ion techniques to improve the compression efficiency [30- by exploiting the redundancy in the signal. In particular, 32], as well as to improve the error-resilient performance CHAPTER 3.6 Real-time communication over networks over lossy networks [33,34]. The goal of the R-D opti- mechanisms to control the rate at which the media are mization algorithms is to minimize the expected distor- transmitted. This helps avoid a potential penalty on tion due to both compression and channel losses subject overuse of bandwidth, which usually leads to quality to the bit-rate constraint. degradation and even service interruption. Typical One example of this area is Intra/Inter-mode bandwidth adaptation techniques include rate control, switching [35-38], where Intra-coded macroblocks are transcoding, scalable coding, and bit stream switching. updated according to the network condition to mitigate temporal error propagation. Another approach is 3.6.3.4 Audio-video synchronization to modify the temporal prediction dependency of motion-compensated video coding in order to mitigate or RTP timestamps from different media streams may ad- stop error propagation. Example implementations in- vance at different rates and usually have independent and clude reference picture selection [27,39-41] and random offsets. Therefore, although these timestamps NEWPRED in MPEG-4 [42,43], where channel feed- are sufficient to reconstruct the timing of a single stream, back is used to efficiently stop error propagation due to directly comparing RTP timestamps from different any transmission error. Another example is video re- media is not effective for synchronization. Instead, for dundancy coding (VRC), where the video sequence is each medium the RTP timestamp is related to the sam- coded into independent threads (streams) in a round- pling instant by pairing it with a timestamp from a ref- robin fashion [27,44]. A Sync-frame is encoded by all erence clock (wallclock) that represents the time when threads at regular intervals to start a new thread se