what is ZigBee

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ZigBee

 

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:-

Table 2.1: ZigBee Characteristics[2]

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.

Figure 2.1: A general ZigBee topologies[3]

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.

       

Figure 1.2:  (a) Star Topology General          

     (b) Star Topology in Terms of WPAN[3]

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:-

Figure1.3: Wireless 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:-

                                             

 

Figure 1.4: Tree topology[4]

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. 

About the Author

I am Kashif Mirza, the founder of Student Training Lab (STL). I am working on projects since 2005 before that I just search things and now I am sharing my knowledge through this plateform.

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