Sunday, 13 October 2013

ZIGBEE


ZIGBEE

Zigbee is a wireless technology used for communication.
It is a low cost and low power consumption technology.
It is developed to meet the low power requirements of the devices.
The zigbee standard is regulated by a group known as the ZigBee Alliance.
Zigbee operates on IEEE 802.15.4 specification, which is the standard for wireless protocol.
    -IEEE stands for Institute of Electrical and Electronics Engineers
    -802 groups denote the section of the IEEE involved in network operations and technologies.
    -15 groups denote wireless networking technologies.
Zigbee and Bluetooth are nearly same technology.  Bluetooth connects large packet user devices whereas Zigbee connects small packet devices.
The data rate of Zigbee is 250 Kbps and operates on the 2.4 GHz ISM band, which is available throughout most of the world.
The maximum range of operation for ZigBee devices are 250 feet i.e., 76 m.
Due to low power consumption Zigbee devices withstand for long period of time.

LAYERS IN ZIGBEE:

The layers in Zigbee are
    1. Physical Layer
    2. MAC Layer
    3. Network Layer

1. Physical Layer

The Physical Layer satisfies the need of low cost.
It uses DSSS (Direct Sequence Spread Spectrum) modulation scheme.
Because of this modulation the circuits used will also be simple thus reduces the cost.

FREQUENCY BANDS AVAILABLE FOR ZIGBEE:

The Zigbee has three frequency bands for its operation.
    -868.3 MHz for Europe, it has one channel at 868.3 MHz and data rate of 20 kb/s
    -915 MHz for North American, it has 10 channels between 902.0 and 928.0 MHz and data of 20 kb/s
    -2.400 GHz for world wide application, it has 16 channels between 2.4 and 2.4835 GHz and data rate of 250 kb/s

The transmitter in 868 and 915MHz bands uses BPSK, whereas the transmitter in 2.450 GHz band uses offset-QPSK.

2. MAC Layer

 

The MAC layer in Zigbee controls the accessing channels by means of CSMA-CA (Carrier Sense Multiple Access with Collision Avoidance) mechanism.
MAC Layer initially transmits beacon frames and provides necessary synchronization for a reliable transmission.
It has two modes, Beacon-enabled (slotted) and Non Beacon-enabled mode (unslotted).
In Beacon-enabled mode, periodic data is transmitted to all devices. The device will wake up for that beacon, check for any messages and then go back to sleep.
In Non Beacon-enabled mode, device has to be awake all the time in order to receive new messages.

3. Network Layer

 

The Network layer in Zigbee tells about addressing mechanisms, security of transmitted frames and relay of frames to their destinations.
Network Layer's security is responsible for authentication, encryption and message integrity.
The way of communication between devices is also included in this network layer.
There are three types of network topologies in Zigbee. They are Star, Cluster Tree and Mesh.
In Star Topology, the nodes have to communicate via the coordinator.
In Cluster Tree Topology, the communication takes place in hierarchical manner. The node that wants to send data should first send it to router, the router will send it to coordinator and then the coordinator will distribute it to corresponding node.
In Mesh Topology, mostly all the nodes are interlinked so any node can communicate with any other. For this advantage only we are mostly using mesh topology.

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