Mesh Wi-Fi Complicates Wireless VoIP Use

Updated: January 25, 2007

An emerging trend in wide-area Wifi gives wireless IP telephony fans one more technical issue to worry about. According to a new In-Stat report, there will be some 99,000 mesh Wifi access points in operation in 2010, up from less than 20,000 last year. And while the quality of wireless VoIP calls may be spotty to begin with, having Wifi APs linked via wireless mesh backhaul can degrade it further. All users can do is hope that Wifi operators build out their mesh networks with voice in mind.

Wifi mesh has a number of benefits for deployments covering large areas and large numbers of access points. Connecting APs to a high-speed Internet gateway via wireless rather than wireline/cable links makes it easier to blanket broad areas with coverage. It's cheaper than running cables to APs on light poles, for example, and can provide substantially higher throughput than connections that rely on 1.5Mbps T1 lines. And because the architecture allows multiple redundant routes, it can be more reliable than landline-based backhaul.

Such advantages make Wifi mesh attractive in a number of situations. One of the most obvious, according to In-Stat analyst Gemma Tedesco, is municipal Wifi installations, which by definition cover a lot of territory and users. University campuses are another promising application, she says. So are government networks for the use of police, fire and transportation agencies, which are themselves often part of broader municipal installations.

The technology has made significant headway around the world, Tedesco observes. Kenya and Macedonia are currently operating multi-city or national wireless mesh broadband networks. Municipal networks are running in cities as diverse as Cittagong, Bangladesh; Moscow, Russia; and Taipei, Taiwan. The Beijing public security bureau is deploying a network in preparation for the 2008 Olympics, with an emphasis on video surveillance. U.S. cities where the technology is in use include Anaheim, Mountain View, and Pasadena, Calif.; Chandler, Gilbert, and Tempe, Ariz.; Philadelphia, Pa.; and Corpus Christi, Texas.

On the other hand, one fact will almost certainly slow the rollout of Wifi mesh over the next few years: There is no industry standard for equipment. Thus any deployment will, for the time being, have to run on gear from only one vendor. That could keep prices relatively high, and make operators hesitate before buying. The top vendors in the market are BelAir Networks, Nortel Networks, SkyPilot Networks, Strix Systems, and Tropos Networks, according to the report.

VoIP could prove one of the better opportunities for Wifi mesh. Tedesco notes that Bangladesh, Kenya and Macedonia are using their networks to provide phone service to subscribers. Doing so is quicker and less expensive than rolling out either wireline or cellular infrastructure to do the job.

But the technology has inherent disadvantages when it comes to carrying voice. The more wireless "hops" a call has to traverse, the more likely it is to suffer increased latency, especially when the network gets busy and the spectrum crowded. The result can be voice quality problems involving factors such as cadence, echo and jitter.

One solution is to use more radios in each access point, rather than having one radio handle both backhaul and user links. Strix, which Tedesco points to as one of the two most voice-oriented Wifi mesh vendors (the other being RoamAD), uses three. One handles the 802.11g client connections in the 2.4Ghz band. The other two, using 802.11a links in the 5Ghz band, separately handle incoming and outgoing connections among APs.

Strix also uses switching rather than routing to channel traffic among the APs. Doing so eliminates the need for propagating and/or maintaining large routing tables. That reduces the overhead required for control signaling, and thus increases efficiency, according to marketing director Kirby Russell. And perhaps most important, Strix APs automatically give VoIP packets top priority, delaying other traffic if necessary.

Strix's approach allows it to keep voice latency between hops at 3 to 5 milliseconds, Russell states, adding that overall latency under 100 milliseconds yields "great" voice quality. It can also deliver throughput as high as 18Mbps to 24Mbps on the ninth or 10th hop away from a 100Mbps Internet gateway, with each node along the way capable of serving 500 to 700 users, he says.

As Wifi VoIP becomes more common, keeping track of the nuances of wireless mesh technology will be a bit much for ordinary users to deal with. And Russell claims there will be many such users in the future. A majority of Strix's customers are interested in providing voice service over their mesh networks, he says, often expecting it to produce significant revenue even when they're offering basic Internet access for no charge.

And that may provide the best hope for those wishing to, say, eliminate their cellular bills entirely by using Skype or other VoIP services from their favorite hotspots. As long as providers think they can make money offering VoIP over Wifi, they will — or should — do whatever is necessary to make sure it works right, even if it means some people will use it for next to nothing.

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