Video multicast over wireless local area networks

Video multicast services over wireless media are expected to grow in importance over the next few years. Multicasting over wireless networks is complicated by the fact that wireless links are error-prone and time varying. In multicast scenarios using the 802.11 wireless LAN protocol, multiple receivers experience widely varying channel conditions and the link layer protocol does not retransmit erroneous or lost packets, potentially resulting in poor video quality. Therefore, it is a key requirement to support quality of service for all the receivers of the multicast video in the desired coverage area while efficiently utilizing the available wireless LAN resources. In this thesis, we investigate some of the aspects of reliable video multicast over WLANs. We design, implement and evaluate multi-group hybrid ARQ (MHARQ), a new and improved adaptive system for reliable video multicast. MHARQ combines the advantages of receiver-driven staggered FEC and hybrid ARQ schemes to compensate the large dynamic range of WLAN channels and to achieve high reliability, scalability and wireless bandwidth efficiency for video multicast. The FEC packets generated by a cross-packet FEC code are divided into multiple streams according to the pre-configured overhead and are transmitted in different multiple IP multicast groups. Certain FEC streams are delayed from the original video stream. The receivers dynamically join/leave the FEC multicast groups based on the channel conditions. For efficient utilization of WLAN bandwidth, FEC data for a multicast group would not be transmitted by the APs in wireless networks if no receiver joins this group. The time shift between the video stream and the FEC streams introduces temporal diversity and compensates for the client join delay and handoff interruption. In addition, when delayed FEC packets are not enough to recover the lost packets, the receivers can send a hybrid ARQ request to the video server. We design a channel estimation algorithm for a receiver to dynamically determine the delayed FEC multicast groups to join and/or send ARQ NACK to request for retransmission. Using the ORBIT radio grid testbed, we have investigated the performance of the proposed MHARQ system with various numbers of users per AP and different numbers of APs per video server. It is demonstrated via real system implementation on ORBIT that MHARQ improves wireless bandwidth efficiency and scalability for reliable video multicast, compared with existing reliable multicast schemes. The experience and insight obtained from implementation are discussed as well.M.S.Includes bibliographical references (p. 55-56)