Based on their experience with the wired network, users' expectations are very high: calls without excessive noise, momentary dropouts or disconnects in mid-call.
Meeting call quality goals requires maintaining a consistently strong signal throughout your facility. The first step is a comprehensive analysis of your facility's radio frequency (RF) environment, which requires:
Any wireless deployment should include an RF analysis, but a VoWLAN deployment also requires the analysis to cover areas of the building that may not have been considered in an earlier wireless deployment (e.g., stairwells or even restrooms), because voice users expect to be able to make or continue calls from anywhere in the building.
"Implementing Wi-Fi for VoIP today demands a level of engineering expertise that goes well beyond what's needed for email and Web surfing," said Joe Bardwell, chief scientist at Connect802 Corp. of San Ramon, California. "The characteristics of a voice call make it necessary to focus on a careful RF analysis of the entire facility and on manufacturers' key specifications for call quality, rather than simply focusing on 802.11 data rates the way one would for simple Internet connectivity."
RF analysis for any wireless deployment requires use of a spectrum analyzer to measure signal strength. Analysis for VoWLAN should also include measurement of R-value, a measure of call quality developed by the International Telecommunications Union (ITU). Specialized tools to measure and display R-value are available from vendors such as AirMagnet and Veriwave.
End-to-end delay, jitter and codec characteristics that are the result of overall network design and the choice of wireless equipment and phone handsets.
Review of network security
Security is a vital concern in any wireless deployment, but VoWLAN creates new vulnerabilities. Many currently available handsets support only the WEP security standard, which has proven to be insufficient protection against intruders. Intruders may crack a WEP password simply to make free phone calls at your expense, but the goal may also be to access your corporate data.
How can you address network security in planning? You may need to add new tests to your ongoing set of penetration tests to make sure your network is not vulnerable to this added threat. For the future, look to the WPA2 standard to provide a much higher level of protection. Handsets supporting WPA2 are just now becoming available.
Investigation of the wireless vendor's voice experience and product features
Key considerations in vendor selection should include:
Meeting latency and jitter requirements involves prioritizing voice traffic above data. IEEE standard 802.11e, finalized in late 2005, adds QoS features to the previous 802.11 standard. Some vendors have now implemented the standard, but others continue to use nonstandard methods that may be equally effective. If possible, test your vendor's performance under realistic conditions. If running your own tests isn't possible, review tests done by a third party.
Access points (APs) can maintain acceptable call quality only by limiting the number of simultaneous calls. From your vendor, find out the following and compare with your expected usage:
Some vendors have added security features intended to address vulnerabilities that come into play with voice. For example, some wireless controllers contain a firewall that identifies a voice call and protects against any attempt to access resources that are not appropriate for a phone call. Find out how your vendor approaches this problem.
Some architectures require re-authentication every time a caller roams from AP to AP, which may result in an unacceptably long gap in the conversation. Other products perform a single authentication in a central controller and exhibit no delay when roaming. Test your vendor's product to determine how it handles roaming.
Adding voice to an existing wireless network may seem easy, but without a careful analysis of your RF environment, network security and your vendor's product capabilities, the result may be unhappy users and possibly a network break-in.
About the author:
David B. Jacobs has more than 20 years of networking industry experience. He has managed leading-edge software development projects and consulted to Fortune 500 companies as well as software startups.
