1 Introduction
Wireless technologies that exist now days are able to
transfer data without cable and are low cost but still there is need of a
technology that can transfer data both cheaply and efficiently. There must be a
technology that don’t consume too much power and don’t need expensive
maintenance. For all of these reasons a new WPAN 802.15.4 was developed. It is
also known as ZigBee.
ZigBee is a
unique protocol as its name implies. The myth behind its name is that, it was
inspired by communication pattern of bees, as bees start dance in zig waggle pattern
when they discover a new food source. In this way bee are able to share
important information about food source.
ZigBee standard provides a set of protocols that communicates
at low data rate and short range while consuming low power. ZigBee based
wireless devices use 868 MHz, 915 MHz, and 2.4 GHz frequency bands to operate.
The maximum data rate that a ZigBee device can support is 250 K bits per
second. ZigBee is used in those applications where low data rate, low cost, and
long battery life is required. In most of applications ZigBee devices spend
most of their time in sleep mode. They consume power only when they are said to
communicate. As a result ZigBee enabled devices are capable to operate for
several years before their batteries need to be replaced
ZigBee standard is a low cost, low power and short
range wireless networking protocol that operates in ISM band. It can be used
for automation and remote control applications. Unlicensed band are not same in
every part of the world but ZigBee supports a band that is unlicensed in whole
world. ZigBee uses BPSK modulation scheme and have ability to receiver even in
very low signal environment.
For channels below 1 GHz, ZigBee standard uses BPSK
and for channels of higher frequencies it uses OQPSK. Purpose of doing so is to
avoid the zero states present in QPSK. It improves efficiency of device. List
of all the modulation schemes data rates and channel used by each of the three
bands is given in the table below:-
|
Frequency Band (MHz)
|
868.3
|
902-928
|
2400-2483.5
|
|
Bandwidth
|
600
|
200
|
5000
|
|
Data rate (kbps)
|
20
|
40
|
250
|
|
Symbol rate (kbps)
|
20
|
40
|
62.5
|
|
Area of use
|
Europe
|
USA
|
World wide
|
|
Frequency stability
|
40ppm
|
40ppm
|
40ppm
|
|
No. of channels
|
1
|
10
|
16
|
1.2 IEEE (802.15.4) Network Model
ZigBee standard has ability to handle up to
65535 nodes in a single network. Three types of nodes are supported by ZigBee
standard.
1.
Coordinator
2.
Router
3.
End Device
These
nodes are explained as under
1.2.1 Coordinator
Co-ordinate is required to initialize the network at
the start. Whatever network topology and number nodes are, there is only one
co-coordinator in a network. Co-coordinator acts as a central hub and is at the
top node of the network in mesh or tree topology. It routes the messages across
the network.
Tasks
performed by co-coordinator are
1.
It always selects a suitable frequency channel for initializing the network
2.
Coordinator always starts the network in every topology.
3.
Coordinator has the capability to allow other types of nodes
to join the network.
1.2.2 Router
Router performs following tasks
1.
It routes a message from one node to the other node.
2.
Child nodes are connected to a network through a router.
In star topology all of above actions are performed by
co-coordinator, so router doesn’t exist in star network.
When we are using Tree and Mesh topologies, functions
of router are
1.
In Tree topology, Routers are usually present in a position
so as it may allow to route the message from one node to other node which may
be present at up or down of the tree.
2.
In a Mesh topology, Routers are always present there where
the routing of a message is required.
The above concept is explained in the diagrams as
under. Routers are in red color.
1.2.3 End Device
Devices present at the end corners of a ZigBee network
are called end devices.
1.
In Star topology, End Devices are the outside nodes of the
network.
2.
In case of Mesh and
Tree topologies, End Devices are the leaf nodes
This concept is explained in the above diagrams, in
which the End Devices are blue color coded, routers are red color and
co-coordinator are sky blue color coded. End device receives and sends
messages. It cannot relay messages hence it cannot add child nodes as router
node can.
2.3 ZigBee Supported Network Topologies
ZigBee network layer manages the network. The network
should either have star or peer to peer topology.
2.3.1 Star topology
Co-coordinator is the main control point of network in
star topology. Every device can communicate to co-coordinator only. In this
type of networking topology, a co-coordinator selects a PAN identity and starts
searching for devices. It don’t allow any other parent or routing device.
1.3.2 Peer to Peer Topology
In this topology, every device can communicate with every
other device. But end device can only communicate with its parent device. All
devices in peer to peer are fully functional devices. Peer to peer topology is further
divided into two topologies that are wireless mesh and tree topology.
1.3.3 Wireless Mesh Topology
In mesh topology each device acts as an independent
device. A wireless mesh network is a special type of ad-hoc network. There is
no end device and router can add more and more devices. Routers can communicate
with their parent and with other routers as well. All of devices used in mesh
networking are fully functional devices. Following figure shows the mesh
network:-
|
1.3.4 Tree Topology
Tree topology consists of co-coordinators. All other
devices are connected to co-coordinator. The co-coordinator is linked with end
devices and routers. A router may be linked with further routers and end
devices. In this topology Childs can only communicate with it parents and vice
versa. The basic drawback of this tree topology is that if a necessary link is
failed there is no alternative route .Following figure shows the tree
topology:-
1.4 ZigBee Applications
ZigBee
has a lot of applications, some of these applications are explained below.
1.4.1 Home Automation
Controlling home appliances remotely and wirelessly is
termed as home automation. It is one of
major application in area of wireless technologies. ZigBee can be used to
control home appliances. Home automation is one of the major application areas
for ZigBee wireless networking. The applications of home automations are
further explained.
1.4.2 Security Systems
A security system consists of several sensors. These
sensors may include motion detector, security cameras and glass-break sensors.
All of these sensors communicate with base station. ZigBee protocol is used in
communicating with base station.
1.4.3 Meter-Reading Systems
Utility meters are read manually every month at home
and industry premises. These readings are added in data base to generate bills.
A ZigBee based automatic reading system can be used instead of manual method.
1.4.4 Light Control Systems
ZigBee can be used to control lights in home or in
commercial building. In a typical light control system, wires are installed to
control the lights. Wires and switches can be replaced by ZigBee modules to
turn on and off the lights.
1.4.5 Hotel Guest Room Access
Magnetic key card systems are widely used in hotels to
access the guest rooms. They can be replaced by ZigBee-based systems. A reader
is installed in door that reads information’s in card and opens the door. Card
readers require wiring through the door. A ZigBee based system can be helpful
in getting rid of wires.
1.4.6 Remote Control
Air conditioners, TVs, DVDs and some other home
appliances are controlled by remote control. Their remote controls use infrared
technology. Infrared technology can provide single way transmission and have
line of sight problem. Infrared do not have ability to penetrate through walls
and objects. ZigBee is a suitable technology that can provide two way communications
and have a good penetration power.
