Bluetooth uses a radio technology for data transmission, as we have already seen. The standard uses an ISM (Medical Scientific Industrial) frequency band that is within 2.4 GHz. To cut short (in part) the controversy, remember that “in the 2.4 GHz band” does not mean 2.45 GHz – the frequency used in microwave ovens – and that it is not nor the resonant frequency of the water. Incidentally, industrial microwave ovens work around 900 MHz. Here I want to tell you about a handy gadget the Best Bluetooth Transmitter , this little device is very useful to connect your TV with headphones wirelessly.
A crowded bandreally far from an oven
Another common technology works in the same band, Wi-Fi (in 802.11b, 802.11g and 802.11n version), and the receivers of many keyboards and mice also use this band of frequency.
Bluetooth works on 79 frequencies, in steps of 1 MHz, between 2400 and 2483.5 MHz. It operates in frequency jumps that is to say that the frequency used changes continuously (every 625 microseconds, 1600 times per second). This feature partly avoids interference (the 2400 MHz band was used by the army for its communications).
The different versions of the Bluetooth standard partially improve the interference management, the version 1.1 adds a signal indicating the power of reception (RSSI) and the version 1.2 uses technology AFH (adaptive frequency hopping) which makes it possible to reduce the interferences with Wi-Fi networks by avoiding jumps between frequencies too close.
Operation: master – slave
Bluetooth works with a master / slave architecture. A device can have three roles: master, slave, or slave parked. A master can administer slaves to form a “piconet”: it is a set of peripherals that can hold 8 members: 1 master and 7 slaves. In addition, there can be up to 255 “parked” slaves.
A piconet ,A scatternet
Then, the piconet networks can be interconnected, a master can be slave to another master. In theory, a “scatternet” can contain up to 72 devices: you can chain up to 10 piconets, but some members (8) must be common to make bridges between piconets: (8 x 10) – 8 = 72.
Three transfer modes
There are three Bluetooth transfer modes, the synchronous link, the asynchronous link, and the SCO (voice) link.
The synchronous connection, Synchronous link
Synchronous links are used to connect two devices that must communicate in both directions at the same speed, such as a link between two computers, or between a PDA and a phone. The synchronous transfer rate is 432 kilobits / sec bidirectional. The data is transmitted continuously and if a problem occurs during the transfer, the data is returned directly. Specifically, this connection mode is not suitable for voice transmissions, for example, because of the possible offset that is unacceptable. In EDR, the speed is tripled, if both devices are compatible (1,296 kilobits / s).
Asynchronous Link (ACL), Asynchronous binding
Asynchronous links are used when one channel requires more speed than the other. For example, a typical Internet connection works in asynchronous mode (we receive a lot more than what we send). The ACL transfer rate is 721 kilobits / sec in one direction and 57.6 kilobits / sec in the other. In the same way as fast synchronous links, asynchronous it can happen that packets are lost. In case of a problem, the packets are sent back immediately, which prevents you from working in real time. As a result, ACL links are not suitable for real-time content transfer (voice, video, etc.). ACL links are used for file transfers and for communications between two devices that require little return: an Internet connection, printing, and so on. As in synchronous,
The voice link (SCO)
Bluetooth offers specialized channels for transferring voice or data, SCO (Synchronous Connection Oriented) channels. One channel offers a transfer rate of 64 kilobits / s. A slave can use 3 SCO channels simultaneously. Unlike ACL links, in case of data loss the SCO link does not return packets. As a result, it allows real time, but with a risk of data loss. Since SCO links are mainly used for voice, the loss of a few bits of data is negligible.
SCO Liaison (voice)
The links between a mobile phone and a headset use an SCO link. For voice transmission, Bluetooth uses CVSDM (Continuously Variable Slope Delta Modulation) coding. Since the channel only provides 64 kilobits / s and the CVSDM does not compress the sound, the quality is quite low: the sound is transmitted in monophonic, with a sampling frequency of 8 kHz and an 8-bit quantization. As a reminder, the CD uses a sampling frequency of 44 kHz in 16 bits and stereo. Concretely, for use “voice” only that is more than enough, but for a musical use it is totally insufficient, which explains the existence of a dedicated Profile (A2DP).
Conclusion, the future of Bluetooth
We hope you liked this file on Bluetooth technology and that it showed you that far from being a cousin of Wi-Fi in slower, Bluetooth is above all a very complete and effective technology.
In the future, in addition to switching to UWB (Ultra Wide Band) in Bluetooth 3.0, Wi-Fi and Bluetooth could be paired. The idea of using Bluetooth for classic uses and automatically switching to Wi-Fi for large transfers was proposed by the consortium that manages the standard (Bluetooth and Wi-Fi paired in the future). In addition, a wired version of Bluetooth seems also provided (From Bluetooth to Wi-Fi or Bluetooth wired?), for professional use when radio waves are prohibited.
In the near future, we should especially see landing devices capable of supporting the standard 2.1 and its simplified management of the twinning: some brands already offer auricles that are paired without using a PIN (between devices of the same brand).