ice versa, it is difficult to recovery. Techniques to recover the damaged areas based decrease the buffering delay without significantly in- on characteristics of image and video signals have been creasing the loss rate. Therefore, packet loss in delay- reviewed in [25]. sensitive, real-time applications is a result of not only a packet being dropped over the network, but also delay 3.6.3.2.2 Active techniques jitter, which impairs communication quality greatly. Due to the aforementioned buffering delay-late loss A different category of error-resilience techniques re- rate trade-off-it is desirable to design smart playout quires the encoder to play a primary role. They are able scheduling mechanisms to reduce the buffering delay. to provide even higher robustness for media communi- Fixed scheduling poses a limitation for this trade-off. In cation over best-effort networks. We refer to these real-time speech communication, more advanced techniques as "active" to differentiate them from those mechanisms use a playout buffer to completely absorb only employed at the client side. delay jitter within talkspurts and dynamically adjust the For speech communication, one widely accepted way schedule between talkspurts [12-16]. Adaptive playout to reduce the effective packet loss observed by the re- scheduling is proposed to allow adaptive schedules even ceiver is to add redundancy to the data stream at the within talkspurts [17], and this idea has also been ex- sender. This is possible without imposing too much extra tended to video streaming [18,19]. An adaptive play out network load since the data rate of the voice traffic is very schedule is able to reduce the latency and the effective low when compared with other types of multimedia and packet loss rate at the same time. data traffic. A common method to add redundancy is forward error correction (FEC), which transmits re- dundant information across packets, where loss recovery 3.6.3.2 Combating losses is performed at the cost of higher latency. The efficiency of FEC s