1. doc.: IEEE 802.11-08/0717r1 Submission July 2008 Graham Smith, DSP GroupSlide 1 802.11 Packets and MPEG Frames Background to Graceful degradation of audio video streams and Intra-Access Category prioritization Date: 2008-07-02 Authors:

2. doc.: IEEE 802.11-08/0717r1 Submission July 2008 Graham Smith, DSP GroupSlide 2 Abstract 1.The typical make-up of various MPEG video streams is determined 2.The relationship to 802.11 packets is noted 3.The results are considered with respect to the objectives of: Graceful degradation of audio video streams Intra-Access Category prioritization of transport streams

3. doc.: IEEE 802.11-08/0717r1 Submission July 2008 Graham Smith, DSP GroupSlide 3 Objectives Two objectives of PAR are: Graceful degradation of audio video streams when there is insufficient channel capacity, by enabling packet discarding without any requirement for deep packet inspection, Intra-Access Category prioritization of transport streams by modifying EDCA timing and parameter selection without any requirement for deep packet inspection The question to be considered is: “What are the relationships between an 802.11 packet, and the various MPEG Frames”

4. doc.: IEEE 802.11-08/0717r1 Submission July 2008 Graham Smith, DSP GroupSlide 4 SDTV and HDTV SDTV –720 x 480 pixels, at 30fps (NTSC) –720 x 576 pixels, at 25fps (PAL) HDTV –1280 x 720 pixels at 50/60fps (720p) –1920 x 1080 pixels at 25/30fps (1080i) –1920 x 1080 pixels at 50/60fps (1080p)

5. doc.: IEEE 802.11-08/0717r1 Submission July 2008 Graham Smith, DSP GroupSlide 5 MPEG2 Basics I – Frame = Intra Coded Frame P – Frame = Predictive Coded Frame B – Frame = Bi-directionally Predicted Coded Frame GOP = Group of Pictures GOP = 8, 12 or 16 Typical sequence for GOP 12 I B B P B B P B B P B B (I) One I, Three P, Eight B Typically P - Frame = 1/3 (I – Frame) B – Frame = 1/2 (P – Frame) Hence in GOP of 12I + (I/3 x 3) + (I/6 x 8) = 3.333I I : P : B = 30% : 30% : 40%

6. doc.: IEEE 802.11-08/0717r1 Submission July 2008 Graham Smith, DSP GroupSlide 6 Example – 1080i 1920 x 1080 = 2073600 pixels/frame 1920 x 1080 x 25 = 5184000 pixels/sec Data Rate = 18Mbps (say) Mean Pixels/bit= 18/5.184000 = 2.88 Ave Bits/Frame= 2073600/2.88 = 720000 Number of bits in GOP (12) = 720000 x 12 = 8640000 Typical Frames I-Frame = 8640000/3.3333 = 2592000 bits P- Frame = 864000 bits B-Frame = 432000 bits

7. doc.: IEEE 802.11-08/0717r1 Submission July 2008 Graham Smith, DSP GroupSlide 7 802.11 Packets Typical Frames – 1080i I-Frame = 2592000 bits P- Frame= 864000 bits B-Frame = 432000 bits 802.11 packet = 7 x 188 (TS) = 1316B = 10528 bits I-Frame = 246.2006 packets P- Frame= 82.06687 packets B-Frame = 41.03344 packets Typical COG (12 frames) 820.6687 Packets = 108000 Bytes 246.2006 Pkts

8. doc.: IEEE 802.11-08/0717r1 Submission July 2008 Graham Smith, DSP GroupSlide 8 Approach from 08/529r2 –VTS stream intra-flow differentiation fields Used for frame differentiation within one video flow, e.g., (Suggested method VTS Flag in TSINFO and b8-15 QoS Control Field) –I,P,B frames in MPEG-2 video flow; –video, audio, FEC packets in one video flow Provide the frame dropping criteria when there is insufficient channel capacity

9. doc.: IEEE 802.11-08/0717r1 Submission July 2008 Graham Smith, DSP GroupSlide 9 As the decoder must predict some frames from subsequent frames, we must have a decode order separate from the presentation order. Presentation Order (coming out of encoder, and as you see it on TV.) I1I1 B1B1 B2B2 P1P1 B3B3 B4B4 P2P2 B5B5 B6B6 P3P3 B7B7 B8B8 Decode Order (going into set-top box buffer) I1I1 P1P1 B1B1 B2B2 P2P2 B3B3 B4B4 P3P3 B5B5 B6B6 …, 2006 Source: MPEG and Digital Video in Modern Video Networks, Sept 13, 2006, JDSU

