Bluetooth is a short-range communication method designed to replace wired cable connections between electronic portable devices. The main features of Bluetooth are robustness, low power consumption, and low cost, and it works in the free 2.4 wireless transmission band.
Bluetooth has two technology systems: Basic Rate (BR) and Low Energy (LE). Both systems include mechanisms for device discovery, connection establishment, and connection. BR system includes Enhanced Data Rate (EDR) Alternate Media Access Control (MAC) And Physical (AMP). The BR system supports connection-oriented or connectionless communication: a basic rate of 721.2 kbps, an enhanced rate of 2.1 Mbps, and a high-speed rate of 24 Mbps if 802.11 AMP is used. Compared with BR/EDR, LE has the characteristics of low cost, low power consumption and low complexity. It is designed for some products with low transmission rate and high power consumption. A Bluetooth system can support one or both.
Between the host and the hardware is the HCI interface (Host Controller Interface). As depicted in the diagram, hardware may contain multiple execution units:
BLE uses frequency hopping technology to increase anti-interference, while supporting the air transmission rate of 1Mbps. BLE uses two multiple access schemes: Frequency division multiple access (FDMA) and time division multiple access (TDMA). FDMA uses a frequency of 2MHz to separate 40 channels: 3 advertising channels and 37 data channels. TDMA technology transmits a data packet within a specified time. Different physical channels are divided into events in which BLE packets are transmitted. There are two kinds of events: advertising events and connection events. Those who send advertisement packets are called advertisers, and those who receive advertisement packets on the advertisement channel but have no intention of connecting to the advertisement initiator are called scanners.
At the beginning of an event, advertisers send an advertisement packet. In the second channel Scanner has a response, followed by a response packet sent by the advertisers. At the beginning of the next event, start again from the first channel.
LE devices may use one-way or broadcast communication throughout the communication, and may also use advertising events to establish paired two-way data communication between two or more devices.
Devices that need to connect to another device are called initiators, which listen for connectable advertisement packets. If the advertiser is using a connectable advertisement event, the initiator initiates a connect request on the physical channel that received the connect packet. If the advertiser accepts the connect request, the advertisement event ends and a new connect event begins. Once the connection is established, the initiator becomes the master and the advertiser becomes the slave. Connect events are used to transfer packets between master and slave devices. Between connection events, frequency hopping techniques are used.
