: Ref. [10] reports that by 2016 there will be some 4 billion Bluetooth enabled devices worldwide (Figure 12.8). Applications include wireless headsets, mouse, keyboard, as well as game con- soles of Sony PlayStation and Nintendo Wii, wireless speakers, and SO on. Bluetooth tech- nology is also used to connect wearable devices, such as heart monitors and pedometers, to smart phones. Bluetooth technology is specified through IEEE under the 802.15.1 family of standards. These Content and services layer devices operate in the same 2.4 GHz ISM band as Wi-Fi devices, and similarly need to overcome inter- Connectivity retail layer ference emitted by other devices operating in the same frequency band. Similar to Wi-Fi systems, interference can be caused by other Bluetooth Infrastructure layer devices, Wi-Fi devices and other ISM equipment active in the same frequency band. In order to operate under this interference, the Bluetooth physical layer Subscriber/end-user uses a technology designed in the 1940s for military communications to avoid enemy jamming. The tech- nology is spread spectrum frequency hopping, in which the carrier frequency is regularly switched over a range of available carrier frequencies. In this way, even if one or more of the carrier frequencies experience jamming (interference), time-averaged interference can remain small. Bluetooth IEEE 802.15.1 standard specifies 79 carriers of 1 MHz bandwidth over which fre- quency hopping may be made. The transmitter/receiver pair uses any one of these 79 carriers once every 1/1500 S. A hop is then made to another carrier frequency based on a pseudo-random sequence arranged between the pair, as illustrated in Figure 12.9. Interference from other Bluetooth transmitter/receiver pairs can be small as the probability of a collision (i.e. two pairs using the same frequency carrier at any time) is 1/79, or approximately -18 dB. As the number of Personal Wireless Communications Systems Frequency 1 MHz 2.4 GHz 2.484 GHz Figure 12.9 Bluetooth frequency hopping in the 2.