Voice Over WLAN Best Practices

VoWLAN deployments can be challenging - VoIP already imposses strict criteria on the network conditions to facilite good call quality & WiFi itself poses additional challenges. Given that a wireless channel is a shared medium, potentially subject to interference from other devices & that the transmit time on a WiFi device is highly variable due to the nature of CSMA/CA, this is a recipe for jitter & packet loss.
Cisco publish a number of guidelines for VoWLAN success, albeit spread across multiple documents, so below is a summary of some of the main points:

• Maximum of 15 or 20 associated devices per AP.
• 5GHz is strongly preferred.
• Noise levels should not exceed -92 dBm with a signal-to-noise ratio (SNR) of 25 dB or higher.
• Signal strength should be -67 dBm or better per AP.
• Minimum 20 to 30 percent overlap of adjacent access points with non-overlapping channels must be considered during design site survey.
• Packet error rate (PER) should not exceed 1%, jitter should be <100 ms & retries should be < 20%.
• To avoid one-way audio issues resulting from different power settings between Wi-Fi IP phones & access points, World mode (IEEE 802.11d) should be configured.
• Traffic Specification (TSPEC) must be enabled for CAC on APs & Platinum QoS for the VoWLAN SSID.
• Channel utilization levels should be kept below 50 percent.
• Cisco Compatible Extensions (CCX) should be enabled on wireless infrastructure, where possible.
• Set the Beacon interval to 100 ms.
• A DTIM of 2 is recommended where possible to save battery life on the IP phones.
• WPA2/AES Enterprise with CCKM or 802.11r is recommended for 792x phones to avoid the need for a complete 802.1x re-authentication when roaming.

Some further useful Cisco documentation can be found in the Enterprise Mobility 8.1 Design Guide, Voice Over Wireless LAN (VoWLAN) Troubleshooting Checklist & the excellent Cisco Live presentation Voice over WiFi - Deployment Recommendations and Best Practices (BRKEWN-2000).