Zigbee is a global open standard wireless technology developed to address the wireless communication between battery-operated devices which are low power and low cost. he Zigbee standard operates on the IEEE 802.15.4 physical radio specification and operates in unlicensed bands including 2.4 GHz, 900 MHz and 868 MHz.

Zigbee devices are of three kinds:

Zigbee Coordinator (ZC):

The most capable device, the Coordinator forms the root of the network tree and might bridge to other networks. There is precisely one Zigbee Coordinator in each network since it is the device that started the network originally (the Zigbee LightLink specification also allows operation without a Zigbee Coordinator, making it more usable for off-the-shelf home products). It stores information about the network, including acting as the Trust Center & repository for security keys.

Zigbee Router (ZR):

As well as running an application function, a Router can act as an intermediate router, passing on data from other devices.

Zigbee End Device (ZED):

Contains just enough functionality to talk to the parent node (either the Coordinator or a Router); it cannot relay data from other devices. This relationship allows the node to be asleep a significant amount of the time thereby giving long battery life. A ZED requires the least amount of memory, and, therefore, can be less expensive to manufacture than a ZR or ZC.

ZigBee Architecture:

Application layer

defines various addressing objects including profiles, clusters, and endpoints. You can see the ZigBee stack layers in the figure above.

Network layer:

It adds routing capabilities that allows RF data packets to traverse multiple devices (multiple "hops") to route data from source to destination (peer to peer).

MAC layer

manages RF data transactions between neighboring devices (point to point). The MAC includes services such as transmission retry and acknowledgment management and collision avoidance techniques.

Physical layer:

It defines how devices are connected to make a network; it defines the output power, number of channels and transmission rate. Most ZigBee applications operate on the 2.4 GHz ISM band at a 250kbps data rate.
The Zigbee 3.0 protocol is designed to communicate data through noisy RF environments that are common in commercial and industrial applications. Version 3.0 builds on the existing ZigBee standard but unifies the market-specific application profiles to allow all devices to be wirelessly connected in the same network, irrespective of their market designation and function. Furthermore, a ZigBee 3.0 certification scheme ensures the interoperability of products from different manufacturers. Connecting ZigBee 3.0 networks to the IP domain opens up monitoring and control from devices such as smartphones and tablets on a LAN or WAN, including the Internet, and brings the true Internet of Things to fruition.