Mesh: Interference in the City?

If you've been to a tech conference recently you'll know that congestion and interference on the 2.4GHz band is a real issue. The sheer number of WiFi signals in the air -- coupled with other devices and appliances using the same public frequency -- can make it hard to get and stay connected to an 802.11 link.(See N+I: Noise Report.)

Now imagine the same scenario on a citywide WiFi mesh network and you'll see why many informed observers are warning that that municipal mesh networks will be plagued by wireless congestion and interference, slowing connection speeds to dial-up levels.

A new report tracking wireless mesh from Heavy Reading lays out the problem. At least one unnamed vendor, says author and senior analyst Patrick Donegan, has expressed "significant concerns" about large-scale wireless LAN mesh deployments in urban areas – particularly in the U.S. "In August 2006, this vendor, which is involved in some large-scale municipal buildouts in the U.S., stated that there will inevitably be fundamental issues in urban coverage, simply because of the number of private consumer and hotspot wireless LAN deployments in some of these areas," writes Donegan. "The vendor referred to these locations causing a very high 'noise floor,' which has a potentially ruinous effect on the network link budget for a city-wide mesh buildout." This scenario doesn't even take into account microwave ovens, Bluetooth radios, and consumer-grade wireless video cameras. All of these devices operate in the 2.4GHz band and are known to cause some degree of interference with WiFi networks. (See Pop-Up Pariahs.)

Some mesh players are aware of the issues but suggest that there are ways to deal with them. Ben Gibson, director of wireless and mobility marketing at Cisco Systems Inc. (Nasdaq: CSCO) says that all interference issues that apply indoors in the office or convention center also apply outdoors -- only more so.

"Factor that times ten when you take it outdoors," he says. "What's needed is real-time RF management."

Radio frequency management systems -- either as part of a WiFi system or a third-party application -- are becoming increasingly common in the enterprise. They allow network managers to swap channels and throttle back radio signals if they are causing interference or bandwidth blowout. Some systems even automate the process so that network admins don't have to be involved.

"Properly-designed WiFi mesh nodes, like Strix's OWS, first avoid interference by self-selecting channels during mesh formation with the least potential for interference," notes Nan Chen, VP of marketing at Strix Systems Inc. "As well, channel re-assignment may occur automatically in response to changing interference conditions."

Moving as much traffic as possible from the 802.11b/g pathways to 802.11a, which operates in the 5GHz band and offers more channels with less traffic on the band, is one way that vendors and operators are looking at avoiding interference issues.

"Most muni WISPs are utilizing a WiFi CPE [customer premises equipment] and in some cases are looking to use .11a to connect the WiFi modem back to the metroWiFi network providing 20 channels to choose from," says Strix's Chen. "Some WISPs have deployed .11a for client access -- with a WiFi CPE -- and are using .11g for the mesh backbone, as another way to eliminate the interference potential."

As it's still early days in the municipal mesh game, just how big an issue WiFi interference is will likely become clearer as densely populated cities such as Philadelphia and San Francisco light up their citywide networks (See SF Net to Go Public?)

— Dan Jones, Site Editor, Unstrung

wlanner 12/5/2012 | 3:39:12 AM
re: Mesh: Interference in the City? All the major indoor players have some form of automatic/dynamic RF management. Any AP today can pick the least congested channel on boot, but the thin AP/controller/switch vendors make adjustments to power and channel in real-time (ok, every 5-15 minutes).

Great, but you have 3 non-overlapping channels (and maybe you can get to 4 if you do it well). Vendors quoted in this article saying "our automatic RF solves it" are pushing fluff (ok, its their marketing people so they don't know they are doing it).

If you have potentially hundreds of other APs, you can't "adjust" your way out of that, you only have 3 choices. Indoors you can lower the power and create smaller cells, but if you don't have a free channel, there isn't much you can do.

So, do you expect an outdoor AP to lower its power until you can't hear it?

I did a site survey in a major city at an entprise in a a high-rise and we saw over 40 other APs. Many from other floors, but most coming into the building from outside (T-mobile, starbucks, home APs, etc...). As Cisco says, it gets even worse outside.

So, the reality of unlicensed spectrum is you are stuck. No amount of automatic power/channel changing will really help. Not to mention have users connected and change the power or channel - see how the clients react when their signal suddenly goes away. It makes life much easier indoors picking the right channel and then adjusting every so often to react to a new AP, but not outdoors in an urban area where there simply is no space.

Muni WiFi sounds good, but its best effort.

Another bigger impact is the enterprise in these cities. What do you do with your enteprise WLAN when somebody puts a bunch of mesh APs outside your door on channels 1,6, and 11 - your network gets impacted.

I know the answer is "hey its unlicensed spectrum", but expect enterprises to push back when it starts hurting them.
Sign In