This dissertation addresses the problem of displaying live continuous media (e.g., digital audio and video) with low latency in the presence of delay jitter, where delay jitter is defined as variation in processing and transmission delay. Display in the presence of delay jitter requires a tradeoff between two goals: displaying frames with low latency and displaying every frame. Applications must choose a display latency that balances these goals.
The driving problem for my work is workstation-based videoconferencing using conventional data networks. I propose a two-part approach. First, delay jitter at the source and destination should be controlled, leaving network transmission as the only uncontrolled source. Second, the remaining delay jitter should be accommodated by dynamically adjusting display latency in response to observed delay jitter. My thesis is that this approach is sufficient to support the low-latency display of continuous media transmitted over campus-sized networks.
Delay jitter at the source and destination is controlled by implementing the application as a real-time system. The key problem addressed is that of showing that frames are processed with bounded delay. The analysis framework required to demonstrate this property includes a new formal model of real-time systems and a set of techniques for representing continuous media applications in the model.
The remaining delay jitter is accommodated using a new policy called queue monitoring that manages the queue of frames waiting to be displayed. This policy adapts to delay jitter by increasing display latency in response to long delays and by decreasing display latency when the length of the display queue remains stable over a long interval. The policy is evaluated with an empirical study in which the application was executed in a variety of network environments. The study shows that queue monitoring performs better than a policy that statically chooses a display latency or an adaptive policy that simply increases display latency to accommodate the longest observed delay. Overall, the study shows that my approach results in good quality display of continuous media transmitted over campus-sized networks that do not support communication with bounded delay jitter.
A PostScript copy of this dissertation is also available in four parts: