Generic RTP Payload Format guaranteeing QoS of Video communication

1. Generic RTP Payload Format guaranteeing QoS of Video communication draft-bsong-fecframe-gfec-00.txt Hao Qin hqin@mail.xidian.edu.cn

2. Summary • Aim: • To reduce the impact of packet loss to a video receiver. • Solution: • Use FEC technique to recover the lost information from the correctly received packets. • Bundle maximum number of output data blocks from an FEC encoder into each RTP packet to maximize the bandwidth utilization and minimize the latency.

3. Unequal FEC protection • Different parts in the video stream may have different importance to the receiver. • reference frames are critical to a decoder to reconstruct following pictures. • prediction frames are used to reconstruct just one picture based on the previous reference frame. • Losses of reference frames may cause far more serious disasters to the video decoder because the video decoder may be forced into a very long time of interruption. • Data with different importance may be unequally protected with different FEC algorithms.

4. Interleaving • Many consecutive data blocks from an FEC encoder may be encapsulated into a single RTP packet, so that loss of an RTP packet means loss of many data blocks. This brings difficulty to an FEC decoder for the insufficient information to reconstruct the source data. • Interleaving should be introduced to solve this problem.

5. Protected by FEC+Interleaving

6. Protected by FEC only • If the data are protected by FEC only, the format of encapsulation is different from the format mentioned above. To mitigate the impact of burst packet loss to the FEC decoder, exactly one FEC block is encapsulated in one RTP packet.

7. Generic payload format for FEC

9. Delay consideration and parameter determination FEC and interleaving reduce the effect of pack losses at the cost of increasing the delay. Large values of N, K or U increase not only the ability of the receiver to be tolerant of packet losses, but also the delay. Tradeoff between the low end-to-end delay and great tolerance for packet losses should be made in the choice of FEC and interleaving parameters.

10. Detecting packet losses To make the FEC decoder work, the position and the amounts of the lost packets in their corresponding FEC unit need to be determined. • If the data are neither protected by the FEC and nor interleaved,the lost packet cannot be recovered. • If the data are protected by the FEC only or by FEC and interleaved, the decoder can detect packet losses by the sequence number, FEC type, FEC subtype and index field.

11. Video encoder FEC encoder Interleaving Encapsulation FEC type FEC subtype Block size C bit Interleaving index Block size RTP Transmission Video decoder FEC decoder Deinterleaving Payload extraction The process flow of the video stream

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