6.1 Bluetooth Standards Today

Bluetooth is a wireless communication technology that is designed for short-range, point-to-point data transfer. Like infrared, Bluetooth is well suited for ad-hoc applications where the presence of a network infrastructure is not available. However, unlike infrared, Bluetooth does not have the limitation of requiring line of sight for communication. Bluetooth uses radio waves in the 2.4 GHz band. Coincidentally, this is also the band used by 802.11b and 802.11g devices. Unfortunately, a lot of domestic appliances also use this 2.4 GHz band, most notably cordless phones and microwave ovens. This means that this 2.4 GHz frequency band (also known as Industrial, Scientific, and Medical, or ISM) is crowded (see Interference Between Bluetooth and Other Devices in this chapter).

Interference Between Bluetooth and Other Devices

If you are operating an 802.11b (or 802.11g) network together with Bluetooth, Bluetooth performance will take a minor, generally unnoticeable hit. Since both operate in the 2.4 GHz band, interference will cause Bluetooth devices to resend their data. Well, if your father is operating his microwave in the kitchen, that is also going to interfere with your wireless network, whether it is Bluetooth or 802.11. When you are deciding where to put your computers and access points, you need to take this into consideration.

To minimize the impact of interference, Bluetooth uses a technique called Spread Spectrum Frequency Hopping . The actual frequency range used by a Bluetooth device is from 2.402-2.480 GHz. Within this range, Bluetooth devices "hop" between the different frequencies with a 1 MHz interval. This essentially gives Bluetooth devices 79 different frequencies to choose from. During a data transfer, the data is divided into packets. Each packet can be sent or received using any of the 79 different frequencies. Per the Bluetooth specification, Bluetooth devices can make up to 1600 hops per second. This essentially reduces the chance that a device will be "jammed" in a fixed frequency. (If the device is experiencing too much interference on the current frequency, you can be sure that it will look for a less busy one in a fraction of a second.)

6.1.1 Piconet

When two Bluetooth devices connect to each other, they create a network called a piconet. A piconet contains a master and one or more slaves. Any device can take the role of a master. The device that initiates the connection (the one that found the other device) is the master. Figure 6-1 shows a piconet containing eight devices (the maximum number allowed in a piconet) with one master and seven slaves.

Figure 6-1. A piconet with one master and seven slaves

I mentioned frequency hopping in the previous section. So how do all the devices in the piconet know which frequency to hop to? The master will establish a frequency-hopping scheme and communicate it to all the slaves. All slaves will then follow the frequency sequence set by the master.

6.1.2 Scatternet

When you have more than eight devices to connect, you can form piconets and join them into a larger network called the scatternet. A scatternet is made up of up to 10 piconets, giving a total of 80 devices. Figure 6-2 shows a scatternet comprising of two piconets. The individual master in the scatternet serves as a communication bridge between devices in each piconet.

Figure 6-2. A scatternet with two piconets

A device can belong to more than one piconet at any one time. It can also be a master in one piconet as well as a slave in another piconet. But a device cannot be a master of more than one piconet.

When a master is initiating a connection with a slave, it is known as paging. The master selects a new frequency for paging every 312.5 seconds, while the slave(s) selects a frequency to listen at every 1.28 seconds. Eventually, the master and slave devices will encounter a common frequency on which communication can take place.

The frequency hopping nature of Bluetooth devices also helps to enhance the secure communication path between two devices. See Section 6.7 in the later part of this chapter for more information.

6.1.3 Bluetooth Device Classes

Bluetooth devices fall into three categories: Class 1, 2, and 3. Most devices in the market today are Class 3. Due to their low power consumption, they operate with a radius of 30 feet (10 meters). For long-range applications, Class 1 devices allow up to 300 feet (100 meters) in operating radius. Table 6-1 shows the range and output of each class.

Table 6-1. Bluetooth device classes

Power class	Range/operating radius	Output power (dBm)
1	300 feet (100m)	20
2	30 feet (10m)	4
3	10cm to 10m	0

What Is dBm?

The unit for output power is dBm, a measure of signal strength in wires and cables at RF and AF frequencies. dBm means "decibels relative to one milliwatt." The output power, S, is defined to be:

S_dBm = 10 log₁₀ P

where P is the power level in milliwatts (mW). By definition, a 1 mW signal has a level of 0 dBm.

You can freely mix devices of different classes in a piconet or scatternet. However, note the range of each device. For example, Figure 6-3 shows a Class 1 Bluetooth access point with an operating radius of 300 feet. The Class 2 (or 3) Bluetooth adapter is within the access point's range. However, the Class 2 (or 3) device does not have an operating range to reach the access point. Hence the two devices are not able to communicate with each other.

Figure 6-3. Mixing Class 1 and 2 devices