The Bluetooth Special Interest
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
Bluetooth uses a frequency-hopping technique in which each transmission lasts for only 625 m . 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
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
Communications can take place in both directions, between master and slave, with each time slot numbered. The range of time slot