Best Practices for Voice over Wi-Fi – Part 1
Voice over Wireless LAN (VoWLAN), also known as “Voice over Wi-Fi” (VoWiFi), delivers the capabilities and functionality of an enterprise telephone system in a Wi-Fi handset. The handset is a WLAN client device, sharing the same wireless network as laptops and other handheld devices.
A Wi-Fi handset requires a continuous, reliable connection as a user moves throughout the coverage area. In addition, voice applications have a low tolerance for network errors and delays. Using a Wi-Fi network for voice can be complex, but there are ways to mitigate complexity with some basic considerations.
For example, Polycom’s VoWLAN certification program is designed to ensure interoperability and maximum performance for enterprise-grade Wi-Fi infrastructure products that support SpectraLink handsets. Full Access Point diversity is critical for improved communication between the wireless handset and AP.
This configuration, using both AP antennas, helps provide low retry rates and improves voice quality.
For each certified product, Polycom provides a VoWLAN Configuration Guide that details the tested hardware models and software versions; radio modes and expected calls per AP; and specific AP configuration steps. VoWLAN Configuration Guides are available on the Polycom website and should be followed closely to ensure a successful deployment.
Based on our experience with enterprise-grade deployments, the following guide provides recommendations for ensuring that a network environment is optimized for use with Polycom SpectraLink 8400 Series Wireless Telephones.
Wireless LAN Considerations
VoWLAN handsets like the SpectraLink 8400 Series handsets utilize a Wi-Fi network consisting of access points (APs) distributed throughout a building or campus. The required number and placement of APs in a given environment is driven by multiple factors, including intended coverage area, system capacity, power output, physical environment, and radio types.
One of the most critical considerations in deployment of SpectraLink handsets is to ensure sufficient wireless signaling coverage. Enterprise Wi-Fi networks are often initially laid out for data applications and may not provide adequate coverage for voice users. Such networks may be designed to only cover areas where data devices are commonly used, and may not include coverage in other areas such as stairwells, break rooms or building entrances – all places where telephone conversations are likely to occur. It is important to consider coverage requirements in areas where a voice conversation may not be as common, such as restrooms and stairways, stairwells & parking areas, for the purpose of emergency planning.
The overall quality of coverage is more important for telephony applications. Coverage that may be suitable for data applications may not be seamless enough to support the requirements of VoWLAN. Most data communication protocols provide a mechanism for retransmission of lost or corrupted packets. Delays caused by retransmissions are not harmful, or even discernable, for most data applications. However, the real-time nature of a full-duplex telephone conversation requires that voice packets be received correctly within tens of milliseconds of their transmission. There is little time for retransmission, and lost or corrupted packets must be discarded after limited retries. In areas of poor wireless coverage, the performance of data applications may be acceptable due to retransmission of data packets, but for real-time voice the audio quality will suffer.
Another factor to consider when determining the coverage area is the device usage. Wireless telephones are used differently than wireless data devices. Handset users tend to walk as they talk, while data users are usually stationary or periodically nomadic. Wireless voice requires full mobility while data generally requires simple portability. Wireless handsets are typically held close to the user’s body, introducing additional radio signal attenuation. Data devices are usually set on a surface or held away from the body. The usage factor may result in reduced range for a wireless telephone as compared with a data device. Therefore, the WLAN layout should account for some reduction of radio signal propagation.
We will continue our discussion with Roaming Coverage in part 2 of this series.