Voice: The Killer App For WLANs

Wireless LANs have been proliferating in the last few years, but many enterprises remain unable to justify installing wireless technology broadly. But voice-over-wireless LAN (VoWLAN) is starting to look like the killer app that will change that.

VoWLAN systems offer a single infrastructure for supporting mobile telephone and data traffic within a facility. That enables companies to realize substantial ROI through lower initial hardware costs and long-term operational support expenses.

Put differently, the benefits of VoWLAN are fairly easy to quantify, which will make WLAN deployment more palatable to accounting folks. Even if a company isn't implementing VoWLAN solutions today, many are deploying wireless LANs with requirements to support VoWLAN in the future.

VoWLAN Architecture

VoWLAN is an extension of voice-over-IP (VoIP) technology. Many companies make use of VoIP in order to have a common network infrastructure for internal telephone and data traffic. VoWLAN completes the picture by adding wireless telephony.One of the key components of a VoWLAN system is a Wi-Fi-enabled telephone. Examples of dedicated VoWLAN phones are Spectralink's NetLink e340 and Cisco's Wireless IP Phone 7920, which interface with 802.11b access points.

Software, sometimes calls softphone software, is also available that converts a wireless-equipped PDA, laptop, or desktop PC into a VoWLAN telephone. Telesym's SymPhone, Xtend Networks' X-PRO and VLI's Gphone are examples of softphone software.

In a VoWLAN system, the phones connect to the network via wireless access points. Some access points need vendor-specific software upgrades from the VoWLAN phone maker to enable VoWLAN operation. In addition, some products make use of voice servers that improve voice quality and management of the network.

At a higher level, the phone communicates directly with an IP PBX (private branch exchange), which understands IP (Internet Protocol) and connects via analog and digital circuit-switched connections to the public switched telephone network (PSTN). The IP PBX manages calls within the facility and interfaces with the PSTN for external calls, which travel across traditional telephone networks. A VoIP gateway can interface VoWLAN phones to a legacy PBX in cases where a company wants to make use of existing, older equipment.

Another scenario is to route calls over the Internet instead of the PSTN. This offers even lower costs because of the savings in long distance telephone charges. For example, a company can subscribe to Vonage, which offers broadband phone service at rates lower than traditional cellular prices. Vonage interfaces with a variety of adapters, such as ones from Linksys and Motorola, that interconnect telephones, PCs, and fax machines to the Internet.

Capacity And Roaming IssuesCapacity and roaming are key issues when building out a wireless LAN to accommodate VoWLAN. That's because a voice signal only needs 8Kbps continuous throughput to achieve quality that's similar to a traditional telephone. However, Internet and 802.11 protocols raise actual data rate requirements to approximately 200 Kbps for VoWLAN. Along with other delays from collisions and other users, there's not much room for voice phone calls.

On solution, particularly if there will be a lot of VoWLAN users, is to use a nearly-forgotten standard for voice-over-WLAN: 802.11a. The 802.11b/g, specification can sustain six or seven concurrent calls per cell. In comparison, 802.11a handles around 25 concurrent calls. As a result, 802.11a works well in areas having a greater density of telephone users.

This approach is not without its problems, though. First, not many VoWLAN phones support 802.11a yet. Also, the installation of an 802.11a network requires a greater number of access points in order to keep access point signal strength high enough for VoWLAN operation.

Access point roaming is a function of the 802.11 protocol, which enables VoWLAN phones to re-associate automatically with access points as the user moves throughout the facility. The timing for the handoffs must be relatively fast, however, in order to assure smooth operations and no dropped calls. Access points are generally fast enough, but the wired distribution system that interconnects the access points may cause additional latencies beyond the limits of the phone. Keep this in mind when designing a VoWLAN system.

Provisions For QualityData is much more forgiving of various levels of network latency than voice. A user browsing the Web, for example, can get by when it takes a second or two longer to load a Web page. However, a streaming voice signal is sent in real-time and the ups and downs of wireless LAN performance can make voice sound awful. Radio frequency interference and higher-than-expected utilization of the wireless LAN in one area can significantly reduce the quality of calls. Coverage holes are also common in enterprise wireless LANs, which causes dropped calls to occur.

There are not yet any Quality of Service (QoS) standards to handle these problems -- the 802.11e standard which addresses QoS, is still under consideration won't be ratified until at least later this year. As a result, most enterprise WLANs can't innately guarantee voice quality.

However, a number of vendors offer either proprietary QoS or subsets of the expected 802.11e standard. In fact, the Wi-Fi Alliance, a trade organization that ordinarily does interoperability testing for WLAN equipment, recently approved a subset of the 802.11e standard. Look for QoS from vendors such as SpectraLink has voice prioritization for its NetLink phones, from vendors like Cisco, Proxim, and Airespace.

Deployment Tips

When installing a wireless LAN for VoWLAN applications, be certain that the access point signal levels are high enough throughout all coverage areas. The signal-to-noise ratios should not drop below 25dB. There should be around 20 percent cell overlap and at least two acceptable access point signals at all user points in the facility.

Also, keep in mind that users will operate their phones from different places than they operate laptops and other devices when connecting to the WLAN. In other words, you need coverage in places such as hallways, staircases, and elevators, which are not common to wireless laptop users.In addition, be certain to set adjacent access points to non-overlapping channels (e.g., channels 1, 6, and 11 in the U.S.). This may not be absolutely necessary for non-VoWLAN and low-utilization wireless applications, but it's critical in VoWLAN solutions to keep the phone traffic in one cell from interfering with phone traffic in another cell. You need to tweak the performance as much as possible to ensure that there's adequate capacity.

Concluding Thoughts

There's an extremely high likelihood that VoWLAN will be a great benefit to your enterprise. However, you must deal with a number of issues that you ordinarily wouldn't deal with when deploying WLANs for more general use.

For instance, until the 802.11e standard is approved, you'll need to deal with proprietary QoS issues. And, as previously discussed, there are some special considerations you must keep into account when considering the architecture -- and 802.11 specifications -- you choose.

However, in most cases, the benefits of VoWLAN far outweigh the challenges of deploying it. And, besides the ROI benefits of VoWLAN, when you build out your WLAN to accommodate voice, it will be available for other applications as well.Jim Geier is a regular contributor to Mobile Pipeline. He is the principal of Wireless-Nets, Ltd. (www.wireless-nets.com), an independent consulting firm focusing on the implementation of wireless mobile solutions and training.