1. Concept

Bluetooth, a new wireless transmission method introduced in 1998, is actually a short-distance wireless communication technology that replaces data cables, and realizes point-to-point or point-to-multipoint information exchange through low-bandwidth radio waves. This network mode, also known as the PersonalAreaNetwork (PAN), is based on multiple micro-networks or mini-networks constructed by exquisite Bluetooth master/accessory. Each micro-network consists of 8 active devices and 255 accessory devices, and multiple micro-networks are connected to form an expanded network, so as to realize communication between various devices conveniently and quickly. It is an open standard to realize wireless transmission of voice and data, and it is a low-cost and short-distance wireless connection technology.

2. Technical features:

The features of Bluetooth technology include: adopting frequency hopping technology to resist signal fading; Fast frequency hopping and short packet technology are adopted to reduce co-frequency interference and ensure transmission reliability; Forward error correction coding technology is adopted to reduce the influence of random noise in long-distance transmission; Use the ISM band of 2.4GHz without applying for a license; Adopt FM modulation mode to reduce the complexity of the equipment. The transmission rate of this technology is designed to be 1MHz, and full duplex communication is carried out in time division mode. Its baseband protocol is a combination of circuit switching and packet switching. One hop frequency sends a synchronization packet, and each packet occupies one time slot, which can also be extended to five time slots. Bluetooth technology supports one asynchronous data channel or three concurrent synchronous voice channels, or one channel that simultaneously transmits asynchronous data and synchronous voice. Each voice channel supports 64kb/s synchronous voice; Asynchronous channels support asymmetric connections with a maximum rate of 721kb/s and a reverse response rate of 57.6kb/s, or symmetric connections with a rate of 432.6kb/s..

3. Protocol:

(1) Establish connection

Before the piconet is established, all devices are in a ready state. In this state, the unconnected device listens for messages every 1.28s, and once the device is awakened, it listens for messages at the preset frequency of 32 hops. The number of frequency hopping varies from region to region, but the frequency of 32 frequency hopping is adopted by most countries. The connection process is initialized by the master device. If the address of a device is known, the connection is established by using Page message; If the address is unknown, the connection is established by using the Inquiry message immediately following the page information. In the piconet, devices without data transmission turn into energy-saving working state. The master device can set the slave device to hold mode, at this time, only the internal timer works; The slave device can also request to switch to the hold mode. Data transmission can be resumed immediately after the device is turned out by holding mode. Hold mode is often used when connecting several piconets or managing low-power devices. Listening mode and sleeping mode are two other low-power working modes. Bluetooth baseband technology supports two connection modes: connection oriented (SCO) mode, which is mainly used for voice transmission; Connectionless (ACL) mode, mainly used for packet data transmission.

(2) error control

The baseband controller adopts three error detection and correction methods: 1/3 forward error correction coding (FEC); 2/3 forward error correction coding; Automatic repeat request (ARQ).

(3) authentication and encryption

authentication and encryption services are provided by the physical layer. Authentication adopts password-response method. During the connection process, authentication may be required once or twice, or it may not be required. Authentication is an important part of any Bluetooth system, which allows users to add trusted Bluetooth devices by themselves. For example, only users' own laptops can communicate through their own mobile phones. The purpose of Bluetooth security mechanism is to provide an appropriate level of protection. If users have a higher level of confidentiality requirements, effective transport layer and application layer security mechanisms can be used.

(4) Software structure

Bluetooth devices should be interoperable. For some devices, interoperability should be realized from radio compatible modules and air interfaces to application layer protocols and object exchange formats. The requirements for other equipment (such as head-mounted equipment, etc.) are much looser. The goal of the Bluetooth project is to ensure that any device with a Bluetooth tag can be interoperable. Software interoperability begins with the multiplexing of link-level protocols, the discovery of devices and services, and the segmentation and reassembly of packets. Bluetooth devices must be able to recognize each other and identify the high-level functions supported by each other by installing appropriate software. Interoperability requires the same application layer protocol stack. Different types of Bluetooth devices have different requirements for compatibility, so users can't expect the headset to contain an address book. The compatibility of Bluetooth means that it has radio compatibility, voice transceiving ability and the ability to find other Bluetooth devices, and more functions should be completed by mobile phones, handheld devices and notebook computers. In order to realize these functions, Bluetooth software architecture will make use of existing specifications, such as OBEX, vCard/vCalendar, HID (Humanized Interface Device) and TCP/IP, instead of developing new specifications. The compatibility of equipment requires that it can adapt to bluetooth profile and existing protocols.

4. Advantages:

Bluetooth transmission is carried out by RF(2.4GHZ) carrier, so it has the basic characteristics of electromagnetic waves, has a large power, can increase the transmission distance, and has no angular and directional restrictions, has the ability to penetrate walls, and can be reflected, mirrored and diffracted between objects. Bluetooth is mainly used for short-distance transmission (up to 1 meters) of data and voice (1Mbps), with very low power consumption, and can connect many components at the same time, with high transmission speed.

5 Disadvantages:

Bluetooth is expensive; RF technology is susceptible to frequency interference; The protection setting of data security due to the characteristics of wall penetration; Bluetooth started relatively late, there is no clear and unified standard at present, and the compatibility problem has not been solved.