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Feature Article
September 2004

Planning for Voice on WiFi Networks


Enterprises of all types are deploying Wireless LANs at an ever-increasing pace. According to In-Stat/MDR, there are currently more than 75 million wireless LANs in use worldwide, with 40 million more estimated to be put into service this year. And for good reason; in a recent survey conducted by NOP World Technology and Cisco Systems, end users estimated that they are as much as 27 percent more productive with access to WLANs as they would be without them. And while the most prevalent applications accessed via WLANs today are data applications such as e-mail and Internet access, Voice over WLANs (VoWLANs or VoWiFi) is quickly gaining traction.

WiFi increases productivity by freeing workers from their wired data network connections; VoWiFi completes the process of untethering workers from their desks including offering mobile access integrated with the enterprise telephone network — something traditional cellular service can’t do. VoWiFi also offers more reliable coverage indoors and higher voice quality than traditional cellular service. One of the biggest benefits VoWiFi has over the mobility offered by cellular is cost — using cellular to achieve mobility while within the grounds of the enterprise is an expensive proposition.

Whether planning an initial deployment or expansion of a WLAN network, the decision-making process should include considerations for carrying voice, whether now or in the future.

One of the first considerations when designing a WiFi network, with or without voice, is which RF technology to use. Today there are three different standards, each with different data rates. The most popular is 802.11b, which operates in the 2.4 GHz spectrum, supporting up to 11 Mbps per channel and up to three non-overlapping channels. 802.11g also operates in the 2.4 GHz spectrum and supports three non-overlapping channels, however it supports a data rate of up to 54 Mbps. 802.11a operates in the 5 GHz spectrum, supports a data rate of up to 54 Mbps and 23 non-overlapping channels. Products that use the newer 802.11g standard are compatible with 802.11b as they both use the same spectrum.

Most VoIP handsets on the market today are manufactured for 802.11b, due to its popularity. However, in order to support high-quality voice, capacity of the WiFi network needs to be taken into consideration. As more access points are installed, the possibility for co-channel interference increases. The more non-overlapping channels can be configured, the more this risk can be mitigated. Since 802.11a supports more non-overlapping channels it is a good choice for high-density environments.

Capacity planning is a key element to ensuring high-quality VoWiFi, however a Quality of Service (QoS) strategy to ensure voice is prioritized over data traffic is also needed. There is a task force working to enhance the current 802.11 standard to expand support for applications with QoS requirements, the much anticipated new standard is called 802.11e, and is expected to be ratified this year.

As often happens when approved standards are so long in the making, the industry seems to have settled on interim versions of the QoS standard. There are two interim standards which correspond with the two methods proposed in 802.11e. Wireless Multimedia Extensions (WME), also known as Prioritized QoS, assigns traffic to traffic classes. Certain classes, such as voice, get priority treatment although the bandwidth is not guaranteed. Up to eight classes are supported. Wireless Scheduled Multimedia (WSM), also known as Parameterized QoS, allows bandwidth to be reserved. Parameterized QoS is needed for applications such as HDTV.

While WME and WSM prioritize traffic over the air, once it hits the wired LAN there must be a scheme to prioritize voice as well. For that, an IP type of service scheme such as DiffServ can be used.

Like their wired counterparts, wireless LANs are vulnerable to numerous different types of security threats. In addition to the threats common to wired LANs, there are aspects of WLANs that make them particularly vulnerable. The transport medium (open air) is harder to lock down than a physical wire. The end devices, by their very nature, are mobile and often connect to foreign networks. VoWiFi implementations add their own complications to the security strategy. Security measures, such as encryption, can add computing requirements causing delay (which is not well tolerated for voice applications) and increase the drain on the battery-powered handsets.

Wired Equivalent Privacy (WEP), the security protocol defined in 802.11b, is widely used today to provide encryption over the air. However, it is also widely accepted that WEP is not as secure as once believed. The 802.11i task force is currently working on a stronger encryption standard, which will hopefully be ratified later in the year. In the meantime, the industry is adopting an interim version of the standard. WiFi Protected Access (WPA), proposed jointly but the IEEE and WiFi Alliance, provides increased security by adding user authentication and stronger encryption.

WPA provides encryption via Temporal Key Integrity Protocol (TKIP), which uses dynamic instead of static keys. WLAN devices can upgrade to support WPA via software or firmware updates. However, the full version of 802.11i will include support for even stronger encryption using Advanced Encryption Standard (AES), which will require a hardware upgrade. 802.11i will also add pre-authentication, which will be needed to prevent signal latency and the dropping of voice content when roaming.

There are many alternatives to choose from when selecting a handset for use with VoWiFi. Options range from softphones installed on laptops to PDAs enabled for voice to wireless handhelds. Several vendors manufacture VoWiFi handsets, and with the addition of a gateway, some handsets will even integrate with existing legacy PBX installations. Depending on the vendor, the supported protocols may include H.323, SIP, or some proprietary variation to work with the most popular PBXs on the market. To date, VoWiFi handsets have been costly, ranging from around $250 at the low end to over $600 at the high end. As with any technology, those prices are expected to drop as the technology gains wider adoption.

There is also another kind of VoWiFi handset coming out soon. Several manufacturers including Nokia, Motorola and NEC have announced dual-mode cellular phones that will support seamless roaming from WiFi to cellular networks. The service also requires a gateway that sits in the core of the cellular carrier’s network, managing subscriber access and handoff between networks. There are many issues carriers need to sort through before the service would be viable, such as what the business model is and how billing will be done. Some industry watchers are predicting that carriers won’t warm up to the idea since roaming to WiFi essentially takes the subscriber off the cellular to the free or lower-cost VoIP network. However, carriers themselves are talking about it, saying they will offer the service as a way to stay competitive.

Broad coverage is a key to making VoWiFi a truly viable application. Fortunately, in addition to cellular-to-WiFi roaming, there are other up-and-coming technologies that can be used to extend the reach of the wireless network. Meshed networks turn hot spots or hot zones into hot regions by passing traffic between access points. One application of mesh networking is to allow users to roam between different private or public hot spots; however, mesh networking is also a great way to backhaul traffic from the wireless to wired network over large campuses without requiring wired connectivity at every access point. Thus, mesh networks allow the installation of access points where it is either impossible or uneconomical to install a wired access point. Many of the mesh solutions either currently or soon to be available provide auto-discovery of new nodes, auto-configuration, and inherent redundancy through multiple network paths.

There have already been a number of well publicized VoWiFi deployments in industries such as healthcare, manufacturing, and education. Those industries have been early adopters of the technology because it solves some of the unique challenges they face such as the need for reliable mobile communication over large campus areas. Now, with standards maturing and handset prices dropping, VoWiFi is poised to move from early adopter stage to mass market adoption by enterprises of all shapes and sizes.

Tracy Venters is vice president of marketing at tekVizion PVS, Inc. For more information, please visit the company online at www.

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