Prospects Brighten for Voice over Wi-Fi

By Ted Stevenson | Feb 8, 2006 | Print this Page
http://www.enterprisenetworkingplanet.com/unified_communications/Prospects-Brighten-for-Voice-over-Wi-Fi-3583711.htm

Wi-Fi (wireless LAN) and VoIP are natural partners. In a world that's become totally accustomed to walking while talking, a wireless connection to a company's converged IP voice/data network makes all kinds of sense. And indeed, voice over WLAN—aka VoWi-Fi, VoFi, and a few other convoluted acronyms—has been available for years. But there have been some barriers to its widespread adoption.

Perhaps foremost has been the absence—until very recently—of a standards-based solution for prioritizing voice traffic on converged WLANs. Why is this an issue? Because of the limited bandwidth of IEEE 802.11 (Wi-Fi) networks, relative to their wired counterparts. When data packets 'contend' for bandwidth with voice packets, the quality of the voice signal plummets.

Also troublesome is the process of roaming—shifting connections between moving clients (phones or other voice-enabled devices) and stationary Wi-Fi access points (transmitters) because a smooth 'hand-off' is hard to achieve.

New technology being demonstrated today by Wi-Fi infrastructure vendor Extricom, addresses both of these issues, in a way that should make VoWi-Fi a more viable choice for many enterprises. In a dual announcement at Demo 06 , Extricom unveils a new hardware configuration and a new technology concept. Not surprisingly, the two go hand in hand.

The hardware in question is the EXSW-2400 Wireless Switch, a device for centrally managing up to 24 "UltraThin" access points (APs). The technology concept is trademarked TrueReuse. To appreciate the details of these announcements we'll need to step back a pace or two and absorb some background information.

First of all, proprietary switches and the resulting central network management have become firmly established in the Wi-Fi world over the past several years, as they make possible many performance improvements in the basic 802.11 technology. Part and parcel of this trend to centralizing network intelligence in the switch has been a move to "thin" APs—the ultimate extension of which makes the AP simply a radio.

Second, we need to understand a bit of basic 802.11 RF (radio frequency) reality: The radio spectrum used by Wi-Fi occupies defined frequency 'bands,' which are subdivided into a limited number of broadcast 'channels.' In a standard Wi-Fi deployment, adjacent APs using the same channels will interfere with each other—reducing effective bandwidth. With the two most common 'flavors' of Wi-Fi—802.11b and 802.11g—there are only three non-overlapping channels, and the standard approach to 're-using' the channels across a network while minimizing 'co-channel interference' is to stagger channel use in a network of APs in a kind of honeycomb configuration. 'Minimize' is key here, since it's impossible to eliminate co-channel interference this way.

Okay, back to Extricom and the EXSW-2400. While other centralized switch architectures are able to control things like broadcast power for all the network APs—thus controlling interference to a degree –Extricom's new switch actually keeps track of each packet and the location of the AP to which it is being dispatched, and in this way, according to vice president of marketing David Confalonieri, allows the total elimination of interference. "We haven't just mitigated these problems, they simply don't exist" Confalonieri told VoIPplanet.com.

In a nutshell, this packet-by-packet micro-management of multiple data streams means that adjacent APs are never broadcasting at the same time, hence, no interference. At the same time, it extends the coverage for each channel on the network to the entire web of APs, creating what Extracom calls a 'channel blanket.' "Every channel is at every AP," Confalonieri explained. "The capacity of that channel is available everywhere in the network."

But there's more. Extricom's APs carry dual radios, and can thus broadcast all three flavors of Wi-Fi—802.11a, b, and g—sequentially. Among other benefits, this allows IT staff to completely segregate different types of traffic (e.g., voice and data) on different radios, completely eliminating the 'contention' issue or need to prioritize traffic.

Furthermore, the switch is smart enough to know which APs are far enough apart so that simultaneous transmission on a given channel simply won't produce any interference. "Imagine that there's a radio station broadcasting at a particular frequency in New York City," Confalonieri said. "If you had a station broadcasting on the same frequency across the river in New Jersey, it might interfere. But you could have a station in Boston reusing this frequency. It's far enough away." This is really the basis for TrueReuse, as it allows simultaneous transmission on the same channel by widely separated APs, multiplying the effective bandwidth of the network by a factor of two to three..

The technical hurdle to roaming—moving around within a network while carrying on an uninterrupted phone conversation—is how to move a connection (or 'association' in Wi-Fi jargon) from one AP to another without a noticeable hiccup or break in the audio link. This is a 'user experience' issue stemming from our expectations of acceptable phone quality—based on traditional wired telephony.

Extricom's switch solves this 'latency' problem by eliminating the distinction between APs. Rather than associating with a sequence of APs, a mobile caller on an Extricom network associates with the central switch itself, which then sends the packets to the appropriate AP, with no need to re-authenticate the client and re-establish the connection on another radio. Zero-latency mobility, as the company's press release terms it.

The demo at Demo, at least according to the plan that Confalonieri described to VoIPplanet in a pre-event briefing, involves broadcasting three separate data streams simultaneously to three separate access points—on the same channel, in the same room. That would be tripling the effective bandwidth for that channel—and for all channels available to the 'mode' of Wi-Fi being used.

Wide availability of the EXSW-2400 is expected early in the second quarter of this year.