transmissions. This delay means the video display must also be delayed, and this will impact the feeling of interaction in a real-time two-way communication. Congestion-Driven Rate Control If the receiver sends side information back to the transmitter to explain the level of loss and hence the congestion it is experiencing, then the rate of generation can be increased or decreased according to network conditions. This not only improves the display of the user, but, if adopted by all, it should achieve better utilization of the network. Error Concealment The key to making video transport over the Internet possible is the ability of the codec to recover from packet loss. Error cancealment can be applied to any of the conventional hybrid DCT codecs and awaits standardization (Ghanbari, 2003). In its simplest form when a codec recognizes that a macroblock has been lost, content from the previous frame is rendered in its place; motion compensation can also be employed if available. This remarkably simple References solution dramatically improves the subjective quality of the video in the event of loss, although it can produce blocking artifacts against neighboring macroblocks that have been updated. Subsequent filtering of the image can alleviate these effects. Conclusions This chapter provided a brief overview of digital video, its transport over packet-based networks, and the difficulties encountered. The high-bitrate nature of digital video dictates the use of high compression; otherwise, networks would have insufficient bandwidth. Com- pression uses a variety of techniques that have resulted from decades of active research. The act of compression has left the bitstream vulnerable to loss because the absence of a single packet can have catastrophic effects on the reconstruction. The Internet itself offers no guarantee of delivery, and the interactive nature of two-way video dictates that applications must tolerate packet loss and delay. For noninteractive applications, overheads in the form of TCP and FEC can be ap