Of the several error control schemes examined in the previous chapter, the ones that apply interactive error control and concealment between the encoder and the decoder are those that provide the most robust solution. The aforementioned latency drawback of these schemes can be eliminated by adopting the error tracking technique or its variants, which do not interrupt the playback to correct the errors. Instead, they apply error concealment at the decoder, sending feedback details about the error to the encoder, which heals the problem in the subsequent frames.
As discussed in paragraph 4.4, the techniques that apply selective retransmission are considered inappropriate for multicast schemes. This claim is challenged today, with multipoint control units , which could group error control feedback information between the encoder and a multicast group of decoders. Furthermore, sophisticated schemes retransmit only selected regions of the picture, keeping the encoder bandwidth and the retransmitted data below a total data rate, decreasing the packet loss probability.
In addition to the above, packetisation can be applied to isolate the packet loss effect, and reduce error propagation significantly. The packetisation scheme of RFC 2190 [Zhu97] is effectively decorrelating the information stored in adjacent packets, so that the loss of one of them does not affect the correct decoding of the rest. It suggests that whenever possible, segmentation of the H.263 bitstream should be done in GOB boundaries and that the optional resynchronisation GOB headers should be always used.
All the techniques discussed above, are fairly complex to implement, especially the interactive error control scheme. In the next chapter, the main focus will be on the effect of packetisation and receiver-based error concealment to the output video quality. These were much more straightforward to implement, however they remain a subset of the decisions taken in this paragraph.