With the emergence of high-speed wireless cellular networks, such as High Speed Downlink Packet Access (HSDPA) or WiMAX, new services with a high bandwidth demand have been introduced. One example is non-interactive video streaming. In contrast to wireline networks, the characteristics of wireless links, such as a time-varying bandwidth and the trade-off between delay and reliability, impose problems to a streamed video. In this paper, we study the impact of delay and losses within a Radio Access Network (RAN) on the video quality at the example of a state-of-the art HSDPA network. The nature of transmission errors is discussed. We investigate how active buffer management strategies within the RAN can alleviate the impact of an unreliable and time-varying link on the video quality, and we discuss the advantages and disadvantages of several proactive and reactive approaches. We further propose a proactive buffer management scheme with data differentiation which significantly increases the video quality by taking into account MPEG frame dependencies. Finally, we show that the structure of the MPEG-4 video data affect the performance of proactive buffer management schemes with data differentiation.