Wednesday, October 27, 2010

The Elements of the Physical Layer of Bluetooth

Author: Jhonder | at : 9:48 PM | Category : , , , |
The physical layer of Bluetooth has several elements starting with the radio frequency. Bluetooth operates in a 2400 - 2483.5 MHz bandwidth, which is the open ISM band (ISM stands for (Industrial, Scientific and Medical). This bandwidth allows it to maximize compatibility of communication worldwide and in fact, the global usage of the ISM spectrum is the key reason why the designers of Bluetooth chose it.

Modulation is another element of the Bluetooth physical layer, which employs “Gaussian Frequency Shift Keying” or GFSK. The index of modulation index is restricted to 0.28 to 0.35, which corresponds to a maximum deviation in frequency of 140 kHz to 175 kHz.

The Bluetooth physical layer also consists of the use of a technique known as Frequency Hopping Spread Spectrum or FHSS, which enables a Bluetooth device to hop from one radio frequency onto another at an incredible speed of up to 1600 hops per second. Bluetooth uses 79 different channels. It is through Frequency Hopping Spread Spectrum that multiple Bluetooth devices do not interfere with each other’s transmissions.

Transmission Power is another aspect of the physical layer of Bluetooth and there are three classes that the technology supports: Class 3 with a 1 milliwatt power and a typical 10 meter range; Class 2 with a 2.5 milliwatt power and a typical 20 meter range and Class 1 with 100 milliwatt power and a 100 meter typical range. In actuality, almost all types of Bluetooth devices support just one of the power classes, Class 3, which is the lowest.

The physical layer is also made up of the link data rate also with the maximum rate supported being 723.2 kb per second and speech coding, which ensures that the quality of speech is acceptable.

These elements come together to make Bluetooth technology work. During any typical Bluetooth operation, two or more devices share a radio channel that is in-sync to one clock and FHSS pattern. One device, the master, serves as the reference point of the synchronization and the rest of the devices in the group are referred to as slaves. This synchronized setup is known as a “piconet”, which is the fundamental mode of communication of Bluetooth technology.

The physical channel of Bluetooth has subdivisions consisting of slots or units of time. Transmission of data happens between devices in the packets that are placed in these time units/slots. The frequency hopping pattern occurs in between the packet’s sending and receiving of data. Each slave is physically linked to the master device but there are no physical links between slaves.

For in-depth study of Bluetooth technology, you can refer to the Bluetooth official website or to other websites dedicated to wireless technologies. There are many online tutorials that you can download to learn more about how this amazing technology works.

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