An emerging killer application for enterprise wireless LANs (WLANs) is voice over IP (VoIP) telephony, which
promises to greatly improve the reachability and mobility of enterprise telephony service at low cost. None
of the commercial IEEE 802.11 WLAN-based VoIP products can support more than ten G.729-quality voice
conversations over a single IEEE 802.11b channel on real-world WLANs, even though the physical transmission
rate is more than two orders of magnitude higher than an individual VoIP connection's bandwidth requirement.
There are two main reasons why these VoIP systems' effective throughput is significantly lower than expected:
VoIP's stringent latency requirement and substantial per-WLAN-packet overhead. Time-Division Multiple Access
(TDMA) is a well-known technique that provides per-connection QoS guarantee as well as improves the
radio channel utilization efficiency. This paper compares the effective throughput of IEEE 802.11, IEEE 802.11e
and a software-based TDMA (STDMA) protocol that is specifically designed to support WLAN-based VoIP applications,
on the same commodity IEEE 802.11 WLAN hardware. Empirical measurements from a VOIP over
WLAN testbed show that the numbers of G.729-quality voice conversations that IEEE 802.11, IEEE 802.11e
and STDMA can support over a single IEEE 802.11b channel are 18, 22 and 50, respectively.