We all know the situation: One moment we are talking to someone on the phone and, the next, the connection cuts out, repeatedly. All you can hear is garbled nonsense, so you hang up in frustration. This will keep happening as long as the telephone or internet connection is poor or unstable. So how can we ensure a good voice quality when using Voice-over-IP (VoIP) telephony, and what factors influence that quality? These questions are of the utmost importance particularly for environments where Voice is transmitted over a wireless network. VoIP telephony is a real-time application that is highly sensitive to delay, jitter or even the loss of data (packets). In a wireless network, all of this can be caused by weak signal strength, range limitations, reflections, or interference from other signals on the same frequency. However, you can be sure of worry-free wireless IP-telephony at home or at your business if you take some basic principles into account.
Planning your network for VoWLAN
A wireless network will only support Voice-over-WLAN (VoWLAN) if it is optimized for voice transmission. Users expect perfect stability from their telephone connection, far more than they do with a conventional Wi-Fi data deployment.
First things first: To establish a functional and stable VoWLAN infrastructure, a considerable amount of effort needs to go into the planning of the Wi-Fi network itself. The most important factor influencing the quality of any VoWLAN is the stability and quality of the network it runs on. Prior to any deployment of wireless networking equipment, an extensive site survey including a thorough radio frequency analysis should be conducted. For example, a spectral analysis of the radio frequencies will help technicians to detect interference and its sources and plan the layout of the network accordingly.
Specialized test equipment and the knowledge of wireless experts help to calculate the optimum wireless coverage. This can be quite a demanding task in high-density areas with large numbers of clients such as, for example, in office buildings or where there are multiple radio signals in places like hospitals.
The second step is to choose the right networking equipment.
The safest choice is dual-radio access points that allow parallel use of the 2.4- and the less frequented 5-GHz band. This enables administrators to separate data traffic from voice traffic by using different bands and, therefore, to keep interference with other Wi-Fi signals to a minimum. If this is not a possibility, for example because existing Wi-Fi networks are based on single-radio access points, there are still a number of options for improving VoWLAN transmission. One example would be to use separate SSIDs and VLANs for the different types of data. Whatever you choose to do, it is important that the VoWLAN hardware (telephones, headsets) support the operated standard and the chosen frequency band.
Quality of Service – It is all about Prioritization
Quality of Service (QoS) is an option in your network configuration which guarantees that certain data packets are prioritized over others and that the bandwidth is shared between the applications accordingly. Products supporting Quality of Service (QoS) are identified by a special Wi-Fi alliance certification.
The standard known as 802.11e refers to Wi-Fi Multimedia or WMM for short (formerly known as Wireless Multimedia Extension, WME). WMM defines four categories—voice, video, best effort and background—and these make up separate queues for prioritization.
The 802.11e standard sets a priority for each IP packet and aims to keep delay times (jitter) below 2 milliseconds, a magnitude that is just inaudible to the human ear. Consequently, the extension of the 802.11 standard to include 802.11e introduces Quality of Service for data transfers over Wi-Fi. 802.11e supports the prioritization of certain data-packet types, which is an important basis for the use of voice applications in wireless LAN networks.
QoS is absolutely essential to achieve optimum quality for voice over WLAN. However, it is not only the network components that need to support QoS; the software solution also has to offer tools and features for optimization.
Usually the network configuration menus feature certain functions for improving Voice-over-WLAN transmission. Legacy network equipment, however, often lacks the features supporting voice data traffic altogether. Most professional hardware vendors have certified their products according to the Wi-Fi Alliance 802.11e standard and developed special software solutions or features within their operating systems especially tailored to enhance voice transmission.
If you want to find out more about how to optimize your wireless network, please take a look at our blog post about our LANCOM Active Radio Control features. http://lancomwire.com/how-to-optimize-your-wi-fi-network/
The MOS value
So how does one determine the quality of a VoWLAN connection? The standard test characterizing the voice quality is known as the Mean Opinion Score (MOS) invented by Bell technologies as an arithmetic mean value of individual measurements and appraisals of several standardized tests. Entering the calculation are the one-way delay or the latency of the connection, packet loss with a metric to include the number of consecutive packets lost, and the amount of jitter (difference in time it takes packets to arrive).
The results of a series of tests are ordered according to a five-level quality scale from 1 (worst value) to 5 (theoretical best value). Formerly the test score comprised of subjective judgments by users who were asked about their perceived voice quality, but nowadays there are special software tools for testing which ensure a certain standard and comparability.
Whoever wants to use VoWLAN for communication should choose hardware which has a MOS score of 4 or higher. Scores below 4 are regarded as “slightly annoying”, meaning that voice quality cannot be guaranteed to reach a sufficient level.
Why this all matters
Wherever VoWLAN solutions are operated, the quality of the calls can be crucial. For example, many hospitals nowadays use voice-over-WLAN communication via handhelds or headsets for flexibility but also in order to reduce the radiation from old DECT telephones or mobile phones. Another use case would be the emergency communication systems inside cable cars or elevators. In both cases a good voice quality is highly important and can be critical in case of an emergency. In these sensitive environments, only professional, certified wireless equipment should be used.
PS: In acknowledgement of the increasing importance of Voice over WLAN, we at LANCOM have invested considerable effort to achieve maximum voice quality for connections established over our wireless network components. You can get am impression of this from the MOS scores achieved by selected LANCOM access points in the corresponding PDF: https://www.lancom.de/en/publications/