is chapter. Packet Loss Effects The impact of packet loss on a piece of video is largely dependent on the sequence of events leading to it and the types of packet that have sustained loss. Figure 3.24(a) shows a situa- tion where predictive information, or P packets, have been lost in an H.263 codec; the effect is to propagate errors because coding is relative. Figure 3.24(b) shows the catastrophic effect of the loss to I packets; the image fails to render subsequent predicted packets, which use these as their basis, SO the compound errors. Countering Loss Effects Because of complexity and the time required to repair losses in the video stream, codecs generally have to accept that loss of data is inevitable and recover from it as best they can. Forward Error Correction The Forward Error Correction technique can be used to help codecs recover from bit errors. To provide a mechanism that allows recovery from error requires that addi- tional information be inserted into the bitstream. This is an overhead or additional expense that will require extra data. In its simplest form, this could be sending multiple Chapter 3 Digital Video Transmission Fig. 3.24 Off-air pictures with packet loss. copies of the same packet; more usually, block or convolutional coding is employed (Lee, 1997). While the techniques are often applied in wireless applications and used for video storage in DVD, FEC is usually avoided in real-time Internet-type traffic because of the overhead. TCP Retransmission Request IP networks permit the inclusion of Transmission Control Protocol (TCP) (Tanenbaum, 2003) packets. TCP provides a method of a connection orientation where packets are assured delivery provided the network remains in an operational state. This is achieved by keeping a record of the packet numbers at the sending and receiving sides. If a packet delivery fails, a request is made for retransmission. The difficulty with this approach is the round-trip time-the time taken for the retransmission request (negative acknowledgement) and the re