A Cross-Layer PLC Algorithm for a Real-time Audio Conferencing System

In this paper, we propose a cross-layer packet loss concealment (PLC) algorithm in order to improve the quality of audio for a real-time audio conferencing system. The proposed algorithm tries to estimate a packet loss rate (PLR) of the network and to request a sender to transmit redundant information for audio decoding when the PLR is assumed to be higher. An audio quality measure implemented on the application layer is used for an estimate of the PLR. In addition, an RTP payload format for the transport layer is defined here to control the degree of redundancy for the PLC algorithm. When lower quality of audio is measured, the packet of the current frame is composed of the bitstream of the current frame and that of the previous frame to help the audio decoder reconstruct the audio signals of the previous frame with better quality than with no redundancy. In order to show the effectiveness of the proposed PLC algorithm, we use MPEG-2 Advanced Audio Coding (AAC) and the ITU-T P.563 as an audio codec and an objective quality measure of audio signals, respectively. It is shown from the experiment that the audio conferencing system employing the proposed cross-layer PLC algorithm improves audio quality significantly under packet loss conditions.