FEATURE: Interference: The Muni-WiFi Bogeyman

Some observers, most recently the Economist, have raised the specter of muni-WiFi network failure due to chaos in the unlicensed spectrum in which they operate. Their concerns include overuse of the frequency band available to WiFi, muni-WiFi networks interfering with home and office WiFi access points and other users of unlicensed spectrum to interfering with muni-WiFi networks.

Interference is more bogeyman than bane. Tropos Networks' experience in more than 300 customer deployments gives us reason to be optimistic that muni-WiFi networks will not be hampered by interference and will not cause significant problems for nearby WiFi networks.

Tragedy of the Commons?
A tragedy of the commons occurs when overuse of a shared resource leads to a reduction in the available quantity of that resource. Some observers have stated that an overabundance of WiFi networks will lead to overuse of the airwaves, resulting in no bandwidth being available for anyone. In other words, they fear that muni WiFi could cause/suffer from a tragedy of the commons.

This fear is unfounded. The WiFi protocol, based on the long-used Ethernet protocol, does not allow overuse to diminish the amount of available bandwidth. This is true for many users sharing the same WiFi network and, importantly, for many WiFi networks sharing the same spectrum. The average amount of available bandwidth per network and user decreases as the number of networks and users increases but the total amount of available bandwidth does not. The WiFi protocol is very efficient at allowing users and networks to share spectrum. WiFi networks don't interfere with one another. By design, they share the airwaves.

Big Stick vs. Little Stick
A more subtle question is what happens when two or more WiFi networks share the same airlink and one of the networks operates at a much higher transmit power than the other(s). In this situation, the lower power access points may be able to hear the higher power mesh routers but not vice versa. In this case, interference could, in theory, occur because the mesh routers could properly listen before talking and still transmit at the same time as lower power network.

Two factors mitigate against this situation. First, most lower-power enterprise and residential WiFi networks are located indoors, while powerful metro-scale WiFi networks are located outdoors. Building walls significantly attenuate WiFi signals. As a result, metro-scale WiFi signals will generally only slightly increase the level of noise that already exists due to other devices that use the 2.4 GHz frequency band.

Second, if the attenuation from walls is low and outdoor signals propagate well to the indoors, then it is likely that signals from indoor networks will likewise make their way outside with little attenuation. Because muni-WiFi routers generally exhibit excellent receive sensitivity, even weak signals from indoor networks will often be received. If this is the case, the indoor network and the metro-scale network will properly participate in the WiFi protocol and interference will be avoided. The enhanced receive sensitivity of muni-WiFi routers helps reduce the potential for interference caused by their higher transmit power.

Non-WiFi Interference
Finally, some commentators fret that interference from other users of the 2.4 GHz band will interfere with metro-scale WiFi networks. A well-designed metro-scale WiFi offers three levels of defense against this possibility.

First, most 2.4 GHz band interference comes from devices such as baby monitors, cordless phones and microwave ovens. Most of these devices are fairly low power and operate indoors. Little interference from them leaks to the outdoors.

Second, most outdoor interference in the 2.4 GHz band comes from point-to-multipoint (P2MP) systems that run along the rooftops of high-rise buildings. Properly installed metro-scale WiFi systems are installed at streetlight level, well below P2MP systems. This spatial separation effectively prevents interference between metro-scale WiFi systems and P2MP systems. In our more than 300 metro-scale WiFi deployments, Tropos Networks generally has seen little interference at streetlight level.

Third, to the extent that there is interference in the 2.4 GHz band at streetlight level, the self-organizing and self-healing properties of mesh networks obviate the problem. As interference crops up, the mesh detects it and automatically routes around it.

These three factors dramatically reduce concerns about non-WiFi interference in metro-scale WiFi networks.

Technology Advances Offer Further Help
Advances in technology will further reduce worries about interference. For example, dynamic channel selection algorithms will allow metro-scale WiFi networks to use the channel(s) with the lowest levels of interference while automatic power control will enable the networks to operate at the lowest possible power levels. Coupled with the existing protections described above, evolving technology will keep the interference bogyman at bay, even as more networks and larger networks are deployed.

Bert Williams is the senior director of marketing with Tropos.