We present a statistics analysis of the picture distortion caused by quantization

channel errors and subsequent error propagation

and propose rate-distortion estimation models which are functions of two parameters

i.e.quantization step and intra refreshing ratio.Based on these R-D models

a joint optimal quantization step and intra refreshing ratio selection algorithm is developed by using Lagrange multiplier theory.Simulation results show that our algorithm can make the encoder adapting itself in time to the variation of the channel bandwidth

and significantly improve the video quality over packet loss networks.