Virtual queue dropping for robust real-time video over IEEE 802.11aa wireless LANs

Stream classification service (SCS) is a novel concept proposed in the newly released IEEE 802.11aa standard for robust audio and video streaming, particularly for graceful degradation of streaming quality during network congestions. Based on intra-access category prioritisation (IACP), SCS is equipped with a pair of access category (AC) queues to differentiate real-time and non-real-time video streams, namely (AC_VI, AAC_VI), and thus introduces the need for an efficient scheduler design; similarly for audio. In this study, we propose a novel cross-layer design called virtual queue dropping (VQD) for packet scheduling between AC_VI and AAC_VI. Driven by a real-time video-packet-importance scheme, VQD adopts conditional priority weighting to enhance the priority of I-packets from AAC_VI so that they can fairly compete with P-packets from AC_VI. Moreover, to achieve graceful degradation during network congestions, VQD (i.e. based on priority coupling among different component queue lengths within the same physical AC queue) is adopted so that the dropping probabilities of lower-priority video packets are not only larger but also affected-and-increased by those of higher-priority ones. Different limits of priority retires of I- and P-packets can avoid unexpected long delays, and an optimal set of retry limits has been found to minimise the performance impact due to real-time constraints. Our results show that VQD outperforms PWD and SCS-CR in various performance metrics, and achieves a win-win game for the dilemma in maintaining priority for AC_VI and fairness for AAC_VI simultaneously.