О. В. Фёдорова е. В. Мезина е. В. Хамис английский язык в сфере информационных технологий и коммуникаций учебное пособие

Being situated beside, Bluetooth device can establish not only connection of a type a point - - to - point, when there are only two devices, but also the point to - many points, when one device works simultaneously with several others. At connection of one device with several by others, the device which serves some connections is called master, and connected devices –slave. To one master can be connected up to seven active slaves. There are can be a set inactive slave, which can not exchange data with master while all channels are engaged, except for active slave (that is, devices which actively exchange data), remain are synchronized with it (Such structure is called piconet).

There can be only one master in one piconet but everyone slave can be master for other devices simultaneously, and form piconet. Several piconets connected thus form scatternet. In scatternet different devices can not only to be at the same time master and slave simultaneously for various piconet, but also only slave for different piconets.

It is possible to familiarize with this structure in the submitted below figure more evidently.

Moreover, in case of necessity any slave can become master in piconet. Previous master thus becomes slave.

Thus, in scatternet can be united so much Bluetooth devices, how much it is necessary. The logic connections can be formed how it is required, and can change somehow, m case of necessity. A unique condition, various piconet included in one scatternet should have the different channels of communication, that is work on various frequencies and have various hopping channel. Hopping is a regular change of frequencies determined in parameters hopping sequence. Specification provides 10 variants hopping sequence, 5 with a cycle in 79 changes and 5 with a cycle in 23 changes. With any hopping sequence of frequency are replaced 1600 hops/ sec. Hopping used to prevent attenuation of a radio signal and interference.

As was already spoken, the automatic installation of connection between Bluetooth devices situated within the limits of reach is one of major Bluetooth features. Therefore, Bluetooth device starts its work in a unfamiliar environment from device discovery, in other words search others Bluetooth devices. For this purpose the inquiry is sent, and the answer to it depends not only on presence in radius of communication active Bluetooth devices, but also from a mode in which there are these devices. On this stage it is possible three basic modes: Discoverable mode- the devices, being in this mode, always answer all inquiries, received by them; Limited discoverable mode- in this mode there are devices which can answer inquiries only limited time, or should answer only at observance of the certain conditions; Non-discoverable mode- the devices, taking place in this mode, as it is visible from the name of a mode, do not answer new inquiries.

But it is not all. Even if device finds out success, it can be in connectable mode or in non-connectable mode. In non-connectable mode the device does not allow to adjust some important parameters of connection, and, thus, though it can be revealed but to exchange the data with it, it will be impossible. If the device is in connectable mode, at this stage Bluetooth device agree among themselves about a used range of frequencies, size of pages, quantity both order of hops, and other physical parameters of connection

If the process of detection of devices has passed normally, new Bluetooth device receives a set of addresses accessible of Bluetooth devices, and it follows device name discovery, when the new device finds out names of all accessible Bluetooth devices from the list Everyone Bluetooth device should have the unique address (like as MAC-addresses at network payments), but at a level of the user not this address is usually used, and name of the device, which can be any, and it is no necessary to be global unique. The name of the Bluetooth device can be length up to 248 bytes, and use code page according to Unicode UTF-8 (at use UCS-2. the name can be short up to 82 symbols). The specification provides, that Bluetooth of the device are not obliged to accept more first 40 symbols of a name of another Bluetooth device. If Bluetooth the device has the limited size and limited computing power screen, quantity of symbols, which it will accept can be reduced up to 20.

Another major Bluetooth features is the automatic connection of Bluetooth devices to services given by others Bluetooth devices. Therefore, after there is a list of names and addresses, the search of accessible services given by accessible devices is carried out service discovery. The reception or granting any services are main courses of Bluetooth creation, therefore for search of possible services there is special protocol named is Service Discovery Protocol (SOP).