Performance Analysis of VoIP over WLAN

Recent disasters such as the Indian Ocean tsunami in 2004, or the 2005 hurricanes Katrina and Rita in U.S.A. have shown the importance of emergency communication systems. This is true both in the preceding phase of the events, to issue warnings and evacuation instructions, as well as during and after catastrophes happen, to coordinate the activity of rescue teams. Dependable communication is also essential in mission-critical and safety-critical systems, or even in normal business environments that require “anytime, anywhere” access to network resources. Unfortunately these recent events have also demonstrated that current communication systems fail too easily under emergency conditions. An alternative is to make a more extensive use of wireless networks (WLANs). Using converged WLANs one may transmit both audio and video information, so that rescuers can communicate with each other and with remote experts, as well as receive data, such as street maps or building floor plans – all crucial for saving lives and preventing losses. Wireless LANs are more stable than other communication infrastructures, as they are decentralized. Since their potential failure is independent, ad-hocWLANs can continue functioning in emergency conditions. The nodes of WLANs are cheap and require little power. Moreover, the potential of using advanced features on WLANs that are not available in traditional communication systems makes it possible to provide probabilistic guarantees of service for emergency responders. Using a priority-enforcement system such users could be given an assured service level, independently of the activity of regular users. In this paper we present our approach to investigating the possibility of using real-time applications, such as Voice over IP (VoIP), on WLANs in a dependable manner. According to the survey we did in [1], this is difficult at the moment for several reasons: i. WLAN QoS parameters (bandwidth, packet loss, delay & jitter) have a high variability in real-world environments, and this has a significant effect on application performance; ii. Existing WLAN QoS mechanisms are only of limited use for managing contention when applications with different QoS requirements, such as VoIP calls and TCP-based data traffic, share the same communication channel; iii. VoIP is a multimedia application that requires timely servicing of the voice traffic; this is a challenging task in WLANs, even when using QoS enforcement, since most currently-implemented QoS mechanisms focus on bandwidth provisioning; iv. Roaming between access points, a typical WLAN event, introduces communication gaps that may even be of the order of seconds, an unacceptable situation for real-time applications. We started analysing application performance over WLAN, and VoIP in particular, using StarBED [2], the large scale network experiment environment of Hokuriku Research Center in Ishikawa, Japan. StarBED is a cluster-based testbed currently employing about 700 PCs. The custom-designed configuration language SpringOS makes it possible to define complex experiments on StarBED in a straightforward manner. The paper is structured as follows. First we present the analysis methodology that we propose for the study of application performance over wireless LANs, and emphasize the specific issues related to VoIP. Then we discuss the use of WLAN emulation, which is a key element of our approach as a complement to real-world tests. Following that we show some illustrative results for our study of VoIP performance on WLAN. The paper ends with a section of conclusions and future work.