28-05-2013, 03:30 PM
Low-Cost Low-Power Radios: Bluetooth and Zigbee
As radios decrease their cost and power consumption, it becomes feasible to embed them in more types of electronic devices, which can be used to create smart homes, sensor networks, and other compelling applications. Two radios have emerged to support this trend: Bluetooth and Zigbee.
Bluetooth2 radios provide short range connections between wireless devices along with rudimentary networking capabilities. The Bluetooth standard is based on a tiny microchip incorporating a radio transceiver that is built
into digital devices. The transceiver takes the place of a connecting cable for devices such as cell phones, laptop nand palmtop computers, portable printers and projectors, and network access points. Bluetooth is mainly for short
range communications, e.g. from a laptop to a nearby printer or from a cell phone to a wireless headset. Its normal range of operation is 10 m (at 1 mW transmit power), and this range can be increased to 100 m by increasing the
transmit power to 100 mW. The system operates in the unlicensed 2.4 GHz frequency band, hence it can be used 2The Bluetooth standard is named after Harald I Bluetooth, the king of Denmark between 940 and 985 AD who united Denmark and
Norway. Bluetooth proposes to unite devices via radio connections, hence the inspiration for its name.
worldwide without any licensing issues. The Bluetooth standard provides 1 asynchronous data channel at 723.2 Kbps. In this mode, also known as Asynchronous Connection-Less, or ACL, there is a reverse channel with a data rate of 57.6 Kbps. The specification also allows up to three synchronous channels each at a rate of 64 Kbps. This
mode, also known as Synchronous Connection Oriented or SCO, is mainly used for voice applications such as nheadsets, but can also be used for data. These different modes result in an aggregate bit rate of approximately 1 Mbps. Routing of the asynchronous data is done via a packet switching protocol based on frequency hopping at
1600 hops per second. There is also a circuit switching protocol for the synchronous data.
Bluetooth uses frequency-hopping for multiple access with a carrier spacing of 1 MHz. Typically, up to 80 different frequencies are used, for a total bandwidth of 80 MHz. 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 ofnpiconets in an area increases, the number of collisions increases, and performance degrades.nThe Bluetooth standard was developed jointly by 3 Com, Ericsson, Intel, IBM, Lucent, Microsoft, Motorola,Nokia, and Toshiba.