Bluetooth: 1 mestre, 7 escravos

Bluetooth: what it is and how it works

#Bluetooth #works

  • How Bluetooth works

  • What is the communication principle

  • Establishing connections

  • Bluetooth profiles

How Bluetooth works

The Bluetooth standard, as well as Wi-Fi, uses the FHSS (Frequency Hopping Spread Spectrum) technique, which consists of cutting the frequency band (2,402 – 2,480 GHz) in 79 channels (called hops or hops) of a width of 1MHz , and transmit using a known channel combination from the cell stations. Thus, when changing channels up to 1,600 times per second, the Bluetooth standard allows you to avoid interference with the signals from other radio modules.

What is the communication principle

Bluetooth is based on a master / slave operating mode. So, it’s called picorede (in English, piconet) the network formed by one device and all devices present in its range. Up to 10 picoredes can coexist in the same coverage area. A master can be connected simultaneously to up to seven active slave devices (255 in parked). In fact, devices in a picorede have a 3-bit logical address, which allows a maximum of eight devices. The devices said in parked are synchronized, but do not have a physical address in the picorede:

Bluetooth: 1 master, 7 slaves

In reality, at any given time, the sovereign device can only connect with one slave at a time. It switches very quickly from one slave to another, to give the illusion of a simultaneous connection for all slave devices.

The Bluetooth standard provides for the possibility of connecting two picoredes between them in order to form a wider network, called chained net, thanks to certain devices that act as a bridge between the two.

Establishing connections

Establishing a connection between two Bluetooth devices follows a relatively complicated procedure that guarantees a certain level of security, according to the following development: Passive mode > Inquisition phase > Synchronization with the access point > Discovery of access point services > Creating a channel with the access point > Pairing with the help of a PIN code > Use of the network.

In normal use, a device works in passive mode. Establishing the connection begins with a phase called inquisition phase, in which the master device sends a connection request to all devices in the range, called access points. All devices that receive the request respond with your address.

The master device chooses an address and synchronizes with the access point according to a technique called paging, which consists of synchronizing your watch and frequency with the access point. Then, a connection is established with the access point, allowing the master device to start a discovery phase of the access point services, according to the SDP protocol (Service Discovery Protocol). At the end of this service discovery phase, the master device can create a communication channel with the access point, using the L2CAP protocol.

According to the needs of the service, a supplementary channel called RFCOMM, functioning above the L2CAP channel, can be established in order to provide a virtual serial port. In fact, certain applications are designed to connect to a standard port, regardless of any hardware. This is the case, for example, with certain road navigation applications designed to connect to any Bluetooth GPS device.

It is possible that the access point integrates a security mechanism, called pairing, which restricts access only to authorized users, in order to guarantee a certain level of waterproofing of the picorede. Pairing is done with the help of an encryption key commonly called PIN code. Thus, the access point sends a pairing request to the master device. Most of the time, this can trigger user intervention to enter the access point’s PIN code. If the received PIN code is correct, the connection is established.

In protected mode, the PIN code will be transmitted encrypted, with the help of a second key, in order to avoid any risk of compromise. When pairing is effective, the master device can use the communication channel thus established.

Bluetooth profiles

The Bluetooth standard defines several application profiles, allowing you to define the type of service offered by a Bluetooth device. Each device can therefore support multiple profiles. Below is a list of the main Bluetooth profiles:

Advanced Audio Distribution Profile (A2DP): Advanced Audio Video distribution profile; Remote Control Profile (AVRCP): profile of multimedia remote control;

Basic Imaging Profile (BIP): basic infographic profile;

Basic Printing Profile (BPP): basic printing profile;

Cordless Telephony Profile (CTP): wireless telephony profile;

Dial-up Networking Profile (DUNP): remote network access profile;

Fax Profile (FAX): fax profile;

File Transfer Profile (FTP): file transfer profile;

Generic Access Profile (GAP): generic access profile;

Generic Object Exchange Profile (GOEP): generic object exchange profile;

Hardcopy Cable Replacement Profile (HCRP): heavy copy replacement profile;

Hands-Free Profile (HFP): hands-free profile;

Human Interface Device Profile (HID): human-machine interface profile;

Headset Profile (HSP): headset profile;

Intercom Profile (IP): intercom profile (walkie-talkie);

LAN Access Profile (LAP): network access profile;

Object Push Profile (OPP): profile for sending files;

Personal Area Networking Profile (PAN): personal network profile;

SIM Access Profile (SAP): access profile to a SIM card;

Service Discovery Application Profile (SDAP): application discovery profile;

Synchronization Profile (SP): synchronization profile with a personal information manager (called PIM – Personal Information Manager);

Serial Port Profile (SPP): serial port profile.

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