The first barrier is lack of Quality of Service (QoS) support. ITU-T G.114 recommends the the maximum one-way voice packet delay be below 150 msec. Because 150 msec is the end-to-end path delay budget, it means the wireless LAN channel access delay must be considerably less than 150 msec. In addition, most voice codec specifications such as G.729 require the packet loss ratio in voice connections to be less than 1% to avoid audible errors. The original IEEE 802.11 WLAN standard only supports a best-effort service model. Almost all existing WLAN deployments operate in the DCF (Distributed Coordination Function) mode, in which each wireless station accesses the shared radio channel using an Ethernet-like medium access mechanism, which is inadequate for real-time voice applications for the following reasons. First, it is impossible to have precise control over the exact transmission timings of voice frames because of collision and random back-off. When the injected traffic load is high, there is more collision and interference, which increases packet loss rate, packet delay and packet delay jitter. Second, it is impossible to prioritize all traffic flows. Consequently, large-volume non-real-time data traffic such as FTP may consume a large proportion of the network capacity, leaving time-sensitive voice traffic to suffer the consequences.
The second technical barrier associated with IEEE 802.11 WLAN is substantial per-packet transmission overhead. Because the maximum transmission rate of an IEEE 802.11b WLAN link is 11 Mbps, in theory an IEEE 802.11b link should be able to support hundreds of VoIP connections if each of them requires only 8Kbps as is the case with G.729. In practice, several benchmarking tests reported that existing VoWLAN products on the market cannot support more than ten concurrent VoIP calls with comparable quality to toll calls over a single IEEE 802.11b channel. A major cause for this gap between theory and practice is the considerable overhead associated with a WLAN packet's transmission. This overhead includes per-packet header bits, link-layer acknowledgment, back-off delay to avoid contention, retransmission due to channel interference and inter-frame spacing for synchronization.