This paper adresses the transmission of VBR MPEG-2 video streams on top of both network adaptation (NAL) and ATM adaptation layers (AAL) for real-time multimedia applications. The NAL, specific to MPEG-2, provides a selective data protection mechanism based on syntactic criteria. The AAL provides a reliable transmission by applying per-cell sequence numbering combined with a selective forward error correction (FEC) mechanism based on burst erasure codes. Studies carried out with constant bit rate (CBR) video streams showed improvements in terms of network performance evaluated by the cell loss ratio (CLR) as well as in terms of user perceived quality compared to the performance obtained with AAL5 under the same network conditions. This paper proposes improvements at the NAL and presents the results obtained for the transmission of variable bit rate (VBR) video streams. To evalute the impact of cell losses at the application level, we apply a perceptual quality measure to the decoded MPEG-2 sequences which allows us to evaluate performance at the user level.
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