Modeling and control of VBR H.261 video transmission over frame relay networks

Abstract

Examines the transmission of variable bit-rate (VBR) H.261 video over a mixed traffic (video/inter-LAN) integrated services frame relay (FR) network. We introduce a modified H.261 codec that produces VBR output and show that parsing of the video bit stream at group of blocks (GOB) boundaries produces variable length FR packets well suited to the network. We demonstrate that GOB-level video traffic requires a more sophisticated statistical model of the resulting data stream than the frame-level models. The transform expand sample (TES) method is used to obtain an accurate model of the autocorrelation and the marginal probability distribution of the bit-rate variations at the GOB level. A simple and effective methodology is introduced for capturing the periodic components that are present in the GOB-level autocorrelation. The methodology is extended to permit simulations of VBR codecs in which the codec quantization step size is adjusted in response to prevailing network conditions. We show that the quality of service requirements of VBR video can be met by using the FR backward explicit congestion notification (BECN) facility in conjunction with a modified H.261 codec whose rate is controlled by the congestion notification. We also show that the performance of the control mechanism is significantly influenced by a subset of network threshold and codec control parameters which are identified using 2/sup k/ factorial analysis techniques. We obtain optimal ranges of values for these parameters using mean field annealing. Variable quantization rate control may be more effective for this purpose than variable frame rate control.