June 2008

Graham Smith, DSP Group

Slide 1

doc.: IEEE 802.11-08/0717r0

Submission

802.11 Packets and MPEG FramesBackground to Graceful degradation of audio video streams and

Intra-Access Category prioritization Date: 2008-06-16

Name Affiliations Address Phone email Graham Smith DSP Group 2941 Sunrise Blvd

#100, Rancho Cordova, CA 95742

916 851 9191 x209

gsmith@dspg.com

Authors:

June 2008

Graham Smith, DSP Group

Slide 2

doc.: IEEE 802.11-08/0717r0

Submission

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

June 2008

Graham Smith, DSP Group

Slide 3

doc.: IEEE 802.11-08/0717r0

Submission

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”

June 2008

Graham Smith, DSP Group

Slide 4

doc.: IEEE 802.11-08/0717r0

Submission

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)

June 2008

Graham Smith, DSP Group

Slide 5

doc.: IEEE 802.11-08/0717r0

Submission

MPEG2 Basics

I – Frame = Intra Coded FrameP – Frame = Predictive Coded FrameB – Frame = Bi-directionally Predicted Coded Frame GOP = Group of Pictures

GOP = 8, 12 or 16 Typical sequence for GOP 12I 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 12 I + (I/3 x 3) + (I/6 x 8) = 3.333I

I : P : B = 30% : 30% : 40%

June 2008

Graham Smith, DSP Group

Slide 6

doc.: IEEE 802.11-08/0717r0

Submission

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

June 2008

Graham Smith, DSP Group

Slide 7

doc.: IEEE 802.11-08/0717r0

Submission

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

I B PB B B B B B BP P

June 2008

Graham Smith, DSP Group

Slide 8

doc.: IEEE 802.11-08/0717r0

Submission

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

June 2008

Graham Smith, DSP Group

Slide 9

doc.: IEEE 802.11-08/0717r0

Submission

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.)

I1 B1 B2 P1 B3 B4 P2 B5 B6 P3 B7 B8

Decode Order (going into set-top box buffer)

I1 P1 B1 B2 P2 B3 B4P3 B5 B6 …

, 2006Source: MPEG and Digital Video in Modern Video Networks, Sept 13, 2006, JDSU

June 2008

Graham Smith, DSP Group

Slide 10

doc.: IEEE 802.11-08/0717r0

Submission

After Sarnoff 1999

Group of Pictures (GOP)

Error Propagatio

n

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

June 2008

Graham Smith, DSP Group

Slide 11

doc.: IEEE 802.11-08/0717r0

Submission

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 1/25th sec

So, which packets can be dropped?Definitely not an I Frame almost 0.5 second error

P and B Frames ?

June 2008

Graham Smith, DSP Group

Slide 12

doc.: IEEE 802.11-08/0717r0

Submission

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!

June 2008

Graham Smith, DSP Group

Slide 13

doc.: IEEE 802.11-08/0717r0

Submission

Packet Loss Experiment

1. Stream various videos, SD, HD, MPEG2, MPEG4, using VLC* over Ethernet and 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 sent2. Number of packets blocked3. Packet Loss

4. Observer(s) note(s) errors in video

Definitely not an I-Frame almost 0.5 second error. P and B Frames ?

*http://www.videolan.org/

June 2008

Graham Smith, DSP Group

Slide 14

doc.: IEEE 802.11-08/0717r0

Submission

Packet Loss Experiment – LIVE!

VLC*

“Blocker”

VLC*

Video File

Screen

“PKTS T/D 397584/45 PER 0.01”

Observed Errors

Compare Observed Errors to Blocked PacketsIdeally, for MPEG2: 30% will be I Frame errors30% will be P Frame errors40% will be B Frame errors

Ethernet

*http://www.videolan.org/

June 2008

Graham Smith, DSP Group

Slide 15

doc.: IEEE 802.11-08/0717r0

Submission

RESULTS

• Video Clip – HDTV ~17Mbps MPEG2 (Corvette)– # of blocked packets

– # of observed errors

• Video Clip – SDTV ~7Mbps MPEG2– # of blocked packets

– # of observed errors

• Video Clip – SDTV ~4Mbps MPEG4– # of blocked packets

– # of observed errors

June 2008

Graham Smith, DSP Group

Slide 16

doc.: IEEE 802.11-08/0717r0

Submission

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?

• Propose that 802.11aa could use VLS* codec as a standard

*http://www.videolan.org/

June 2008

Graham Smith, DSP Group

Slide 17

doc.: IEEE 802.11-08/0717r0

Submission

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?