Application layer QoS for videotelephony

Abstract

Home broadband bandwidth will be a variable commodity as customers run multiple applications through their home hubs simultaneously. The reaction of some of these applications (e.g. Web browsing or e-mail) to congestion or packet loss will be governed by the control mechanisms within the transfer control protocol (TCP/IP). Retransmissions and rate control will slow down TCP applications when in a congested state, but data will, in general, be reliably transmitted. However, applications which are more sensitive to delay, such as voice over IP and videotelephony, are generally sent for delay considerations using the real time protocol (RTP) and the underlying unreliable user datagram protocol (UDP/IP). These protocols deliver timely data but with the downside that any congestion will lead to packet losses which will propagate up to, and need to be managed by, the application layer. This paper reports on efforts to define an application layer quality-of-service framework for the video component of videotelephony over RTP/UDP leading to an initial reference design for specifying videotelephony products. The error resilience features of the latest video-coding standard, H.264, are examined, as are modifications to existing RTP Internet recommendations to allow effective feedback-based repair for this new codec. Results using RTP early feedback and H.264 reference picture selection in a multi-reference configuration will also be presented.