As radios decrease their cost and power consumption, it becomes feasible
to embedded 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.
Bluetooth 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
and 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 handset. Its normal range of operation is 10m (at 1mW transmit
power), and this range can be increased to 100m by increasing the
transmit power to 100mW.The system operates in the unlicensed 2.3GHz
frequency band; hence it can be used worldwide without any licensing
issues. The Bluetooth standard provides l 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 57.6Kbps.The
specification also allows up to three synchronous channels each at a
rate of 64Kbps.This mode also known as Synchronous Connection Oriented
or SCO,is mainly used for voice applications such as headsets, but can
also be used for data. These different modes result in an aggregate bit
rate of approximately 1Mbps.Routing of asynchronous data is done via a
packet switching protocol based on frequency hopping at 1600hops per
second. These are also a circuit switching protocol for the synchronous
data.
Bluetooth uses frequency-hopping for multiple access with a
carrier spacing of 1Mhz,Typically,up to 80 different frequencies are
used, for a total bandwidth of 80Mhz.At any given time, the bandwidth
available is 1Mhz,with maximum of eight devices sharing the bandwidth.
Different logical channels can simultaneously share the same 80MHz
bandwidth. Collision 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 collision increases, and performance degrades.
The ZigBee radio specification is designed for lower cost and
and power consumption than Bluetooth. The specification is based on the
IEEE 802.15.4 standard. The radio operates in the same ISM band as
Bluetooth, and is capable of connecting 255 devices per network. The
specification support data rates of up to 250Kbps at a range of up to
30m.These data rates are slower than Bluetooth, but in exchange the
radio consumes significantly less power with a larger transmission
range. The goal of ZigBee is to provide radio operation for months or
years without recharging, thereby targeting applications such as sensor
networks and inventory tags.
A TECHNOLOGY FOR WPAN: BLUETOOTH
Bluetooth
wireless technology is a low-cost, short-range, radio links between
mobile PCs, mobile phones, and other portable devices. The Bluetooth
specifications are released by the Bluetooth Special Interest Group
(SIG), an industry group consisting of industrial leaders in the
telecommunications, computing, and networking.
The Bluetooth system operates in the 2.4 GHz industrial, scientific, and medicine (ISM) band. It is based on a low-cost, short-range radio link integrated into a microchip, enabling protected ad hoc connections for wireless communication of voice and data in stationary and mobile environments. It enables use of mobile data in different ways for different applications. Due to its low-cost target, it can be envisaged that Bluetooth microchips will be embedded in all consumer electronic devices. The characteristics of the Bluetooth technology offer wide room for innovative solutions and applications that could bring radical changes to everyday life. Let us imagine a PDA (with a Bluetooth microchip) that automatically synchronizes with all the electronic devices in its 10 meter range when you arrive at your home. Your PDA can, for example, automatically unlock the door, turn on the house lights while you are getting in, and adjust the heat or air conditioning to your preset preferences. But not only the home can become a more comfortable environment when the access to information is fast and easy. Let us imagine arriving at the airport and finding a long queue at the check-in desk for seat assignment. You can avoid the queue using a hand-held device to present an electronic ticket and automatically select your seat.
The BLUETOOTH Device:
A Bluetooth unit consists
of a radio unit operating in the 2.4 GHz band. In this band, 79
different radio frequency (RF) channels that are spaced 1 MHz apart are
defined. The radio layer utilizes the frequency hopping spread spectrum
(FHSS) as its transmission technique. The hopping sequence is a
pseudorandom sequence of 79 hop length, and it is unique for each
piconet. The FHSS system has been chosen to reduce the interference of
nearby systems operating in the same frequency range (for example,
IEEE802.11 WLAN) and make the link robust. The nominal rate of hopping
between two consecutive RF is 1600 hop/sec. A time division duplex
(TDD) scheme of transmission is adopted. The channel is divided into
time slots, each 625 _s in length, and each slot corresponds to a
different RF hop frequency. The time slots are numbered according to
the Bluetooth clock of the master. The master has to begin its
transmissions in even-numbered time slots. Odd-numbered time slots are
reserved for the beginning of the slaves’ transmissions. The
transmission of a packet nominally covers a single slot, but it may
last up to five consecutive time slots. For multislot packets, the RF
hop frequency to be used for the entire packet is the RF hop frequency
assigned to the time slot in which the transmission has begun. The RF
change reduces the interference from signals coming from other radio
modules. There are two types of physical links that can be established
between Bluetooth devices: a synchronous connection-oriented (SCO)
link, and an asynchronous connectionless (ACL) link. The first type of
physical link is a point-to-point, symmetric connection between the
master and a specific slave. It is used to deliver delay-sensitive
traffic, mainly voice. In fact, the SCO link rate is 64 Kbit/s and it
is settled by reserving a couple of consecutive slots for
master-to-slave transmission and immediate slave-to-master response
References:
Wireless communication
By
Andrea Goldsmith
Stanford
University
Images:
Google Images
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