Voice over Wireless LAN
(VoWLAN or Wireless VoIP)
What is voice over wireless LAN?
Voice over wireless LAN (also known as VoWLAN and Wireless VoIP) is the use of voice over IP applications over an IEEE 802.11 network. It has emerged as a leading new application that is being deployed over corporate wireless LANs.
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Can a WLAN network handle VoIP and non-voice data traffic simultaneously?
Yes. A properly designed WLAN can handle traditional data traffic, such as e-mail and web browsing, simultaneously with media applications such as voice and video. This gives rise to a number of applications which can result in substantial cost savings to corporate users. An example is the use of Voice over IP in WLAN, allowing the corporation to utilize its existing data infrastructure to carry voice calls between campuses, eliminating much of the cost associated with the traditional phone service.
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What impairments affect the quality of voice traffic in WLAN?
Voice traffic in a WLAN suffers from a number of unique impairments not common to non-IP networks, as VoIP frames have to traverse an inherently unreliable medium. Due to network congestion, some frames may be dropped. While non real-time traffic (such as e-mail) may recover from such a loss using frame re-transmission, the additional delay caused by re-transmission will in most cases make the conversation unintelligible or will cause talker overlap (one person interrupts the other's speech because of long delay). Even if frames are not lost, additional delay is imposed by IP networks. In most implementations, voice samples are streamed across an IP network using the Real Time Transport Protocol (RTP). This protocol accumulates voice samples for up to 30 ms to reduce the amount of overhead transmitted with the voice stream. In addition, as every voice packet may experience different delay due ever changing network loading, as well as due to the fact that successive packets may not even follow the same route from source to destination, jitter may occur. Jitter causes gaps between successive frames, which may result in “broken” conversations.
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How is voice quality guaranteed in a mixed WLAN network?
To guarantee voice quality in a mixed (traditional data and voice) WLAN, the two different types of traffic need to be handled, or prioritized, differently. As the WLAN becomes congested, various network components (APs and routers) need to assign unique priority schemes to these services and essentially "starve" the data traffic from network resources, to ensure that the voice traffic does not suffer from unacceptable delay and jitter.
To verify traffic prioritization is handled correctly, WaveTest generates streams emulating a variety of network protocols, and measures the one-way latency, frame loss and jitter experienced by each type of traffic.
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What is the impact, on a voice call, of a wireless VoIP client roaming between Access Points during the call?
When a wireless client roams to another Access Point, a process of disassociation, searching for new Access Points and reassociation with a new Access Point takes place. Without security enabled, this process can be fast enough to not interfere with voice call quality. However, security for voice clients is just as important as it is for data clients. With strong security like WPA or WPA2 (IEEE 802.11i) enabled, roam time increases due to the additional steps needed for re-authentication. Roam times over 50 msec will cause noticeable degradation in voice quality. New standards such as 802.11r and 802.11k are in the works to create a fast, secure method of roaming for latency sensitive applications.
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