ABSTRACT

Enhancing Voice over IP Quality through use of Synchronised Time

Hugh Melvin, Dept. of Computer Science, University College Dublin (UCD) Ireland

Delivering PSTN-like quality over current best-effort Internet infrastructure presents many technical challenges for VOIP. Much research in recent years has focused on receiver-based approaches that adapt to varying network conditions in order to optimise playout quality. In this presentation, the author describes a new receiver-based approach that implements a hybrid adaptive-fixed playout strategy by integrating synchronized time via NTP into the receiver playout algorithm. Such an approach can deliver significantly better quality than existing adaptive techniques particularly when the underlying network is not heavily congested and end-to-end delays are not excessive. Some initial results from a testbed system are presented using the ITU-T E-model to quantify improvements. Implementing NTP within a VOIP system does not however address audio-card timing discrepancies that lead to distortion of delay estimates and buffer under/overrun. A mechanism is presented that utilises NTP along with the RTP support protocol RTCP to minimise the ill-effects of such audio skew and improve the overall robustness of the playout system.

Hugh Melvin is a PhD candidate at the Dept. of Computer Science, University College Dublin (UCD) Ireland. At UCD, he works within the Performance Engineering Laboratory group (See http://www.cs.ucd.ie/staff/lmurphy/home/pel_ucd.htm). His research interests include Internet Multimedia, Time Synchronization and RealTime Systems. He lectures also at the Dept. of Information Technology, National University of Ireland, Galway (NUI,G) (See http://www.it.nuigalway.ie). Prior to 1997 he worked for almost a decade in the Power Engineering business.

Maintained by rbennett@cs.ucl.ac.uk