Voice over Wireless LANs

By Jim Geier

March 28, 2003

The ability to supporting voice communications such as telephone traffic over a wireless LAN can provide significant benefits. Learn why and how you should go about implementing voice over wireless LANs.

When most people think of wireless LANs (WLANs), they generally only consider transferring data while using applications such as a Web browser, e-mail client, for file transfer, etc. It's possible, however, to use a WLAN as the transport system for carrying telephone traffic from mobile users as well.

Applications and Benefits

A significant benefit of mixing telephone traffic with data on a WLAN is to provide mobility and make use of a common infrastructure. The support of a common system for both data and voice traffic is generally simpler and less expensive than two separate entities.

For example, many warehouses utilize two-way radios operating in licensed frequencies to provide communications among clerks and supervisors. Mobile communication is needed in this environment due to nomadic users and the large size of the facilities. In many cases, warehouses utilize WLANs for providing wireless data collection in inventory and shipping/receiving applications. By using mobile phones designed to interface with the WLAN, a warehouse can avoid the need for two-way radios and the corresponding operational management functions.

The use of WLAN phones within the offices of an enterprise is compelling as well. In this case, a company can avoid the need to wire and rewire telephone outlets as the company size shifts. In fact, users could utilize an 802.11 phone to make very inexpensive long distant phone calls through the Internet.

Key Components

In addition to a WLAN backbone consisting of access points and a distribution system, a key component for implementing voice over WLANs is a telephone equipped with an 802.11 radio. The primary players in this market are Symbol and Spectralink, which have partnered with private branch exchange (PBX) vendors to sell their products. These phones, however, are somewhat pricey at about $500 each.

A company called Telesym has software that turn a PocketPC-based personal digital assistant (PDA) into a telephone. This can be a less expensive alternative if you already have PDAs on site. In addition, you only need to carry one device for multiple purposes.

Most voice over WLAN systems also requires a gateway or access point enhanced to handled to handle special bandwidth control requirements of voice traffic. For example, SpectraLink's gateway allows the phones to work with Cisco's Call Manager IP phone. The gateways will add cost to the system if you plan to effectively support voice traffic.

Despite these costs, however, an 802.11-based phone system is generally less expensive to install and support than a wired system. Of course you'll need to pay specific attention to the potential problems from WLANs, such as radio frequency (RF) interference and denial of service attacks.

Performance Requirements

Performance should be your primary consideration when installing a WLAN system that supports voice. 802.11b is only capable of running three uncompressed audio streams smoothly. As a result, it's important that the system compress the audio signals before transmission in order to increase the number of supported audio streams.

Another way to increase the number of audio streams is to utilize a higher performing standard, such as 802.11a for the WLAN backbone. 802.11a has the capacity to handle approximately four times as much voice traffic as 802.11b. The problem with 802.11a, though, is that other non-voice users of the network may only be equipped with 802.11b. As a result, consider installing access points that include both 802.11a (for voice users) and 802.11b (for data users).

A WLAN can support voice if you implement the system with high performance and quality of service (QoS) in mind. The problem is that the 802.11 standard doesn't support QoS yet. The 802.11e group is currently working on a QoS upgrade, but the ratification of the standard is still a ways off. 802.11e will prioritize traffic on the network, making data give way to voice packets.

Until 802.11e is available, WLANs need to deploy proprietary QoS mechanisms to enable effective blending of voice and data. That's a problem when trying to support voice traffic over a public WLAN. It's not practical for public hotspot operators to mandate the use of a particular QoS method because of the corresponding requirement of a common vendor radio card or specific software for each client.

Jim Geier provides independent consulting services to companies developing and deploying wireless network solutions. He is the author of the book, Wireless LANs and offers workshops on deploying wireless LANs.

Join Jim for discussions as he answers questions in the 802.11 Planet Forums.

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