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Special Focus
January 2005

Will Voice Over WLAN Go Mainstream In 2005?
Seven Key Deployment Issues To Consider

BY Lawrence Gary

We humans are a mobile species and, thanks to the interoperability brought about by the 802.11 standards for wireless LANs (WLANs), we’ve wasted little time in shedding the wires that have bound us to our data networks in the home, at work, and on-the-go. So-called WiFi networks based on those 802.11 standards are popping up everywhere. In the U.S. alone, more than 12 million homes have them; almost half of all enterprises have installed them in some limited fashion; and nearly 20,000 public “hotspots” are now to be found in airports, cafes, and fast-food outlets.

As this WiFi phenomenon has grown, another disruptive technology has emerged alongside it: Voice-over-IP. VoIP has turned the telecom industry on its head by hollowing out time-worn business and pricing models. Now accounting for just one percent of phone users worldwide, VoIP is expected to capture more than a quarter of all phone traffic by 2008, with much of that traffic effectively free between users sharing the same network. Interestingly — and not so coincidentally — 20 percent of new wireless installations are anticipated to be driven by VoIP adoption by year-end 2005.

It seems inevitable then that these two technologies would merge to create a new application: Voice-over-WLAN. VoWLAN can enable businesses to respond to customers or clients faster, resolve issues more quickly, accelerate processes and decision-making, and reduce errors caused by miscommunications (i.e., “the telephone game”). It can even increase job satisfaction by reducing phone tag and enabling employees to respond better and faster to customer or client issues.

Prepping For VoWLAN Deployment
Before considering the key issues involving a VoWLAN deployment, it’s important to understand some WLAN basics. WLANs operate in the unlicensed frequency bands the ISM (Industrial, Scientific and Medical) band at 2.4 GHz and the U-NII (Unlicensed National Information Infrastructure) band at 5 GHz.

Three variants of the 802.11 standard — b, g, and a — are used in WLANs, with 802.11b products the first to emerge several years ago. It supports three independent 11 Mbps channels at 2.4 GHz and uses direct sequence (DS) and frequency hopping (FH) for modulation. Although DS allows higher data rates than FH, it also uses more power in mobile client devices.

Also operating in the 2.4 GHz band and backwards compatible with 802.11b is the 802.11g standard, which can deliver up to 54 Mbps and is fast gaining momentum as single-chip implementations of it are being used in laptop PCs. It uses Orthogonal Frequency Division Multiplexing (OFDM) for modulation.

The 802.11a standard operates in the less crowded 5 GHz band, uses OFDM and can achieve data rates up to 54 Mbps. Its downside is that it’s not backwards compatible with “b” and its radio coverage is less, so more access points must be installed in a given area.

The most important goal for deploying an enterprise VoWLAN is to deliver the voice quality, reliability, and functionality similar to what users expect from their wired business phones. Here are seven key issues to consider in providing a VoWLAN application that meets their expectations:

WLAN Coverage
Just as wired LANs must be upgraded for VoIP to account for QoS, reliability, and security concerns, chances are that an existing WLAN will also need upgrading. For starts, data-only WLANs are not designed for mobile use of data devices. VoWLAN applications, on the other hand, need to be designed for mobile use and need blanket coverage of the entire premise, not just cubicles and offices but lobby areas, stairwells, conference and break rooms, even bathrooms.

Also, mobile VoWLAN phone users will typically walk while they talk, moving among various access points (APs). And unlike data devices that are held or used at arms length or greater distances from the body, users tend to hold phones to their ears, so additional radio signal attenuation is introduced.

For all these reasons, WLAN APs must be positioned with sufficient overlapping coverage throughout the operating environment to eliminate dead spots between them. Then, adjacent APs need to use different radio channels to avoid interference between them. The 802.11b standard provides three non-interfering channels (1, 6 and 11 for North America), so mapping AP coverage must account for APs within range of each other to be set to non-interfering channels.

In addition, all APs should be set to the same power output and data rates. Variances in power could cause interference between a higher and lower powered AP. Differences in data rates could result in wireless phones may not associate with the closest AP if a more distant one supports a higher data rate.

