Problem: Wireless LAN problems
Wireless LANs can introduce several types of problem to Voice over WLAN/WiFi calls (VoWLAN, VoWiFi). Problems may be due to handoffs between Access Points that introduce gaps in the speech path, areas of low signal strength that result in high rates of jitter or packet loss, high rates of jitter or delay due to congestion.
The link layer of a Wireless LAN uses retransmission to increase the reliability of the link. As the delay on a wireless link is very short these retransmissions are generally not noticeable although they can reduce effective throughput. Under conditions of low signal strength this results in increased retransmission and hence an increase in jitter. Delay spikes can also occur during changes in transmission rate.
Impact
High rates of packet loss or high levels of jitter resulting from congestion (which causes excessive numbers of packets to be discarded by the receiving Voice over IP end system's jitter buffer) lead to degraded voice quality. Gaps in speech due to handoffs between Access Points will cause occasional gaps in speech or transient call quality problems.
Resolution
Emerging IEEE802.11 standards address various aspects of WLAN performance:
(i) Higher speed LANs (IEEE802.11a, g, and later n) provide more bandwidth and capacity
(ii) QoS support (IEEE802.11e) gives voice traffic priority and provides bandwith allocation
(iii) Fast handoffs (IEEE802.11r) minimise the size of gaps
Tools: VQmon
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