Model for variable bit rate video transmission over an ATM network

The transmission of video over variable bit rate channels is a challenging problem. The requirements for an acceptable video quality are typically negotiated between the application layer and the network. The encoder of a variable bit rate video application can adjust several of its parameters to meet the requirements of the network. The main aim is to decrease the loss, delay variation. Network resources have to be properly assigned to meet a required service request. Network buffers need to be controlled carefully, since buffers are responsible for delay and loss distribution. On the other hand, during video transmission, the state of buffers at the encoder output, and decoder input are also very important. Since a buffer overflow or underflow indicates inappropriate resource distribution, it is essential to know when and how buffer overflow and underflows occur. The proposed research is based on the encoder and decoder buffer boundary definitions introduced in Nguyen et al. 97. The bit rate of the video is adjusted so that the encoder and decoder buffers neither overflow or underflow at any instance during the transmission. This is guaranteed by estimating the state of the decoder buffer at a fixed frame rate, which in turn is derived for the variable channel rates for the duration of transmission. Both the encoder and decoder buffer fullness information is used as a feedback to the encoder. The encoder, upon receiving this state information adjusts the bit rate for the subsequent frames. This research uses the analytically proven boundaries for the variable bit rate encoder and decoder provided and the behavior is simulated using several video sequences. Simulation results provide a better understanding of the boundary definitions and their practical applications. In addition to understanding the dynamic behavior in terms of the bit rates, this investigation also addresses the impact of the changes in the frame rate and other scalability factors controlled by the encoder. The results show that the boundary definitions, which in turn are used for rate control of the encoder and decoder, improve the quality of the video received.