A comprehensive site survey can help optimize all these factors.

WLAN Capacity
While data traffic tends to be sporadic and bursty, voice QoS can’t tolerate delays, so it’s critical to plan WLAN data throughput capacity carefully.

The number of simultaneous calls a single AP can support varies depending on the codec and data rates used. For example, an 802.11b AP using the common G.711 codec can generally support up to 12 simultaneous calls at 11 Mbps but only seven calls at 2 Mbps. The G.729 codec can support roughly 50 percent more calls at these respective data rates.

Overall, WLAN needs to be engineered for overall voice usage (measured in units of Ehrlangs, with a single Erhlang equal to the traffic generated by a single phone call lasting one hour) weighed against the probability of call blocking. If an AP can support a maximum of 12 simultaneous calls, then moderate calling intensity (0.15 Erhlangs) with one percent blocking probability would translate into about 40 users supported per AP.

System Infrastructure
In addition to APs, a VoWLAN system requires Ethernet switches, a VoIP gateway, a WLAN controller, a PBX to the PSTN and messaging servers, CAT 5 10/100 Base-T wiring, and hard or soft phones for end-users.

At the physical layer, Ethernet switches are recommended over hubs to allow adequate bandwidth and limit traffic collisions. IP addresses can be assigned to handsets either statically or dynamically, but if the latter, a DHCP service is needed.

Subnets are logical boundaries between network segments that can terminate an active call for all practical purposes should the handset’s user roam beyond the originating subnet. This issue is addressed by a new device called a WLAN controller that enables fast, secure handovers across subnets and provide needed scalability for large physical environments.

End-user devices need to be selected depending on the environment and business requirements. “Hard” phones tend to share the form factors of cell phones. But some are hardened for industrial purposes. And some have push-to-talk capabilities. At a minimum, they should have a clear display, multichannel capabilities and long battery life. “Soft” phones are software clients for laptops or PDAs that provide VoWLAN connectivity.

Quality Of Service
Whether deploying a VoWLAN application over a voice-data WLAN or an all-voice WLAN, treating all packets equally will result in jitter and delay in voice quality, so a critical provision for voice packet prioritization must be made.

Early 802.11 standards lacked any QoS mechanisms, so vendors hatched their own proprietary schemes. Soon the 802.11e protocol will be released which should allow for greater interoperability (and lower prices) among vendor equipment.

All in all, managing QoS is all about optimizing the balance between bandwidth, signal strength, and device power conservation while maintaining seamless, full-featured enterprise communications with minimal jitter, delay, or packet loss.

WLANs have long had a reputation for being inherently insecure, but despite security shortcomings in early standards and implementations, vast improvements in the encryption of air interfaces and authentication mechanisms have made WLANs secure for both voice and data across the enterprise.

While WPA and WEP have provided interim solutions, 801.11i has formally standardized WiFi security. In addition, wireless users can be placed into a separate security domain similar to users connecting remotely via VPN mechanisms.

Availability And Reliability
Speaking of end-user expectations, VoWLAN applications must provide the same high availability and reliability of wireline communications. All network components must have a fail-over strategy in case of an outage, and the network itself must have a fallback in case of power loss. Handset power consumption and battery life also need to be accounted for.

A VoWLAN application needs proper ongoing administration to ensure the continuing optimization of all the previous considerations. A WLAN administrator’s toolkit should include the means for hardware and device management as well as operational performance and troubleshooting. A combination of tools provided with system components and from third-parties are required.

With these deployment issues so stated, the allure of VoWLAN applications to a network administrator may have dimmed a bit. And let’s face it: VoWLANs may not be for every enterprise — today. But they will be soon enough as more and more WLANs are deployed in lieu of wired LANs, as buildings are remodeled and new ones erected without end-point wiring, as workers become ever more mobile, and as WLAN system elements are further standardized and costs are driven down. Will 2005 be the year? Maybe not for full-blown ubiquity, but certainly it’ll be much further on its way.

Lawrence Gary is product manager, desktop products at Siemens Communications, Inc. For more information, please visit the company online at

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