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Piconets and Scatternets

As mentioned earlier, the basic unit of networking in Bluetooth is a piconet, consisting of a master and from one to seven active slave devices. The radio designated as the master makes the determination of the channel (frequency-hopping sequence) and phase (timing offset—that is, when to transmit) that will be used by all devices on this piconet. The radio designated as master makes this determination using its own device address as a parameter, while the slave devices must tune to the same channel and phase. A slave may only communicate with the master and may only communicate when granted permission by the master. A device in one piconet may also exist as part of another piconet and may function as either a slave or master in each piconet (see Figure 1). This form of overlapping is called a scatternet. Figure 2 contrasts the piconet-scatternet architecture with other forms of wireless networks.

Figure 1 Master/slave relationships.

Figure 2 Wireless network configurations.

The advantage of the piconet/scatternet scheme is that it allows many devices to share the same physical area and make efficient use of the bandwidth. A Bluetooth system uses a frequency-hopping scheme with a carrier spacing of 1 MHz. Typically, up to 80 different frequencies are used, for a total bandwidth of 80 MHz. If frequency hopping isn't used, a single channel would correspond to a single 1 MHz band. With frequency hopping, a logical channel is defined by the frequency-hopping sequence. At any given time, the bandwidth available is 1 MHz, with a maximum of eight devices sharing the bandwidth. Different logical channels (different hopping sequences) can simultaneously share the same 80 MHz bandwidth. Collisions will occur when devices in different piconets, on different logical channels, happen to use the same hop frequency at the same time. As the number of piconets in an area increases, the number of collisions increases, and performance degrades. In summary, the physical area and total bandwidth are shared by the scatternet. The logical channel and data transfer are shared by a piconet.

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