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Bluetooth Technical
information |
Communication & Connection
A Bluetooth device playing the role of the "master" can communicate with up to 7 devices playing the role of the "slave". This network of "group of up to 8 devices" (1 master + 7 slaves) is called a piconet.
At any given time, data can be transferred between the master and 1 slave; but the master switches rapidly from slave to slave in a round-robin fashion. (Simultaneous transmission from the master to multiple slaves is possible, but not used much in practice). Either device may switch the master/slave role at any time.
Bluetooth specification allows connecting 2 or more piconets together to form a scatternet, with some devices acting as a bridge by simultaneously playing the master role in one piconet and the slave role in another piconet. These devices have yet to come, though are supposed to appear next year (2007).
Setting up connections
Any Bluetooth device will transmit the following sets of information on demand
Device Name
Device Class
List of services
Technical information eg: device features, manufacturer, Bluetooth specification,
clock offset
Any device may perform an "inquiry" to find other devices to which
to connect, and any device can be configured to respond to such inquiries. However
if the device trying to connect knows the address of the device it will always
respond to direct connection requests and will transmit the information shown
in the list above if requested for it. Use of the device's services however
may require pairing or its owner to accept but the connection itself can be
started by any device and be held until it goes out of range. Some devices can
only be connected to one device at a time and connecting to them will prevent
them from connecting to other devices and showing up in inquiries until they
disconnect the other device.
Every device has a unique 48-bit address. However these addresses are generally not shown in inquiries and instead friendly "Bluetooth names" are used which can be set by the user, and will appear when another user scans for devices and in lists of paired devices. Most phones have the Bluetooth name set to the manufacturer and model of the phone by default. Most phones and laptops will only show the Bluetooth names and special programs are required to get additional information about remote devices. This can get confusing with activities such as Bluejacking as there could be several phones in range named "T610" for example. On Nokia phones the Bluetooth address may be found by entering "*#2820#". On computers running Linux the address and class of a USB Bluetooth dongle may be found by entering "hciconfig hci0 class" as root ("hci0" may need to be replaced by another device name).
Air interface
The protocol operates in the license-free ISM band at 2.45 GHz. In order to avoid interfering with other protocols which use the 2.45 GHz band, the Bluetooth protocol divides the band into 79 channels (each 1 MHz wide) and changes channels up to 1600 times per second. Implementations with versions 1.1 and 1.2 reach speeds of 723.1 kbit/s. Version 2.0 implementations feature Bluetooth Enhanced Data Rate (EDR), and thus reach 2.1 Mbit/s. Technically version 2.0 devices have a higher power consumption, but the three times faster rate reduces the transmission times, effectively reducing consumption to half that of 1.x devices (assuming equal traffic load).
Bluetooth differs from Wi-Fi in that the latter provides higher throughput and covers greater distances but requires more expensive hardware and higher power consumption. They use the same frequency range, but employ different multiplexing schemes. While Bluetooth is a cable replacement for a variety of applications, Wi-Fi is a cable replacement only for local area network access. A glib summary is that Bluetooth is wireless USB whereas Wi-Fi is wireless Ethernet, both operating at much lower bandwidth than the cable systems they are trying to replace.
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