The ZigBee or ZigBee/IEEE 802.15.4 protocol is a specification created for wireless networking. It includes hardware and software standard design for WSN (Wireless sensor network) requiring high reliability, low cost, low power, scalability and low data rate. ZigBee-style self-organizing ad-hoc digital radio networks were conceived in the 1990s, but the IEEE 802.15.4-2003/ZigBee specification was ratified on December 14, 2004. And only half year later the ZigBee Alliance announces availability of Specification 1.0 (on June 13, 2005).
A WSN consist in inexpensive wireless sensor which are capable of collecting, storing, processing environmental information, and communicating to neighbors nodes. For example, a WSN, at home can be used for light control, heating ventilation air conditioning (HVAC), security monitoring, and emergency event detection. And for industrial control can be used to improve the current manufacturing control system, detect unstable situations, control production pipelines, and so on.
ZigBee provides ultra low consumption and efficiency (thanks to the adaptable duty cycle, the low rate rates and the low coverage radio), and enable large scale networks for the WPN, making it one of the most convenient standards for this purpose.
The ZigBee Alliance
The ZigBee alliance objective is to work on the interoperability issues of IEEE 802.15.4/ZigBee protocol stacks. There are three levels of membership: Adopter, Participant, and Promoter.
Before start being more technical about ZigBee, let’s introduce the IEEE 802.15.4 standard. Basically what It does is to specify the physical and data link layer protocols for low-rate wireless personal area network (LR-WPAN) and it was defined in 2003.
Devices in a LR-WPAN can be classified as full function device (FFDs) and reduce function devices (RFDs). One device is designated as the PAN coordinator which is the responsible for maintaining the network and managing other devices
It also defines two types of network topologies: star topology and peer-to-peer topology.
The star pattern is structured, where the coordinator of the network will necessarily be the central node. Peer-to-Peer networks can form arbitrary patterns of connection, and their extension is limited by the distance between each pair of nodes.
It can operate on channel 868 MHz in Europe, 916 MHz in America and Australia and 2400 MHz worldwide, also some other were added.
Is the layer that provides the data transmission capabilities. There are three different states defined for the devices: Transmitting, receiving and sleeping. This allows to the device to save energy when duty cycles are defined and the device sleeps. It also characterize the link quality/strength of a received signal of a link - Link Quality Indication (LQI).
Is the layer in charge of data transference between the nodes of the network. It introduces - on the MAC sublayer - the superframe structure. The superframe is defined by the time between two beacons send by the coordinator of the network. The superframe can be divided between active period and inactive period, this - together with the sleeping state- allows to save energy during the inactive periods.
The beacons are used also to synchronizing with other devices, announcing the existence of a PAN, and informing pending data in coordinators.
The ZigBee protocol uses the IEEE 802.15.4 standard for the first two layers and adds the network and security, and framework protocol on top of it.
The network layer provides reliable and secure transmission among devices.
It defines three types of devices:
Also it defines three types of network topology: Star, tree and mesh.
In the star topology there is no presence of ZigBee Router. There is only a ZigBee Coordinator and zero or more ZigBee End Devices.
In the case of the tree, or cluster tree, topology the ZigBee Coordinator and also the ZigBee Router can announce beacons. The ZigBee Router can define dynamics duty cycle per cluster allowing devices to go to sleep not depending on the ZigBee Coordinator but on a more close ZigBee Router. The device would choose a slot and transmit its beacon to avoid collision. It also enables guaranty bandwidth (GTS).
Finally in the mesh topology there is no synchronization, therefore no beacon enable mode, meaning that the ZigBee Coordinator and the ZigBee Routers must be always on. And because of the dynamics of the network there is no bandwidth guaranty.
On the last ZigBee release there were several new features and improvements. Some of the most importants are:
Support of two kinds of security models, the Centralized Security Network and Distributed Security Network.
Incorporation of new specifications such as:
There are several advantages of using the ZigBee protocol when comparing with other protocols for WSN.