What is ZigBee ?

An Executive's Overview

The ZigBee Specification, based on IEEE Standard 802.15.4, has been developed and defined to enable a wireless network which provides for low cost, low power, reliable devices for monitoring and control.  An example application would be a home heating solution that has no wires between the thermostats and the furnace.  This allows the thermostat to be placed in the most advantageous spot for the home owner as opposed to the easiest place to run a wire.  Further if it turns out that this is the wrong spot it is easily changed.  Just move it.  Need another thermostat in another place? Add it!  ZigBee wouldn't be used to transfer media (emails, images or movies) from one device to another.

Why wouldn't 802.15.4 by itself be enough?  This standard talks about how the radios work but little about the "language" they speak between them.  The ZigBee spec as created by the alliance defines that language and thus allows products from different manufacturers to run on the same network.

Some Background

Over the years there have been many communications standards developed to suit various requirements.  While the technologies have advanced due to varying applications’ needs, higher levels of semiconductor integration and performance the architectural foundations for these standards were based on the 7 layer OSI model.  (An EXCELLENT explanation can be found at Webopedia.com ). 

The ZigBee standard is no exception and is also based on this model.  The difference between this standard and previous ones is the enhancements and extensions used to tailor it to the needs of the identified market.  These needs included:

• Wireless operation
• Low cost per node
• Low power
• Self organizing
• Ease of installation

Low cost implies large volumes and this requires customer confidence in the technology.  An open standard is a key to that confidence.  It allows customers to choose from a number of vendors and insures interoperability.

The target marketplace for this standard is low data rate sensing and control. An example would be a burglar alarm system. Typically such an installation requires many sensors which detect intrusion such as a window breaking or a door opening. The majority of the installation costs are in wiring these sensors to the control unit. A ZigBee based system makes installation a snap. A system can be developed which makes customer installation easy and practical.

Another example would be hand held measuring devices.  ZigBee ’s low power and small size are ideally suited to this application.  The ability of the network to self organize is crucial to the mobility requirements of hand held measuring devices which typically are moved around a tool room or a plant.

The ZigBee Specification

The ZigBee Specification defines a protocol stack architecture that is based on a widely used and accepted network model: namely the Open Systems Interconnection (OSI) seven-layer model. ZigBee builds on the low level physical (PHY) and medium access control (MAC) layers defined in the IEEE 802.15.4 standard by providing the network, application, and security frameworks necessary for wireless environments.

In the network layer (NWK), ZigBee defines the mechanisms used for joining and leaving a wireless network, to enforce any required security, and route data along the network to their intended destinations. Consistent with these functions, features for discovering and maintaining routes between nodes, starting a network, assigning network addresses, and other aspects of management reside in the network layer.

The application layer (APL) of ZigBee provides a higher level abstraction set that includes a set of application defined objects each capable of providing a unique service hosted by a ZigBee device (a node that implements the ZigBee protocols). ZigBee maintains binding data that matches devices based on application services provided and needed, and forwards messages between them. ZigBee discovers the devices and their roles in the network, determines the application services that each provides, and establishes secure connections between them. The application layer defines up to 240 endpoints: a mechanism that maps to an application service similar to the way TCP/IP defines ports for access services on the Internet. So while IEEE 802.15.4 node addresses references the entirety of a network node, ZigBee augments this addressing with an endpoint such that individual services can be referenced: a very powerful model for application development.

Profiles are used to define the services that a ZigBee implementation supports. A manufacturer supplies the profile of a specific device that includes the messages, their formats, and the actions performed based on those messages. This is the cornerstone of distributed application interoperability in ZigBee: insuring that devices, possibly from different manufacturers, can communicate and cooperate in some end-user application.

An association between endpoints is maintained by coordinator nodes using a binding mechanism: a logical link between endpoints similar to a TCP/IP connection. These bindings can many-to-many in nature to allow, for example, one switch to control multiple lights and/or multiple switches to control one light. The binding notion also allows short hand addressing to be used rather than requiring full IEEE (64+ bits) and ZigBee (endpoint etal) addressing. This can be important for simple devices having limited memory and processing power. ZigBee also supports broadcasting messages to all devices, used primarily during the device and service discovery and for application specific purposes.

ZigBee includes a ZigBee device object (ZDO) that acts as a base class of functionality for a ZigBee node. It satisfies the common requirements of all applications operating on a ZigBee device including discovery (device and service), binding (the connections between application objects), and security (key) management.

ZigBee supports three network topologies: star, tree and mesh. In a star topology, the network is controlled a single coordinator device that is responsible for all network management services. Other end devices perform the application specific processing and communicate only with the coordinator. Tree and mesh networks also utilize a coordinator for starting the network, but the network is typically extended through the use of router devices. In tree networks, the routers forward data using a hierarchical strategy with communication occurring up (many-to-one) and down (one-to-many) a tree structure. Mesh networks permit full peer-to-peer communications without restricting the communication paths between coordinators and routers.