1. July 2008
doc.: IEEE yy/xxxxr0
July 2008
Packets and MPEG Frames Background to Graceful degradation of audio video streams and Intra-Access Category prioritization
Date:
Authors:
Graham Smith, DSP Group
Graham Smith, DSP Group

2. The typical make-up of various MPEG video streams is determined
July 2008
doc.: IEEE yy/xxxxr0
July 2008
Abstract
The typical make-up of various MPEG video streams is determined
The relationship to packets is noted
The results are considered with respect to the objectives of:
Graceful degradation of audio video streams
Intra-Access Category prioritization of transport streams
Graham Smith, DSP Group
Graham Smith, DSP Group

3. The question to be considered is:
July 2008
doc.: IEEE yy/xxxxr0
July 2008
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 packet, and the various MPEG Frames”
Graham Smith, DSP Group
Graham Smith, DSP Group

4. Thanks for understanding
July 2008
Disclaimer
I am not an MPEG expert. The data in this presentation has been gathered from various readily available sources from the internet.
This is intended as background information, I am not making any specific points
I have tried to find typical numbers in order to find typical relationships that should help us when we consider certain parameters and features
If I have made any mistakes that give a false result please point them out
Thanks for understanding
Graham Smith, DSP Group

5. SDTV and HDTV SDTV HDTV 720 x 480 pixels, at 30fps (NTSC)
July 2008
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)
Graham Smith, DSP Group

6. MPEG2 Basics I : P : B = 30% : 30% : 40%
July 2008
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 12 I + (I/3 x 3) + (I/6 x 8) = 3.333I
I : P : B = 30% : 30% : 40%
Graham Smith, DSP Group

7. Number of bits in GOP (12) = 720000 x 12 = 8640000 Typical Frames
July 2008
Example – 1080i
1920 x = pixels/frame
1920 x 1080 x 25 = pixels/sec
Data Rate = 18Mbps (say)
Mean Pixels/bit = 18/ = 2.88
Ave Bits/Frame = /2.88 =
Number of bits in GOP (12) = x 12 =
Typical Frames
I-Frame = / = bits
P- Frame = bits
B-Frame = bits
Graham Smith, DSP Group

8. 802.11 Packets Typical Frames – 1080i I-Frame = 2592000 bits
July 2008
Packets
Typical Frames – 1080i
I-Frame = bits
P- Frame = bits
B-Frame = bits
packet = 7 x 188 (TS) = 1316B = bits
I-Frame = packets
P- Frame = packets
B-Frame = packets
Typical COG (12 frames)
Pkts
Packets = Bytes
Graham Smith, DSP Group

9. VTS stream intra-flow differentiation fields
July 2008
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
Graham Smith, DSP Group

10. … Presentation Order (coming out of encoder, and as you see it on TV.)
July 2008
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 I1 P1 B1 B2 P2 B3 B4 P3 B5 B6 …
Decode Order (going into set-top box buffer)
Source: MPEG and Digital Video in Modern Video Networks, Sept 13, 2006, JDSU
, 2006
Graham Smith, DSP Group

11. Error on a single frame can propagate spatially and temporally
July 2008
Group of Pictures (GOP)
Error on I Frame
Error Propagation
After Sarnoff 1999
Error on a single frame can propagate spatially and temporally
Source: MPEG and Digital Video in Modern Video Networks, Sept 13, 2006, JDSU
Graham Smith, DSP Group

12. Effect of Losing one Packet
July 2008
Effect of Losing one Packet
Example 1080i
I Frame = bits = /1080/2 = 1200 bits/line
One lost packet = bits = lines and 12 frames affected (0.48sec)
P Frame = bits = 400 bits/line
One lost packet = lines, 2 or 3 frames effected
2.4%? of picture for 0.08sec
B Frame = bits = 200 bits/line
One lost packet = 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 ?
Graham Smith, DSP Group

13. MPEG-2 Loss of a single TS video Packet
July 2008
MPEG-2 Loss of a single TS video Packet
Source=Nortel
Single B-frame IP packet loss
(1 frame affected)
Single I-frame IP packet loss
( 14 frames affected)
Packet Loss
Impact 3.75 Mbps stream: B packet loss vs. I packet loss
Need to look at a real video!
Graham Smith, DSP Group

14. “Acceptable” Packet Loss is dependable upon the Video Data Rate
July 2008
Packet Loss
Data Rate, Mbps 2 4 6 8 12 16 20
Pkt Loss Rate,
1 lost/min
0.0088%
0.0044%
0.0029%
0.0022%
0.0015%
0.0011%
0.0009%
“Acceptable” Packet Loss is dependable upon the Video Data Rate
Graham Smith, DSP Group

15. Packet Loss Requirement
July 2008
Packet Loss Requirement
Assuming :
Specification of 1 visible error per 15 minutes
That only 30% of lost packets are ‘visible”
Data Rate, Mbps 2 4 6 8 12 16 20
Pkt Loss if
30% visible
0.0019%
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?
Graham Smith, DSP Group

16. Packet Loss Experiment
July 2008
Packet Loss Experiment
Stream various videos, SD, HD, MPEG2, MPEG4, using VLC* over Ethernet (or WLAN).
Program blocks packets, randomly, at a preset percentage e.g. 10% to 0.01% packet loss
Program keeps count of
Total packets sent
Number of packets blocked
% Packet Loss
Observers note errors in video
How do observed errors compare to blocked packets? *
Graham Smith, DSP Group

17. Packet Loss Experiment – LIVE!
July 2008
Packet Loss Experiment – LIVE!
Observe Errors
Ethernet (cross over)
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
“PKTS T/D /45 PER 0.01” *
Graham Smith, DSP Group

18. Video Clip – #1 ~16Mbps MPEG 2 (269961 Packets)
July 2008
RESULTS
Video Clip of 3 minutes
Video Clip – #1 ~16Mbps MPEG 2 ( Packets)
# of blocked packets
# of observed errors
Video Clip – #2 ~8Mbps MPEG2 ( Packets)
Video Clip – #3 ~4Mbps MPEG4 (62849 Packets)
Graham Smith, DSP Group

19. Is it reasonable that 802.11aa could use VLC* codecs as a “standard”?
July 2008
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 aa could use VLC* codecs as a “standard”? *
Graham Smith, DSP Group

20. Conclusions Results of tests show ? Quality?
July 2008
doc.: IEEE yy/xxxxr0
July 2008
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?
Graham Smith, DSP Group
Graham Smith, DSP Group