10. doc.: IEEE 802.11-08/0717r1 Submission July 2008 Graham Smith, DSP GroupSlide 10 After Sarnoff 1999 Group of Pictures (GOP) Error Propagation Error on I Frame Error on a single frame can propagate spatially and temporally Source: MPEG and Digital Video in Modern Video Networks, Sept 13, 2006, JDSU

11. doc.: IEEE 802.11-08/0717r1 Submission July 2008 Graham Smith, DSP GroupSlide 11 Effect of Losing one Packet Example 1080i – I Frame = 2592000 bits = 2592000/1080/2 = 1200 bits/line –One lost packet = 10528 bits = 4.3866 lines and 12 frames affected (0.48sec) –P Frame = 864000 bits = 400 bits/line –One lost packet = 26.32 lines, 2 or 3 frames effected 2.4%? of picture for 0.08sec –B Frame = 432000 bits = 200 bits/line –One lost packet = 52.64 lines but only 1 frame effected 4.8%? of picture for 1/25 th sec So, which packets can be dropped? Definitely not an I Frame almost 0.5 second error P and B Frames ?

12. doc.: IEEE 802.11-08/0717r1 Submission July 2008 Graham Smith, DSP GroupSlide 12 MPEG-2 Loss of a single TS video Packet Packet Loss Impact 3.75 Mbps stream: B packet loss vs. I packet loss Single B-frame IP packet loss (1 frame affected) Single I-frame IP packet loss ( 14 frames affected) Source=Nortel Need to look at a real video!

13. doc.: IEEE 802.11-08/0717r1 Submission July 2008 Graham Smith, DSP GroupSlide 13 Packet Loss Data Rate, Mbps2468121620 Pkt Loss Rate, 1 lost/min0.0088%0.0044%0.0029%0.0022%0.0015%0.0011%0.0009% “Acceptable” Packet Loss is dependable upon the Video Data Rate

14. doc.: IEEE 802.11-08/0717r1 Submission July 2008 Graham Smith, DSP GroupSlide 14 Packet Loss Requirement Assuming : –Specification of 1 visible error per 15 minutes –That only 30% of lost packets are ‘visible” Data Rate, Mbps2468121620 Pkt Loss if 30% visible0.0019%0.0010%0.0006%0.0005%0.0003%0.0002% Virtually “zero packet loss” requirement. Need to do everything to get the I-Frame through: Extra Retries on I-Frame packets? Highest Priority QoS? FEC? Questions: How much better compared to HCCA on complete stream? Does dropping P and B Frame packets help? How many P and B Frame dropped packets are visible?

15. doc.: IEEE 802.11-08/0717r1 Submission July 2008 Graham Smith, DSP GroupSlide 15 Packet Loss Experiment 1.Stream various videos, SD, HD, MPEG2, MPEG4, using VLC* over Ethernet (or WLAN). 2.Program blocks packets, randomly, at a preset percentage e.g. 10% to 0.01% packet loss 3.Program keeps count of 1.Total packets sent 2.Number of packets blocked 3.% Packet Loss 4.Observers note errors in video How do observed errors compare to blocked packets? *http://www.videolan.org/

16. doc.: IEEE 802.11-08/0717r1 Submission July 2008 Graham Smith, DSP GroupSlide 16 Packet Loss Experiment – LIVE! “PKTS T/D 397584/45 PER 0.01” Observe Errors Compare Observed Errors to Blocked Packets Ideally, for MPEG2: 30% will be I Frame errors 30% will be P Frame errors 40% will be B Frame errors Ethernet (cross over) *http://www.videolan.org/

17. doc.: IEEE 802.11-08/0717r1 Submission July 2008 Graham Smith, DSP GroupSlide 17 RESULTS Video Clip – #1 ~17Mbps MPEG 2 –# of blocked packets –# of observed errors Video Clip – #2 ~7Mbps MPEG2 –# of blocked packets –# of observed errors Video Clip – #3 ~4Mbps MPEG4 –# of blocked packets –# of observed errors

18. doc.: IEEE 802.11-08/0717r1 Submission July 2008 Graham Smith, DSP GroupSlide 18 Video Codec Points The VLC* video decoder is software, is this a fair representation of a video decoder? –VLC* is very popular media player/recorder for laptops/PCs –VLC* is free Some decoders may have certain error masking features, do we assume their presence? Is it reasonable that 802.11aa could use VLC* codecs as a “standard”? *http://www.videolan.org/

19. doc.: IEEE 802.11-08/0717r1 Submission July 2008 Graham Smith, DSP GroupSlide 19 Conclusions Results of tests show ? Quality? –Is dropping B or P frames a good idea? –Is having a lower priority on P or B Frames a good idea? –What observable error rate is “acceptable”? –Is any degradation due to the WLAN acceptable?