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Bluetooth

Piconet

Channels

Bluetooth

There is a common misconception that  Bluetooth and wi-fi equipment are the same, or that they are to some extent compatible. The confusion is perhaps understandable, since Bluetooth equipment uses the same 2.4GHz band as most wireless networking equipment, and its function is much the same. It provides a wireless link between two pieces of electronic equipment. Despite the superficial similarities, the purpose of Bluetooth is very  different to that of wi-fi equipment.

Wi-fi is an extension of the Ethernet networking system, and when using wi-fi equipment you have what is essentially an Ethernet based network.

A typical wi-fi network will include some wired connections using standard Ethernet ports and cables. Bluetooth is not intended for

networking, although it could probably be used to provide networking. Using Bluetooth equipment in this fashion would be doing things the hard way though, and wi-fi equipment performs this task much more efficiently.

The real purpose of Bluetooth is to link two devices, and it is a general-purpose system which is designed to accommodate practically any two pieces of equipment that could be usefully linked. Bluetooth is used for such things as connecting a computer to a printer, a laptop PC to a desktop computer, and a notebook PC to a mobile phone. It can even be used for a non-digital application such as cordless headphones and hands-free headsets.

Wireless links have been used with PCs for many years in the form of IrDA infrared links. The main problem with the infra-red approach was that it was basically just a short range line of sight system, although for many purposes this was perfectly adequate. When uploading from a digital camera or a laptop computer to a PC for example, the two devices would probably be more or less side-by-side anyway. Another limitation of IrDA is that it is based on a standard serial port, and it has the relatively low bit rates associated with this type of port. The maximum transfer rate is usually about 115kbits (0.115Mbits) per second, and in some cases is much less than this.

Bluetooth is designed to be a sort of universal version of an IrDA link with a higher level of performance. It is designed to operate with low levels of power consumption, making it suitable for use with small portable devices such as phones and palmtop PCs. With a maximum transfer speed of only about 1 Mbit per second, Bluetooth is not particularly fast by modem computing standards. It is only about one tenth as fast as an 802.11b link for instance. The transfer speed is adequate for many purposes though, and it is nearly ten times faster than an IrDA link.

The range of a Bluetooth device is about 10 metres, and because it uses a radio link rather than infrared, the signal is able to pass through many types of object. It can be blocked by large pieces of metal though.

It is possible to obtain signal repeaters that can boost the range to 100 metres, but Bluetooth is normally used in applications where a range of 10 metres is adequate.

Although Bluetooth devices have a very short operating range, they still need security measures in order to keep hackers at bay. Security measures are built into the system, and Bluetooth gadgets have unique identifying codes that can be used to prevent any unauthorized use. For instance, a Bluetooth mobile phone could be set up so that it would only connect to the Internet when used with your own Pocket PC.

Piconet

When Bluetooth devices lock-on to each others´signals they form what is termed a piconet, or a small network in other words. One gadget initiates contact, and this is the master device. This unit negotiates all data transfer paths between devices in the network, and there can be up to seven slave units in addition to the master. In practice it is unlikely that eight units would be networked using Bluetooth, and in most cases it is used with just two devices. However, it could be used in a situation such as a Bluetooth enabled printer being fed from several PCs.

Any Bluetooth device has the wherewithal to control a piconet, and the master – slave relationship has to be flexible so that the equipment can adjust to suit practically any situation. With a simple swap of data between two devices, the one that initiates the connection becomes the master device. Any other units that join the piconet will automatically operate as slaves.

This simple scheme of things does not suit all situations, and it would not work in the printer setup mentioned previously. The first computer to access the printer would become the master unit, but for this network to operate properly it is necessary for the printer to always be the master device. This enables the printer to properly control the flow of print jobs. The way around this is for a master device to be temporarily demoted to slave status when it links with a device that must have master status.

Channels

Bluetooth uses the same 2.4GHz band as 802.11b and 802.11g wi-fi units, but the channeling is arranged differently. The lower bit rate of Bluetooth permits a larger number of channels to be accommodated, and there are some 79 separate channels. There is no need to select a channel, since a system of channel hopping is used. Device “hop” 1600 times a second in an attempt to avoid conflicts.

The system used with Bluetooth is called Adaptive Frequency Hopping (AFH), and it is designed to avoid interference from any devices that use the 2.4GHz band, and not just other Bluetooth equipment. As signals within the band come and go, Bluetooth devices will adapt to the changing situation in an attempt to make the best possible use of any free channels.

Bluetooth had quite a long gestation period, an when first launched its level of take-up was somewhat underwhelming. It is backed by the Bluetooth Special Interest Group, which has over 2000 members including huge companies such as Intel and IBM. It is gaining in popularity, but has yet to achieve its creators´aim of becoming a worldwide standard for short-range communications without cables. It seems likely that it will do so in due course.

Incidentally, Bluetooth is named after a Harold Bluetooth Gormson. He is not one of the brains behind the system, but was in fact the monarch of Denmark just over a thousand years ago. Apparently, he managed the difficult task of uniting two Baltic states, which seems to a rather tenuous link to modern wireless technology.