Wireless embedded networks hold great promise for deployment in residential and commercial building-automation, industrial plant monitoring, and other wireless sensing and control applications. An association of corporations called the ZigBee Alliance is developing a standard for low-cost, low-power wireless embedded networking.
Version 1 of the ZigBee specification is set for release in the fourth quarter, and a number of platforms based on the standard are expected to be available around the same time. The specification provides network and application support services operating on top of the IEEE 802.15.4 standard of the media-access control layer and physical layer. ZigBee software may be implemented in microcontrollers for 802.15.4 radio chips.
The software employs a suite of technologies to enable scalable, self-organizing, self-healing networks that can manage a variety of traffic patterns. The standard is well-suited to applications such as lighting, heating and cooling controls; industrial building and automation; and medical device monitoring. The ZigBee Alliance’s long-term goal is to enable a scalable, low-cost, embedded infrastructure based on interoperable platforms and profiles that will let devices communicate in ways that have been impractical until now.
The ZigBee standard defines three types of devices: ZigBee coordinator devices; ZigBee router devices; and ZigBee end devices. Every network must contain only one ZigBee co-ordinator.
The primary responsibility of the coordinator is to set up the parameters for building a network and to start that process, including choosing a radio-frequency channel, a unique network identifier and a set of operational parameters.
ZigBee routers can be used to extend the range of a network by acting as relays between devices that are too far apart to communicate directly.
ZigBee end devices do not participate in routing.
All these devices can take on other roles an application requires.
As routers and end devices are integrated into a network, they obtain information about the network from the co-ordinator or from any router that already has joined. This information lets other devices set their operational parameters to reflect those of the network, and so join it.
A ZigBee router is given a block of network addresses to subdivide and distribute, and when a wireless device or other end device joins the network, it is given one of these addresses.
A ZigBee router uses tree routing, which takes advantage of the tree-structured addressing in making routing decisions. With tree routing, a device does not have to maintain memory-intensive routing tables or perform any additional over-the-air operations to discover routes, hence minimizing network traffic.
But tree routing follows the structure of a tree rather than taking the shortest path, and routes that are longer than necessary generate extra traffic and are more likely to fail.
To improve routing efficiency, the ZigBee algorithm also lets routers discover shortcuts. Each router that wishes to exploit shortcuts must maintain a table containing pairs of the form D,N, in which D is a destination address and N is the address of the next device on the path to that destination. The rule for routing is simple, “if you have a shortcut use it, otherwise use the tree.”
The simple request/response protocol whereby shortcuts are discovered under ZigBee is derived from a routing algorithm called the Ad hoc On Demand Distance Vector.
ZigBee networks are simple to install because they form autonomously. Furthermore, the combination of tree routing and table-driven routing provides an operational flexibility and a range of price/performance options to developers that support the ZigBee Alliance’s goals of low-cost scalable network infrastructure.
