1. Concept : Bluetooth (BLUETOOTH), a new wireless transmission method introduced in 1998, is actually a short-range wireless communication technology that replaces data cables, realizing the exchange of information between point-to-point, or point-to-multipoint connections by means of low-bandwidth radio waves. This network mode, also known as Private Area Network (PAN, PersonalAreaNetwork), is based on multiple micro-networks or mini-networks constructed by delicate Bluetooth masters/subsidiaries, each of which consists of 8 active and 255 subsidiary devices, and multiple micro-networks are connected to form an expanded network, which makes it easy and fast to realize all kinds of device communication between them. It is an open specification for realizing wireless transmission of voice and data, and is a low-cost, short-range wireless connection technology.
2. Technical characteristics: Bluetooth technology features include: frequency hopping technology, anti-signal degradation; fast frequency hopping and short packet technology to reduce the co-frequency interference, to ensure the reliability of the transmission; the use of forward error correction coding technology, to reduce the impact of the random noise of the long-distance transmission; the use of the 2.4GHz ISM band, without the need to apply for a license; the use of FM modulation to reduce the complexity of the device. FM modulation reduces the complexity of the equipment. The technology is designed for a transmission rate of 1 MHz, full-duplex communication in a time-division mode, and its baseband protocol is a combination of circuit-switched and packet-switched. One synchronous packet is sent at a hopping rate, with each packet occupying one time slot, which can be extended to five time slots. Bluetooth technology supports one asynchronous data channel or three concurrent synchronous voice channels, or one channel that transmits both asynchronous data and synchronous voice. Each voice channel supports 64kb/s synchronous voice; the asynchronous channel supports asymmetric connections with a maximum rate of 721kb/s and a reverse answer rate of 57.6kb/s or symmetric connections with 432.6kb/s.
3. Protocol:
(1) Establishment of Connection
Before the establishment of a micro-micro network, all devices are in a ready state. In this state, unconnected devices listen for messages every 1.28s, and once a device is woken up, it listens for messages on a pre-set frequency of 32 hopping frequencies. The number of hopping frequencies varies from region to region, but the 32 hopping frequencies are used by the vast majority of countries. The connection process is initialized by the master device. If the address of a device is known, a Page message is used to establish the connection; if the address is unknown, an Inquiry message immediately following the Page message is used to establish the connection. In a micro-micro network, devices with no data transmission move to an energy-saving operating state. The master device can set the slave device to the hold mode, at this time, only the internal timer works; the slave device can also request to transfer to the hold mode. Data transmission can be resumed immediately after the device is transferred out of the hold mode. The hold mode is often used when connecting several pico networks or managing low-power devices. Listening mode and sleep mode are two other low-power operating modes. Bluetooth baseband technology supports two connection methods: the connection-oriented (SCO) method, mainly used for voice transmission, and the connectionless (ACL) method, mainly used for packet data transmission.
(2) Error Control
The baseband controller uses three types of error detection and correction methods: 1/3 forward error correction coding (FEC); 2/3 forward error correction coding; and automatic request retransmission (ARQ).
(3) Authentication and Encryption
Authentication and encryption services are provided by the physical layer. Authentication is done using a password-response method, and may be required once or twice during the connection process, or no authentication is required. Authentication is an important part of any Bluetooth system and allows users to add their own trusted Bluetooth devices, e.g., only the user's own laptop can communicate through the user's own cell phone. The purpose of Bluetooth security mechanisms is to provide an appropriate level of protection, and if the user has a higher level of confidentiality required, effective transport and application layer security mechanisms can be used.
(4) Software Architecture
Bluetooth devices should be interoperable, from radio-compatible modules and air interfaces up to application layer protocols and object exchange formats for some devices; for others (e.g., headset devices, etc.) the requirements are much less stringent. The goal of the Bluetooth Initiative is to ensure that any device with a Bluetooth tag can operate interchangeably. Software interoperability begins with link-level protocols for multiplexing, device and service discovery, and packet segmentation and reorganization. Bluetooth devices must be able to recognize each other and identify the high-level functionality supported by each other by installing the appropriate software. Interoperability requires the use of the same application layer protocol stack. Different types of Bluetooth devices have different requirements for compatibility, and users do not have the luxury of an address book contained within the headset. Bluetooth compatibility means that it has radio compatibility, the ability to send and receive voice and the ability to discover other Bluetooth devices, and more functionality to be accomplished by cell phones, handhelds and laptops. In order to realize these functions, Bluetooth software architecture will make use of existing specifications, such as OBEX, vCard/vCalendar, HID (Human Interface Device) and TCP/IP, rather than developing new specifications. Device compatibility requires the ability to adapt to Bluetooth specifications and existing protocols.
4. Advantages: Bluetooth transmission is carried out through the RF (2.4GHZ) carrier, so it has the basic characteristics of electromagnetic waves, there is a greater power, can increase the transmission distance, and there is no angle and directional limitations, with through the wall, can be reflected between the object, the mirror set up, bypassing shot. Bluetooth is mainly used for short distance transmission (up to 10 meters) data and voice (1Mbps), power consumption is very low energy, while being able to connect many components, fast transmission speed.
5. Disadvantages: Bluetooth cost is very high; RF technology is susceptible to frequency interference; through-the-wall characteristics of the data security protection set the problem; Bluetooth started late, there is no clear, unified standards, compatibility issues have not yet been able to solve.