Wireless Netw (2006) 12:511–522 DOI 10.1007/s11276-006-6549-7 An adaptive delay and synchronization control scheme for Wi-Fi based audio/video conferencing Haining Liu · Magda El Zarki Published online: 8 May 2006 C  Springer Science + Business Media, LLC 2006 Abstract The prevalence of the IEEE 802.11b technology has made Wi-Fi based Audio/Video (AV) conferencing ap- plications a viable service. However, due to the “best-effort” transport service and other unpredictable factors such as user mobility, location and background traffic, the transport chan- nel behavior often fluctuates drastically. It thus becomes rather difficult to configure an appropriate de-jitter buffer to maintain the temporal fidelity of the AV presentation. We propose in this paper an adaptive delay and synchroniza- tion control scheme for AV conferencing applications over campus-wide WLANs. Making use of a distributed timing mechanism, the scheme monitors the synchronization er- rors and estimates the delay jitters among adjacent Media Data Units (MDUs) in real-time. It piece-wisely controls the equalization delay to compensate for the delay jitters experi- enced by MDUs in a closed-loop manner. We investigate the performance of the proposed scheme through trace-driven simulations. We collected network traces from a production campus-wide IEEE 802.11b WLAN by emulating real con- ferencing sessions. Simulation results show that the scheme is capable of dynamically balancing between synchroniza- tion requirements and latency requirements in all scenarios. Small synchronization phase distortions, low MDU loss per- centages and low average end-to-end delay can be achieved simultaneously. In particular, compared with solutions us- ing a static setting, the proposed scheme is able to achieve a reduction of around 100ms in end-to-end delay with the same amount of MDU losses under some media-unfriendly situations. H. Liu () · M. E. Zarki Donald Bren School of Information and Computer Sciences University of California, Irvine, CA 92697 e-mail: {haining, magda}@ics.uci.edu Keywords Wi-Fi · AV conferencing · Adaptive delay · Synchronization control 1. Introduction The tremendous success of the IEEE 802.11b technology and the popularity of portable computing units (e.g., PDAs) have made Wi-Fi based real-time multimedia applications a real- ity. For example, with Wi-Fi based campus-wide broadband wireless coverage, end users can enjoy on-demand stream- ing of audio or video, or even engage in mobile AV con- ferencing using PDAs on the go within the covered area. However, due to the lack of QoS support from the network infrastructure, end users often suffer from poor quality assur- ance. A typical packet-based AV conferencing application works as follows. Audio and video signals are periodically captured at the sender side, fragmented into MDUs, encap- sulated and transported across the network in succession to the receiver. Received MDUs are then decoded and rendered to the end users in real-time. As such, different from data applications that usually only concern data fidelity, AV con- ferencing applications also require the preservation of the temporal fidelity, which calls for appropriate synchroniza- tion control to restore the original temporal orderings among the transmitted MDUs, and the stringent control of the la- tency between the two communications parties to ensure acceptable user interactivity. Without guaranteed transport performance, currently the only viable solution to the syn- chronization control is to employ a receiving buffer at the receiver end to equalize the variable network delay that each MDU experienced. Since the bandwidth consumption of an AV conferencing session is much smaller than the nomi- nal bandwidth provided by IEEE 802.11b technology (a few hundred kbps vs. up to 7.7 Mbps), one may argue that a Springer