Solutions to Performance Problems in VoIP Over a 802.11 Wireless LAN

Abstract

Voice over Internet Protocol (VoIP) over a wireless local area network (WLAN) is poised to become an important Internet application. However, two major technical problems that stand in the way are: 1) low VoIP capacity in WLAN and 2) unacceptable VoIP performance in the presence of coexisting traffic from other applications. With each VoIP stream typically requiring less than 10 kb/s, an 802.11b WLAN operated at 11 Mb/s could in principle support more than 500 VoIP sessions. In actuality, no more than a few sessions can be supported due to various protocol overheads (for GSM 6.10, it is about 12). This paper proposes and investigates a scheme that can improve the VoIP capacity by close to 100% without changing the standard 802.11 CSMA/CA protocol. In addition, we show that VoIP delay and loss performance in WLAN can be compromised severely in the presence of coexisting transmission-control protocol (TCP) traffic, even when the number of VoIP sessions is limited to half its potential capacity. A touted advantage of VoIP over traditional telephony is that it enables the creation of novel applications that integrate voice with data. The inability of VoIP and TCP traffic to coexist harmoniously over the WLAN poses a severe challenge to this vision. Fortunately, the problem can be largely solved by simple solutions that require only changes to the medium-access control (MAC) protocol at the access point. Specifically, in our proposed solutions, the MAC protocol at the wireless end stations does not need to be modified, making the solutions more readily deployable over the existing network infrastructure. Index Terms-Capacity, IEEE 802.11, quality of service (QoS), voice over Internet Protocol (VoIP), wireless local area network (WLAN). I. INTRODUCTION V OICE OVER Internet Protocol (VoIP) is one of the fastest growing Internet applications today [1]. It has two fundamental benefits compared with voice over traditional telephone networks. First, by exploiting advanced voice-compression techniques and bandwidth sharing in packet-switched networks, VoIP can dramatically improve bandwidth efficiency. Second, it facilitates the creation of new services that combine Manuscript

VoIP over WLAN: What about the Presence of Radio Interference?

“…Moreover, they have to operate in the presence of unintentional spurious signals from electronic devices that either use this band, like cordless phones, microwave ovens, baby monitors, security cameras, or operate in adjacent frequency bands, like a number of wireless appliances whose distribution in modern houses, public and professional contexts is by now widespread. Some authors tried to investigate on the effects of interference on voice quality in a VoWLAN conversation (Wang & Mellor, 2004;Wang & Li, 2005;Garg & Cappes, 2002;El-fishawy et al, 2007;Prasat, 1999;Hiraguri et al, 2002). For instance, in (Wang & Li, 2005) the coexistence of Transmission Control Protocol (TCP) and VoIP traffic in a WLAN has been studied in terms of delays and performance loss.…”

“…Some authors tried to investigate on the effects of interference on voice quality in a VoWLAN conversation (Wang & Mellor, 2004;Wang & Li, 2005;Garg & Cappes, 2002;El-fishawy et al, 2007;Prasat, 1999;Hiraguri et al, 2002). For instance, in (Wang & Li, 2005) the coexistence of Transmission Control Protocol (TCP) and VoIP traffic in a WLAN has been studied in terms of delays and performance loss. In (Garg & Cappes, 2003), experimental studies have been shown on the throughput of IEEE 802.11b wireless networks for user diagram protocol (UDP) and VoIP traffic.…”

On Maximizing VoIP Capacity and Energy Conservation in Multi-Rate WLANs

“…Unfortunately, in practice, the VoIP capacity of WLANs is very low. For example, Wang et al [1] showed that IEEE 802.11b and 802.11g WLANs are only able to support 12 and 60 VoIP sessions respectively. The main reason is the high overheads associated with the transmission of small VoIP packets.…”

“…Besides that, the constant bit rate nature of VoIP sessions exacerbates collisions, which in turn causes the Rate Adaptation Algorithm (RAA) at the sender to transmit at a lower data rate. As a solution, the authors of [1] propose to combine small VoIP packets and multicasts only one aggregated frame to stations, which has the affect of increasing VoIP capacity by 100%. This solution, however, considers all stations to be awake at all times.…”

“…A critical problem that arises when using WMM-PS, and similar PSMs, is that they negate the performance benefits of frame aggregation as stations are required to be awake at all times to receive aggregated packets. Hence, APs are only able to aggregate packets on a per station basis, which means all downlink transmissions will again incur the high cost associated with transmitting small VoIP packets [1]. To this end, in the next section, we propose an algorithm that restores the benefits of packet aggregation.…”