Personal Area Network
In the early 1990's, Ericsson Mobile Communications sought to do away with the seemingly endless mass of cables that was needed to connect a computer to its peripherals. In 1994, Ericsson began work on a wireless technology called Bluetooth. Since then many companies have adopted Bluetooth technology, making it the standard for a small, low power, cheap radio chip to wirelessly link computers to printers, PDAs, keyboards, etc.

Bluetooth, however, is not limited to use by a single PC and its peripheral devices. Bluetooth is also designed to support the creation of ad hoc networks between any Bluetooth enabled devices. For example, it would be possible to connect a mobile phone to a nearby printer, printers to other printers, or your television to a hi-fi receiver. A chip could be placed inside a cargo container and used to track a variety of information. The uses for Bluetooth are as limitless as the designers' creativity.

Bluetooth provides robust communication over relatively short distances (about 10 meters) between devices which require sporadic contact with one another. It is not intended to replace high traffic networks such as LANs, WANs, and backbone cables.

How Bluetooth Works

The Wireless Channel

The Bluetooth radio operates in the globally available ISM band of 2.4 GHz. This ensures that Bluetooth enabled devices will be able to communicate anywhere in the world. To avoid interference in this noisy radio frequency environment, Bluetooth uses fast frequency hopping in timeslots. By frequently hopping to a new frequency, the link can avoid spending too much time in particularly noisy range.

Frequency hopping

It also uses short packets, which, together with the frequency hopping, reduces the effects of electro-magnetic disturbances , such as microwave ovens or cordless phones. Other coding techniques are used to minimize the impact of random noise in the channel.

Network Architecture

Bluetooth was specifically designed to support ad hoc networks. That is, different devices can be connected for varying amounts of time. At the simplest level, the architecture of a Bluetooth network is called a piconet, which is defined by as:

A network of devices connected in an ad hoc fashion using Bluetooth technology. A piconet is formed when at least 2 devices, such as a portable PC and a cellular phone, connect. A piconet can support up to 8 devices. When a piconet is formed, one device acts as master, and the others act as slaves for the duration of the piconet connection. A piconet is sometimes called a PAN (personal area network).

In a piconet all communication is between master and slaves. Slave units can not talk to each other. Nor is the master responsible for routing information between slaves within a piconet. If two slaves wish to communicate, they must form a new piconet. A device may be a slave in several piconets, but may only be master of one. This interconnection of piconets is called a scatternet.

Bluetooth scatternet

Devices which belong to more than one piconet will spend certain timeslots on each network. A device may choose to be "parked" or on "hold" in its original network, or it may choose to leave it altogether. Either way, Bluetooth is designed to support exactly this type of dynamic reconfiguration.

How Devices Communicate

Bluetooth uses frequency hopping in the 2.4 GHz ISM band. There are 79 hop frequencies spaced 1 MHz apart from 2.402 GHz to 2.480 GHz. The hopping sequence is determined by an algorithm involving the master's device address. All devices in a piconet will follow this same sequence.

Establishing a piconet is done through a series of paging and scanning procedures. A device can determine what Bluetooth devices are within range and request a connection if it so chooses. The device initiating the connection becomes the master and the other the slave by default, although a master/slave switch be done if the slave is not already a master of another piconet.

The master takes turns communicating with each slave in a periodic fashion. Time is divided into 625 us slots. The master transmits at the beginning of even-numbered slots, while slaves may transmit only at the beginning of odd-numbered slots. Thus, it is important that all units be closely synchronized to the master's clock.

The master can maintain both synchronous (SCO) and asynchronous (ACL) connections with its slaves. In an SCO connection, the channel is reserved for two time slots for communication between the master and a particular slave. The period of these reserved slots is set by the master, and may be a function of priority. This is similar to circuit switching. Each slave gets a turn. The time slots are allocated whether or not either master slave has anything to transmit at that time. ACL transmissions may occur in between the time slots reserved for SCO links. In the illustration below, notice that the SCO link between master and slave 1 occurs every six time slots. In between these slots the master may transmit ACL packets to any slave.

SCO and ACL links

Each time slot is associated with a frequency in the hopping sequence. Therefore, if a transmission lasts 3 time slots, the next frequency will be the third one in sequence from that of the last transmission. That is, frequencies will be skipped if a packet takes more than 625 us to transmit, or if no one has anything to transmit at all. The illustration below shows three possible scenarios.

Hop sequence

Future Outlook

When Ericsson first introduced its Bluetooth technology in 1998, it was touted to revolutionize the way consumers use and interact with mobile computing and communications devices. There was a tremendous amount of hype. Several companies, including Ericsson, Intel, Motorola, and IBM, joined the Bluetooth Special Interest Group (SIG) to monitor and promote Bluetooth development. Unfortunately one of BLuetooth's biggest selling points, its low cost, was never realized. Until very recently a Bluetooth chip would have cost device manufacturers around $10. This was too high a price for the cost sensitive mobile electronics industry. Bluetooth took a huge step backwards in early 2001 when Microsoft said that its then new Windows XP would not support Bluetooth.

A break for Bluetooth came in June of 2002 when Texas Instruments announced that it would be releasing a new Bluetooth chip early next year for under $4. These declining component costs and a booming interest in all things wireless has led to a resurrection of Bluetooth technology. What was once a technology on the demise, is now the foundation for the newest wave of personal mobile computing and communications capabilities.

According to the Business2.0 article entitled "Bluetooth to Break Through Gum Line", research from IDC indicates that revenues from Bluetooth related products, such as chips and memory, will grow from $76.6 million in 2001 to $2.6 billion in 2006, with widespread adoption beginning next year. Cell phone handsets are expected to account for nearly 51% of these revenues, with headsets for handsets accounting for the next largest piece of the pie. With statements like the one below from the PalmOS website, it is not hard to believe such predictions.

As an associate member of the Bluetooth Special Interest Group, Palm is committed to helping Bluetooth become the standard for a new style of computing that will extend the way people use their handheld computers.

Bluetooth and 802.11

The IEEE 802.11 standard specifies two types of networks: ad hoc, and client/server. Bluetooth is based on the standard for ad hoc networks. The main difference betweeen Bluetooth and Wireless LANs based on the IEEE 802.11 is that Bluetooth is intended for transmitting small amounts of data (about 1 Mbps) over short distances (10 meters), while WLANs can support higher datarates (2 to 11 Mbps) over much larger distances (up to 100 meters). Perhaps the most defining distinction between the two is that Bluetooth is specifically designed to support rapid and dynamic reconfiguration. This is not how LANs work. In addition, Bluetooth is cheaper to insert into devices and consumes less power, which makes it ideal for the usage model of the personal area network. Thus it appears as though Bluetooth and 802.11 will ultimately coexist.