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Our Expertise:
Bluetooth is a low-speed (up to 700Kbps), low power standard originally developed to
allow notebooks, PDAs and mobile phones connect to each other in order to synchronise data and authenticate users
in public areas, such as hotels, airports and restaurants. Bluetooth is also used for many wireless devices for
PCs, such as printers, keyboards, mice, camcorders, projectors and many other components.
Bluetooth-compliant peripherals are quickly becoming more and more available, and sales have grown significantly
since Bluetooth's entrance to the market in 2000. You can find out what Bluetooth products are available on the
official Bluetooth website at
http://www.bluetooth.com.
Bluetooth devices use a 2.4GHz frequency range (as do Wi-Fi/IEEE 802.11b devices). In order to prevent interference
with Wi-Fi, Bluetooth uses a signalling method known as Frequency Hopping Spread Spectrum, which operates by switching
the exact frequency used during a Bluetooth session 1,600 times over the 79 channels used by Bluetooth. Unlike Wi-Fi,
which is used to allow devices to be connected to the network at all times, Bluetooth is designed to allow temporary,
ad-hoc connections in which two devices connect only long enough to transfer data and then break the connection.
Despite the frequency-hopping nature of Bluetooth, studies have shown that Bluetooth (up through version 1.1) and
IEEE 802.11b devices can interfere with each other, particularly at close range (under 2 metres) or when users attempt
to use both types of wireless networking at the same time. Although 802.11g has not been specifically studied, it uses the
same frequencies as 802.11b, and interference between 802.11g and Bluetooth can take place under similar circumstances.
Interference reduces throughput and in some circumstances can cause data loss.
Bluetooth 1.2 was recently released and this adds adaptive frequency hopping to solve interference problems when devices
are more than 1 metre (3.3 feet) away from each other. However, close-range interference has not yet been prevented.
IEEE has developed 802.15.2, a specification for enabling coexistence between 802.11b/g and Bluetooth. It can use various
time-sharing or time-division methods to enable coexistence. However, these specifications are not yet part of typical
802.11b/g implementations.
Advantages of Bluetooth:
- It's wireless - no need to carry wires around with you everywhere just to connect peripherals to your notebook
or PDA.
- It's inexpensive and it's getting cheaper every few months.
- Automatic detection - There is no need to tell the Bluetooth device how to connect to another device, they
will automatically detect each other and talk accordingly.
Bluetooth Specifications (from the Bluetooth Website)
- The devices in a piconet share a common communication data channel. The channel has a total capacity of 1 megabit
per second (Mbps). Headers and handshaking information consume about 20 percent of this capacity.
- In the United States and Europe, the frequency range is 2,400 to 2,483.5 MHz, with 79 1-MHz radio frequency (RF)
channels. In practice, the range is 2,402 MHz to 2,480 MHz. In Japan, the frequency range is 2,472 to 2,497 MHz with
23 1-MHz RF channels.
- A data channel hops randomly 1,600 times per second between the 79 (or 23) RF channels.
- Each channel is divided into time slots 625 microseconds long.
- A piconet has a master and up to seven slaves. The master transmits in even time slots, slaves in odd time slots.
- Packets can be up to five time slots wide.
- Data in a packet can be up to 2,745 bits in length.
- There are currently two types of data transfer between devices: SCO (synchronous connection oriented) and ACL
(asynchronous connectionless).
- In a piconet, there can be up to three SCO links of 64,000 bits per second each. To avoid timing and collision
problems, the SCO links use reserved slots set up by the master.
- Masters can support up to three SCO links with one, two or three slaves.
- Slots not reserved for SCO links can be used for ACL links.
- One master and slave can have a single ACL link.
- ACL is either point-to-point (master to one slave) or broadcast to all the slaves.
- ACL slaves can only transmit when requested by the master.
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