LC (link controller).
(10) When the device does not need to transmit or receive data but still needs to maintain synchronization, set the device to pause mode. The device in pause mode periodically activates and tracks synchronization and checks page messages.
(11) Establish network connection. Before the connection within the piconet is established, all devices are in standby state. In this mode, the unconnected unit periodically "listens" for information every 1.28 seconds. Whenever a device is activated, it listens to the 32 hopping frequencies planned for the unit. The number of hopping frequencies varies by geographic region, with 32 being the most common number for most countries except Japan, France, and Spain. The master initiates the connection procedure by sending a page message if the address is known, or an inquiry message followed by a page message if the address is unknown. In the initial paging state, the master sends a series of 16 identical page messages on the 16 hopping frequencies assigned to the paged unit. If there is no response, the master continues to page on the remaining 16 frequencies in the order in which they are activated. The maximum delay for a slave to receive a message from the master is twice the activation period (2.56 seconds), and the average delay is half the activation period (0.6 seconds). Inquiry messages are primarily used to find Bluetooth devices such as shared printers, fax machines, and other similar devices whose addresses are unknown. Inquiry messages are similar to page messages, but require an additional burst period to collect all responses.
Bluetooth also supports power-saving modes if the connected devices in the piconet have not transmitted data for a long period of time. The master can put the slave into hold mode, in which only an internal counter is working. The slave can also actively request to be put into hold mode. Once the unit in hold mode is activated, data transfer will immediately resume. Hold mode is generally used when connecting several piconets or for low-power devices such as temperature sensors. In addition to hold mode, Bluetooth also supports two other energy-saving working modes: sniff mode and park mode. In sniff mode, the slave reduces the rate at which it "listens" to messages from the piconet, and the "breathing" interval can be adjusted appropriately according to application requirements. In park mode, the device remains synchronized with the piconet but no data is transmitted. Devices working in park mode abandon their MAC addresses, occasionally listen to messages from the master and restore synchronization, and check broadcast messages. If we rank these working modes in ascending order according to energy-saving efficiency, they are: sniff mode, hold mode, and pause mode.
(12) Connection type and data packet type. The connection type defines which type of data packet can be used in a particular connection. Bluetooth baseband technology supports two types of connections: Synchronous Connection Oriented (SCO), which is mainly used to transmit voice; Asynchronous Connectionless (ACL), which is mainly used to transmit data packets.
Different master-slave pairs in the same piconet can use different connection types, and the connection type can be changed arbitrarily within a stage. Each connection type can support up to 16 different types of data packets, including 4 control packets, which is the same for SCO and ACL. Both connection types use TDD (time division duplex transmission scheme) to achieve full-duplex transmission.
SCO connection is a symmetrical connection that uses reserved time slots to transmit data packets. After the connection is established, the master and slave can send SCO data packets without being selected. SCO data packets can transmit both voice and data, but when transmitting data, they are only used to resend the damaged part of the data.
The ACL link is a directional transmission of data packets, which supports both symmetrical and asymmetric connections. The master is responsible for controlling the link bandwidth and determining how much bandwidth each slave in the piconet can occupy and the symmetry of the connection. The slave can only transmit data when it is selected. The ACL link also supports receiving broadcast messages sent by the master to all slaves in the piconet.
(13) Authentication and confidentiality. The Bluetooth baseband part provides users with protection and information confidentiality mechanisms at the physical layer.
Authentication is based on a "request-response" algorithm. Authentication is a key part of the Bluetooth system. It allows users to establish a trust domain for personal Bluetooth devices, such as only allowing the owner's own laptop to communicate through the owner's own mobile phone. Encryption is used to protect the personal information of the connection. The key is managed by the upper layer of the program. The network transmission protocol and application can provide users with a strong security mechanism.
3.4 Software (Protocol) Unit
The Bluetooth baseband protocol combines circuit switches and packet switches for voice and data transmission. Each channel supports a 64kb/s synchronous link. The asynchronous channel supports asymmetric links of up to 721 kb/s in either direction and 57.6 kb/s in the return direction, and can also support symmetric links of 43.2 kb/s. Therefore, it can handle very large data rates on cellular systems quickly enough. Generally speaking, its link range is 100mm~10m; if the transmission power is increased, its link range can be extended to 100m. The Bluetooth software architecture specification requires that Bluetooth-compliant devices support a basic level of interoperability. This level of compliance is determined by different applications.
Bluetooth devices need to support some basic interoperability feature requirements. For some devices, this requirement involves the wireless module, air protocol, application layer protocol and object exchange format. The Bluetooth 1.0 standard consists of two files: one is called Foundation Core, which specifies the design standard; the other is Foundation Profile, which specifies the interoperability criteria. But for other devices, such as headphones, this requirement is much simpler. Bluetooth devices must be able to recognize each other and load corresponding software to support higher-level performance of the device.
Bluetooth has different requirements for different levels of devices (such as PCs, handhelds, mobile phones, headphones, etc.). For example, you cannot expect a Bluetooth headset to provide an address book. But mobile phones, handhelds, and laptops need more features. The software (protocol) structure must have the following functions: setup and troubleshooting tools; automatic identification of other devices; replacing cable connections; communicating with peripherals; audio communication and call control; commercial card transactions and directory network protocols.
The software (protocol) unit of Bluetooth is an independent operating system and is not bundled with any operating system. Bluetooth specifications applicable to several different commercial operating systems are being improved.
In recent years, mobile communications have developed rapidly, and portable computers such as PDAs (LAPTOP), notebook computers (NOTEBOOK), handheld computers (HPC) and PDAs have also developed rapidly. The rapid development of the Internet has also led to an increasing demand for various data information transmissions other than telephone communications.
Bluetooth technology connects various portable computers and cellular phones with radio, making computers and communications more closely integrated, allowing people to exchange and transmit data information anytime and anywhere. Therefore, the computer industry and the mobile communications industry attach great importance to Bluetooth technology, believing that it will have a huge role in promoting future wireless mobile data communication services. It is expected that wireless data communication services will grow rapidly in recent years. Bluetooth technology is considered to be one of the most significant advances in wireless data communications.