ZigBee-WiFi Compatibility, and Nodes at 500 m

WiFi and ZigBee Compatibility

Some people say WiFi and ZigBee can work side by side; others say they cannot. What's the truth?

Signed, Matchmaker

Wise Guy: Some technical details within the 802.11 (WiFi) and 802.15.4 (over which the ZigBee protocol runs) standards affect coexistence. As you can see in Figure 1, 802.11 takes quite a bite out of the spectrum available at 2.4 GHz. On top of that, 802.11 lays down one whopping big radiated power footprint. The situation gets even worse when multiple WiFi networks, each operating at different WiFi channels, are all functioning in the same physical area. Just imagine how Figure 1 would change if three more WiFi networks—say on channels 17, 21, and 24—took portions of the allocated 2.4 GHz ISM band. Little room would remain for an 802.15.4 network.

 Wise Guy
Wise Guy

Still, if you are using a single WiFi channel, there is available spectrum for an 802.15.4 radio to work. A frequency-hopping spread-spectrum system may perform better than a fixed carrier direct-sequence spread-spectrum system in the presence of WiFi. In any case, with careful channel selection, you can get a WiFi and 802.15.4 system to coexist in the same physical space.

Thousands of Nodes

How can I use ZigBee for intelligent maintenance of thousands of nodes located at the average distance of 500 m? Is there any other low-cost solution?

Thanks, Knowledge Explorer

Wise Guy: A protocol that runs on top of 802.15.4-compliant transceivers, ZigBee does not specify a maximum node-to-node distance. It can use the maximum RF power allowed by local regulations (1 W at 2.4 GHz in the U.S., but because of safety concerns we have settled at 100 mW—the same as Europe). This gives you ~100 m (~300 ft.) line of sight (LOS).

You may have heard that ZigBee can employ repeaters, gateways, and coordinators; it does allow for multihop networking. But you'll need many repeaters between the sensing nodes to achieve that 500 m separation and still maintain the network.

Figure 1. Frequency use within the 2.4 GHz band, showing 802.15.4 channels 11–16 (blue) with an overlay of an 802.11 channel (red). WiFi radiates about 10 X as much power as 802.15.4
Figure 1. Frequency use within the 2.4 GHz band, showing 802.15.4 channels 11–16 (blue) with an overlay of an 802.11 channel (red). WiFi radiates about 10 X as much power as 802.15.4

As for maintenance, a number of ZigBee-compliant products include software that show operational/maintenance values as well as which nodes are communicating with which other nodes. They typically allow you to set an alarm so you'll know if a node winks out. While we will not recommend any particular vendor (WINA is a neutral party), the latest ZigBee Alliance presentations at www.zigbee.org can help you find what works best for you.

ZigBee is the lowest cost standard solution for bi-directional communication. Other low-cost options follow the proprietary protocols of the vendors. But providing helpful direction for you will require more information about the application and your operating environment. Here are some things I'd need to know:

  • 1. The data rate you require per node, in Kbps or Mbps
  • 2. How often you need to take readings
  • 3. If you envision a polling scheme, and the number of nodes per network cluster segment (which leads to aggregate data rate per cluster)
  • 4. Number of nodes per gateway/coordinator
  • 5. If the gateway is to be IP-addressable
  • 6. The subnetwork cluster configurations; that is, how networks of networks are organized hierarchically where coordinators of the nested networks communicate
  • 7. Whether your environment allows for LOS or requires you to work without LOS
  • 8. Whether the environment includes metallic objects
  • 9. If you have or envision using other wireless devices in the same physical area
  • 10. The battery life you require
  • 11. Gateway/coordinator security specifications (are you just relying on the security within the devices, or must you answer IT-centric security questions, too?)

See, simple answers don't apply since we're no longer limited to old school point-to-point radio links. But once you have addressed these questions, you can use your answers as a shopping list in talking with vendors.

Wise Guy is the problem-solving persona of WINA, the Wireless Industrial Networking Alliance (www.wina.org). Send your questions to wiseguy@questex.com.