Softspeak: Making VoIP Play Well in Existing 802.11 Deployments

Voice over IP (VoIP) in 802.11 wireless networks (WiFi) is an attractive alternative to cellular wireless telephony. Unfortunately, VoIP traffic is well known to make inefficient use of such networks. Indeed, we demonstrate that increasing handset deployment has the potential to cripple existing hotspot and enterprise WiFi networks. Our experiments show that VoIP halves the available TCP capacity of an 802.11b hotspot when six to eight VoIP stations share the medium, and effectively extinguishes TCP connectivity when ten VoIP stations are present. Further, we show that neither the higher data rates of 802.11a/g nor the 802.11 standard for quality of service, 802.11e, fully ameliorate the problem. Instead, the problem is rooted in WiFi’s contentionbased medium-access control mechanism and considerable framing overhead. To remedy this problem, we propose Softspeak, a pair of backwards-compatible software extensions that enables VoIP traffic to share the channel in a more efficient, TDMA-like manner. Softspeak does not require any modifications to the WiFi protocols and significantly reduces the impact of VoIP on TCP capacity while simultaneously improving key VoIP call-quality metrics. Results show improvements in TCP download capacity of 380% for 802.11b and 25-200% for 802.11g.