Mesh networks: ZigBee provides mesh for wireless nets

By Control Engineering Staff October 3, 2006

ZigBee networks connect control nodes and target nodes wirelessly in a mesh structure. Each node communicates with its nearest neighbors, so command signals propagate across the network via multiple paths rather than directly. Should one path become unusable (because a node fails or a communication link becomes blocked), the command can reach its target via other paths.

Oak Brook, IL— If you visualize the topologies of most popular networks, “net” is not really the image that springs to mind. Ethernet “net”-works, for example, resemble stars. Wireless networks, being predominantly Ethernet-based, follow this same pattern.

An emerging standard for wirelessly interconnecting controlled components, called ZigBee , breaks that tradition. ZigBee’s purpose is to provide a means of passing low-level control signals in industrial, commercial, and residential spaces. In a ZigBee-“wired” office building, for example, light switches would not be hard-wired to the lights they turn on and off. Instead, all the lights, computers, and other electric and electronic appliances would be linked by spread-spectrum radio signals into a common “mesh” network.

The advantage of a mesh is that control signals can propagate through the net from node to node. Thus, the control point node need not have a direct link to all of the controlled nodes. The controller can pass its signals to its nearest neighbor nodes, which then pass the signal to their nearest neighbors until the signal reaches the target node or nodes. While all of the nodes in the net “see” the message, only the target nodes respond. The others passively hand it off.

A person flipping on a light switch might see no difference from what happens today: flip the switch and all the lights in a certain area would come on; flip it off, and they go off. Facilities engineers, however, would see a big difference. With today’s technology, changing the pattern of which lights come on with what switches involves a lot of pulling cables through a lot of cableways in crawl spaces. With ZigBee, however, it’s simply a matter of reprogramming the individual lights to take their on/off commands from different switches. In fact, since ZigBee is a self-configuring wireless standard, reprogramming those lights is a matter of (wirelessly) uploading the new configuration data through a ZigBee-enabled computer engaged in the network. Those involved don’t have to get their fingers dirty unless they’ve spilled lunch on their keyboard.

The only difference system users might notice would be a strange lack of technicians at the tops of ladders with their heads poked above the dropped-ceiling!

ZigBee is not some futuristic fantasy standard. The original ZigBee standard, ratified by ZigBee Alliance members in December 2004, enables a wide range of products. The ZigBee alliance consists of some 200 corporate members in 26 countries spanning six continents. These members include giants in the controls industry, such as Siemens , Philips , Eaton , and Schneider Electric/TAC who have already shipped ZigBee-enabled products.

Recently, Brent Hodges, vice president marketing and business development of the ZigBee Alliance, told Control Engineering about some major enhancements to the ZigBee standard that the Alliance expects to make available during first-quarter 2007. They fall into four areas:

Grouping devices —OEMs may create groups of devices, while allowing individual devices to belong to multiple groups. This simplifies configuring networks by allowing large groups of devices (such as all the lights on one floor of a building) to be connected to a control point with one command, rather than having to connect them individually.

Easy maintenance —ZigBee technology prevents a single point-of-failure on the network and allows for easy replacement or repair of devices through a simple process of storing a device’s information onto a nearby device. While a failed node may no longer function (that is, a certain light may not turn on), control signals have alternate paths to bypass that node, so the net as a whole functions.

Targeted broadcasts —Broadcasts of commands can be specified for specific types of devices: routers, “awake” or “sleeping” devices. For example, a signal might turn on all of the lights in a certain group that are initially in an off state without affecting those that are already on. This feature reduces RAM requirements, lowering the total cost of the components for ZigBee products.

Over-the-air setup —Opens the door for an array of new setup tools to facilitate adding devices to a network. The setup tools can be used to bind specific devices, such as a light fixture and a corresponding switch, and professional installers may use the tools to modify a network on a larger scale.

Enhanced ZigBee continues use of mesh networking and digital spread spectrum technology to communicate over the globally available 2.4 GHz frequency. In certain countries, 868/915 MHz technology is also available. The protocol is designed to accommodate more than 65,000 devices on a single network. Simplicity of design and operation ensures consumers of all backgrounds can install products and maintain their network in minutes. In fact, Hodges pointed out that consumer applications represent the largest class of ZigBee-compliant products currently available, and he expects this trend to continue.

Two certification levels are available for ZigBee-compliant products:

ZigBee compliant platform —Testing ensures radio/microprocessor combinations and modules adhere to Alliance specifications and offer reliable and robust wireless networking prior to development into a product by an OEM; and

ZigBee certified product —Designed specifically for end-user products, this testing ensures devices work as promised, allowing consumers or other users to buy with confidence.

Certification programs are administered by respected, independent laboratories to ensure devices perform to the standard. To date, hundreds of ZigBee-ready products are in the marketplace, tens of products are in the certification process. Thirteen ZigBee-compliant platforms are ready for OEMs to incorporate into products.

Organizations seeking immediate access to the new ZigBee standard, or assistance with developing products based on ZigBee, can join the Alliance by clicking here .

Search on ZigBee or wireless atop any page at for more on these topics.

—Control Engineering Daily News Desk C.G. Masi , senior editor