Evaluating the use of clock frequency ratio estimators in the playout from video distribution networks

Abstract: As traditional TV-broadcasters utilize the Internet to transport video streams, they often employ third party distribution networks to ensure that the Quality of Service of the packet stream remain high. In the last step of such a distribution network, a playout scheduler will schedule the packets so that their intervals are as close as possible to the intervals with which they were initially sent by the source. This is done with the aim to minimize the amount of packet delay variation experienced by the final destination. Due to the source and distribution network not always being synchronized to the same reference clock, reconstructing the packet intervals back into the initial values is subject to the issue of clock skew; the clocks run at different frequencies. In the presence of clock skew, each packet interval will be reconstructed with a slight error, which will accumulate throughout the packet stream. This thesis evaluates how clock frequency ratio estimators can be implemented as part of the playout scheduler, allowing it to better reconstruct the packet intervals in the face of clock skew. Two clock frequency ratio estimators presented in the literature are implemented as a part of playout schedulers, and their use in the context of a video distribution network is evaluated and compared to other playout schedulers. All in all, four of the considered playout schedulers employ clock frequency ratio estimation, and four do not. The playout schedulers are tested on a test bed consisting of two unsynchronized computers, physically separated into a source and a destination connected via Ethernet, to ensure the presence of clock skew. The source generates a video stream, which is sent to the destination. The destination is responsible for packet interval reconstruction and data collection, that allows for comparison of the eight playout schedulers. Each playout scheduler is evaluated under three different network scenarios, each network scenario with increasing amounts of packet delay variation added to the packet stream. The results show that the Cumulative Ratio Scaling with Warm-up scheduler, which employs a clock frequency ratio estimator based on accumulating inter-packet times, performs well under all three network scenarios. The behaviour of the playout scheduler is predictable and the frequency ratio estimate seems to converge towards the true clock frequency ratio as more packets arrive at the playout scheduler. While this playout scheduler is not perfect, its behaviour shows promise in being extended.