Page 471 - From GMS to LTE

Bluetooth and Bluetooth Low Energy 457

piconet. The channel bitmap is then sent to all devices of the piconet and thus, all
members of the piconet are aware of how to adapt their hopping sequence. The standard
does not specify a single method for channel assessment. Available choices are the
Received Signal Strength Indication (RSSI) method or other methods that exclude a
channel because of a high packet‐error rate. Bluetooth 1.2 also offers dual‐mode devices,
equipped with both a WLAN and a Bluetooth chip, to inform the Bluetooth stack as to
which channels are to be excluded from the hopping sequence. In practice, this is quite
useful, as the device is aware which WLAN channel has been selected by the user, and it
can then instruct the Bluetooth module to exclude 25 consecutive channels from the
hopping sequence.
As Bluetooth has been designed for small, mobile, battery‐driven devices, the stand-
ard defines three power classes. Devices like mobile phones usually implement power
class 3 with a transmission power of up to 1 mW. Class 2 devices send with a transmis-
sion power of up to 2.5 mW. Class 1 devices use a transmission power of up to 100 mW.
Only devices such as some Universal Serial Bus (USB) Bluetooth sticks for notebooks
and PCs are usually equipped with a class 1 transmitter. This is because the energy
consumption as compared to a class 3 transmitter is very high and should therefore be
used only for devices where energy consumption does not play a critical role. The dis-
tances that can be overcome with the different power classes are also quite different.
While class 3 devices are usually designed to work reliably over a distance of 10 m or
through a single wall, class 1 devices can achieve distances of over 100 m or penetrate
several walls. The range of a piconet also depends on the reception qualities of the
devices and the antenna design. In practice, newer Bluetooth devices have a much‐
improved antenna and receiver design, which increases the size of a piconet without
increasing the transmission power of the devices. All Bluetooth devices can communi-
cate with each other, independently of the power class. As all connections are bidirec-
tional, however, it is always the device with the lowest transmission power that limits
the range of a piconet.
Security plays an important role in the Bluetooth specifications. Thus, strong authen-
tication mechanisms are used to ensure that connections can be established only if they
have been authorized by the users of the devices that want to communicate. Furthermore,
encryption is also a mandatory part of the standard and must be implemented in every
device. Ciphering keys can have a length of up to 128 bits and thus offer good protection
against eavesdropping and hostile takeover of a connection.

7.3 Piconets and the Master/Slave Concept

As described previously all devices which communicate with each other for a certain
time form a piconet. As shown in Figure 7.2, the frequency‐hopping sequence of the
channel is calculated from the hardware address of the first device that initiates a con-
nection to another device and thus creates a new temporary piconet. Therefore, devices
can communicate with each other in different piconets in the same area without dis-
turbing each other.
A piconet consists of one master device, which establishes the connection, and up to
seven slave devices. This seems at first to be a small number. However, as most Bluetooth
applications require only point‐to‐point connections as described in Section 7.1, this