Wireless Bluetooth interface

Introduction to Bluetooth

Wired interfaces have long dominated the connection of peripherals to computers. For many types of connections a wireless solution would be easier to handle the communication with small battery operated devices, for example to connect your mobile phone to a PC to synchronize the phone book or for headsets to listen to your favorite music.

Infrared serial connections have been around for a long time. The first mass use of IR serial connections was for the remote control of the television set. IR connections are good for this specific type of operation where there is a line of sight between the two devices and the data transfer speed is low. For connections at higher speed—for example to transfer high quality audio—or for situations where there is no direct line of sight like with portable headsets, IR is not the best technical solution. With infrared light there is also the problem that the communication can easily be intercepted by other devices, or even tampered with. One game children like to do is standing outside of the house with a second remote control and zapping through the available channels on television while mom and dad are sitting in the house not knowing what is happening to their TV. This kind of interception and hostile takeover should not be possible with low-power datatransmission for computers. This is why two developers of Ericsson Mobile Platforms came up with a low-power solution using radio transmission called Bluetooth.

Bluetooth technology considerations

To circumvent some of the known problems with IR communication, Bluetooth uses frequency-hopping spread spectrum technology. With this technology the sender and receiver switch continuously their frequency. This makes it more difficult to intercept the signal and it is also a good system to avoid frequency bands where the signal has interference from other devices in the neighbourhood. Radio equipment in general also allows higher bandwidth rates than infrared communication devices.

Bluetooth was developed with many different uses in mind, ranging from phone book transfer from mobile phones, audio from headsets and position data from GPS receivers. Because of this the internal protocol running on Bluetooth systems is very versatile. Because of the many small, advanced and relatively cheap devices targetted by Bluetooth, you could easily call it the gadget interface.

Bluetooth has been developed for low power, low costs situations where data transfers are necessary. The low power requirement has two good reasons. First of all battery powered devices will work longer on their batteries if less power is consumed by the data transfer technology. Low power also means less radius of the radio signal. If the radius of the signal is smaller, more devices can operate in the same frequency bands with higher bandwidth available for each devices, and the chance that the signal is intercepted by third parties is lower. For some situations longer reach may however be necessary. For this reason three power classes for Bluetooth devices have been defined.

Bluetooth power classes
Class Max power Range
1 100 mW 100 m
2 2.5 mW 15 m
3 1 mW 8 m

High power devices can communicate with low power devices, but in general the low power device will define the maximum allowed distance between the two peers.

Bluetooth communication speeds

As with all communication interfaces, fast is never fast enough. Therefore a steady increase in speed is seen with each new release of the Bluetooth standard. Although the system has never been developed for high speed connections, current Bluetooth systems communicate in the megabit per second range. The design speeds of several versions of the standard are grouped in the following table.

Bluetooth communication speed
Version Year Data rate
1.2 2003 1 Mbit/s
2.0 + EDR 2004 3 Mbit/s
3.0 + HS 2009 24 MBit/s

Bluetooth in the real world

The first version 1.0 of Bluetooth was not really user friendly. Although the system uses a protocol where small packets are sent in independent slots, the first version only allowed one device to be connected with a Bluetooth host at any moment. In the 1.1 version this was corrected to 8 devices which could communicate simultaneously with one host system.

Despite these early startup problems, Bluetooth is now available on a large number of devices including:

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