Completed MSc Student: Conor Cahir

Congratulations to Conor Cahir, who has successfully completed his MSc by Research dissertation on Approaches to Adaptive Bitrate Video Streaming.

The abstract of Conor's dissertation reads:

In this work, I use ns-3 simulations to compare and evaluate different approaches to web based adaptive bitrate (ABR) video streaming. In particular, I look at the difference between client pull and server push based approaches, the effects of media formatting parameters such as chunk duration and number of encoding rates, and the implementation of bandwidth estimation and request scheduling strategies. I find that client pull applications with a 2 second chunk duration are very inefficient with bandwidth compared to applications using a server push based approach. The reasons for this stem from the effect of frequent idle periods at chunk boundaries, which are absent with server push, on the behaviour of TCP. Increasing the chunk duration to 10 seconds makes a significant difference to client pull applications and allows them to perform at a level much more comparable with server push applications. I also find that ABR applications in general are vulnerable to suffering from encoding rate instability, a result that echoes findings from a number of recent studies. This problem seems to stem from the difficulty of selecting a suitable encoding rate based on transfer rates observed at the application layer. Effective remedies for encoding rate instability include ensuring that the system is not over provided for in terms of the number of available encoding rates, and using an averaging function, such as the harmonic mean, over a series of recent transfer rates in order to filter out short term fluctuations in the estimate of available bandwidth. I also show that a simple request scheduling strategy can be used to avoid over buffering and the associated problems, but that periodic request scheduling can introduce further problems related to fairness when multiple ABR flows compete. Finally, I show that a hybrid of client pull and server push, which I call pull selective, can offer a useful compromise between the two, by matching the performance characteristics of server push whilst maintaining the low server overheads and scalability attributes of client pull.

Conor's work was supported by Cisco, as part of our project on Architecture and Protocols for Massively Scalable IP Media Streaming.