Doc.: IEEE 802.11-08/0717r1 Submission July 2008 Graham Smith, DSP GroupSlide 1 802.11 Packets and...
-
Upload
blaze-norris -
Category
Documents
-
view
214 -
download
0
Transcript of Doc.: IEEE 802.11-08/0717r1 Submission July 2008 Graham Smith, DSP GroupSlide 1 802.11 Packets and...
July 2008
Graham Smith, DSP Group
Slide 1
doc.: IEEE 802.11-08/0717r1
Submission
802.11 Packets and MPEG FramesBackground to Graceful degradation of audio video streams and
Intra-Access Category prioritization Date: 2008-07-02
Name Affiliations Address Phone email Graham Smith DSP Group 2941 Sunrise Blvd
#100, Rancho Cordova, CA 95742
916 851 9191 x209
Authors:
July 2008
Graham Smith, DSP Group
Slide 2
doc.: IEEE 802.11-08/0717r1
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
July 2008
Graham Smith, DSP Group
Slide 3
doc.: IEEE 802.11-08/0717r1
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”
July 2008
Graham Smith, DSP Group
Slide 4
doc.: IEEE 802.11-08/0717r1
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)
July 2008
Graham Smith, DSP Group
Slide 5
doc.: IEEE 802.11-08/0717r1
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%
July 2008
Graham Smith, DSP Group
Slide 6
doc.: IEEE 802.11-08/0717r1
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
July 2008
Graham Smith, DSP Group
Slide 7
doc.: IEEE 802.11-08/0717r1
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
July 2008
Graham Smith, DSP Group
Slide 8
doc.: IEEE 802.11-08/0717r1
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
July 2008
Graham Smith, DSP Group
Slide 9
doc.: IEEE 802.11-08/0717r1
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
July 2008
Graham Smith, DSP Group
Slide 10
doc.: IEEE 802.11-08/0717r1
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
July 2008
Graham Smith, DSP Group
Slide 11
doc.: IEEE 802.11-08/0717r1
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 ?
July 2008
Graham Smith, DSP Group
Slide 12
doc.: IEEE 802.11-08/0717r1
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!
July 2008
Graham Smith, DSP Group
Slide 13
doc.: IEEE 802.11-08/0717r1
Submission
Packet Loss
Number of Packets Lost per Hourversus Packet Loss Rate
020406080
100120140
0.0
01
0.0
02
0.0
03
0.0
04
0.0
05
0.0
06
0.0
07
0.0
08
0.0
09
0.0
10
0.0
11
0.0
12
0.0
13
0.0
14
0.0
15
0.0
16
0.0
17
0.0
18
0.0
19
0.0
20
Packet Loss Rate %
No
. Pac
kets
Lo
st p
er H
ou
r
2Mbps
4Mbps
8Mbps
12Mbps
16Mbps
20Mbps
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
July 2008
Graham Smith, DSP Group
Slide 14
doc.: IEEE 802.11-08/0717r1
Submission
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 if30% visible 0.0019% 0.0010% 0.0006% 0.0005% 0.0003% 0.0002% 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?
July 2008
Graham Smith, DSP Group
Slide 15
doc.: IEEE 802.11-08/0717r1
Submission
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/
July 2008
Graham Smith, DSP Group
Slide 16
doc.: IEEE 802.11-08/0717r1
Submission
Packet Loss Experiment – LIVE!
VLC*
“Blocker”
VLC*
Video File
Screen
“PKTS T/D 397584/45 PER 0.01”
Observe 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 (cross over)
*http://www.videolan.org/
July 2008
Graham Smith, DSP Group
Slide 17
doc.: IEEE 802.11-08/0717r1
Submission
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
July 2008
Graham Smith, DSP Group
Slide 18
doc.: IEEE 802.11-08/0717r1
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?
• Is it reasonable that 802.11aa could use VLC* codecs as a “standard”?
*http://www.videolan.org/
July 2008
Graham Smith, DSP Group
Slide 19
doc.: IEEE 802.11-08/0717r1
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?
– What observable error rate is “acceptable”?
– Is any degradation due to the WLAN acceptable?