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Contention-based wireless networks make use of random access delay in the medium access control (MAC) layer to share the channel efficiently among users. This channel access scheme poses challenges to video traffic which is delay-sensitive. When channel access is limited, how to optimally allocate channel access and judiciously drop video packets remains an open issue. In this paper, we investigate the optimal channel access allocation and preemptive packet drop strategy for scalable video packets using contention-based MAC. We first establish a relationship to associate the characteristics of video packets, the cumulative distribution function of channel access delays and the expected video quality at the receiver. Based on the relationship, we define a video utility function that explicitly takes into account cumulative distribution of channel access delay over contention-based wireless links, video packet length and truncation, packet loss rate and quality impacts of SVC layers. As the analytical model of the video utility function is rather complex for optimization, we further propose an approximation model for the video utility function to transform the optimization problem into a convex problem which is a solvable by using the dual decomposition method. Simulation results confirm that our proposed method achieves better PSNR performance over the existing approach, especially when the total channel access for the video traffic is limited.