General Overview of Bluetooth

The original Bluetooth version 1 specifications have been updated several times. The current version of the Bluetooth specification is version 2.0, which adds support for enhanced data rate (EDR) transmissions at up to 3Mbps, compared to 721Kbps maximum with Bluetooth version 1.2. Version 2.0's EDR achieves higher speed by using phase shift keying (PSK) modulation instead of the Gaussian frequency shift keying (GFSK) method previously used.

Bluetooth technology uses lower-power transmissions and therefore is limited in the distance it can coverup to about 10 meters. A more powerful version of Bluetooth allows for higher-power transmissions that can range up to 100 meters. Instead of creating a new technology from scratch, some parts of the Bluetooth specification were borrowed from existing technologies. Some of the more important ones include the following:

• Frequency-hopping spread spectrum (FHSS) Bluetooth hops at 1,600 hops per second, using 79 frequencies each separated by 1MHz, over the total spectrum allowed in the 2.4GHz range. Both asynchronous communication (at 721Kbps) and synchronous communication (at 432.6Kbps) are supported to provide for both voice and data transmissions.

• Motorola's Piano This technology allows the formation of small ad hoc networks, sometimes referred to as personal area networks (PANs). Although ad hoc networks are also used by other wireless technologies, Bluetooth forms ad hoc networks within just a small area, usually up to 10 meters.

• Digital Enhanced Cordless Telecommunications (DECT) This specification was adopted for the voice and telephony applications that Bluetooth can provide.

• Object Exchange Protocol (OBEX) This technology was borrowed from the IrDA (Infrared Data Association). It allows for data exchanges such as synchronizing address books between a Bluetooth-enabled PDA and a PC, for example, or for exchanging electronic business cards.

Bluetooth uses a frequency-hopping technique in which each transmission lasts for only 625m. This means that data is sent over one radio frequency for just this short time. After that, the radio frequency changes and another small amount of data is transmitted. Because of this small allocation of time for each transmission, a transmission of just a simple message is sent over the air by dividing it into many smaller bits of information, and sending these small discrete units over a preset pattern of ever-changing radio frequencies (see Figure 22.1).

Figure 22.1. Data is sent in smaller units, each of which uses a different radio frequency.

In Figure 22.1 you can see that a Bluetooth device delivers data that needs to be sent to the radio transmitter. The radio transmitter breaks the message into many smaller units of information. Each of these units (A, B, C, and so on) is sent using a small window of time, each using a different radio frequency (1, 2, 3, and so on). At the receiving end of the transmission, each unit of data sent on different frequencies is combined back into the original data and passed on to the receiving device.

This is, however, a simplistic illustration. In a typical situation, more than one device is transmitting or receiving at the same time. Thus, the time slots (A, B, C) may not be contiguous, but are shared by all devices. So in reality a transmitter might send data using one time slot, and then wait for a few time slots before sending the next unit of data. Each unit of data, however, is sent using a different radio frequency, hence the term frequency hopping.

At the receiving end of the transmission, each small unit of data is received on a separate radio frequency, and each unit of data for each time slot is reassembled into the full message of data that was transmitted before it is passed to the receiving device. Because both the transmitting end and the receiving end of the communications know in advance what time slots will be allocated, and what frequencies will be used, it is a simple matter to keep track of multiple devices transmitting/receiving at the same time. This is because each device will be allocated different time slots, and thus different frequencies, for each data transmission.

A simple Bluetooth network consists of a single master and up to seven slaves. Transmissions take place based on a frequency-hopping scheme decided on by the master, and all members of a piconet (discussed in further detail in the next section) use the same frequency-hopping pattern. Thus, it's possible to have multiple piconets within close proximity of each other because each piconet uses a different hopping pattern among the 79 available frequencies. When a device joins a piconet, the address of the master device is sent to the slave in a special packet called a frequency-hop synchronization packet (FHS packet). The hopping pattern is calculated based on the address of the master node. The master device's clock is used to determine which particular point in the hopping sequence is the current one, and all slaves keep track of the difference between their own clocks and the master's so that they can all hop along together.

Communications can take place in both directions, between master and slave, with each time slot numbered. The range of time slot numbers is from 0 to 2271. The master device can start transmissions in even-numbered time slots, whereas slaves can start transmissions in odd-numbered slots. To provide for larger data transfers, up to five consecutive slots can be used. However, for these five slot transmissions, the data is transmitted on the same frequency, determined by the frequency to which the hopping pattern is set when the first packet is transmitted.