1. Video over 802.11 Research Date: 2007-05-10 Authors: May 2007 May 2007
doc.: IEEE yy/xxxxr0
May 2007
Video over Research
Date:
Authors:
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Alex Ashley, NDS Ltd
Alex Ashley, NDS Ltd

2. May 2007
doc.: IEEE yy/xxxxr0
May 2007
Abstract
A description of our research in to streaming video over IEEE networks in the domestic environment.
Alex Ashley, NDS Ltd
Alex Ashley, NDS Ltd

3. May 2007
Project History
Started looking at video distribution within the home about 5 years ago
Aim of allowing every home to distribute video to any room in the house and allow every home to do so simultaneously
Considered both wired and wireless options
Wired
Re-use existing (satellite) cable
Installing Ethernet twisted pair cabling
Wireless
Looked at many options, e.g. DVB-T, UWB, DECT, IEEE
was the closest fit at a sensible price point
Alex Ashley, NDS Ltd

4. May 2007
doc.: IEEE yy/xxxxr0
May 2007
RF Study
Commissioned a paper study in to RF propagation within the home T R
LOS
Alex Ashley, NDS Ltd
Alex Ashley, NDS Ltd

5. RF Simulations Static modelling for RF in the home environment
May 2007
RF Simulations
Static modelling for RF in the home environment
Link losses in various types of home
Required transmit power
Zone of interference outside of each home
Number of channels required for every home to be active simultaneously
Ray-tracing
How realistic are the Rayleigh model assumptions in the static model?
Dynamic modelling
Using mathematical model to calculate link losses and apply this to a dynamic model of the MAC
Allows dynamic simulations of a variety of home layouts and options within
Alex Ashley, NDS Ltd

6. Static Modelling Results
May 2007
Static Modelling Results
Digital AV services can be successfully and safely transmitted in typical UK housing stock
Frequency allocation to avoid interference with nearby homes is a big issue
Alex Ashley, NDS Ltd

7. Ray Tracing Modelling Results
May 2007
Ray Tracing Modelling Results
Significant amount of signal at receiver is multi-path
Using Rayleigh channel modelling in the static model was the correct choice
Compared to dynamic aspects such as noise and movement, differences between ray-tracing results and static model results were minor
Alex Ashley, NDS Ltd

8. Dynamic Modelling Results
May 2007
Dynamic Modelling Results
2.4GHz becomes over-crowed at fairly modest traffic levels when every home operates simultaneously
Difficult to reliably stream a couple of streams at 3MBit/sec or higher
5GHz has problems reaching whole home
On some channels due to regulatory
power limits
Building materials attenuation much
higher at 5GHz
Alex Ashley, NDS Ltd

9. Dynamic Modelling Results (2)
May 2007
Dynamic Modelling Results (2)
EDCA becomes non-linear at about 2 x 3 MBit/sec
Alex Ashley, NDS Ltd

10. Real-World Tests 4 different access points and 2 different NICs
May 2007
Real-World Tests
4 different access points and 2 different NICs
In our homes and those of friends & family
Reception characteristics were recorded for each access point in 4 different rooms and also whilst mobile
Packet error rate
Bit error rate
Error rate post FEC correction, using only valid packets
Error rate post FEC correction, also using packets with invalid FCS
Alex Ashley, NDS Ltd

11. Real-World Test Results
May 2007
Real-World Test Results
Marked difference between 11g and 11a
Frames received, but with incorrect FCS more common at 5GHz
In frames with incorrect FCS, roughly 1-2% of bytes are in error
At 2.4GHz packets tended to be completely lost
Similar performance amongst 11g equipment
Higher gain aerial improved BER but only small PER improvement
Antenna diversity produced results similar to 11a / 11g trend
Errors seam to occur in bursts
Alex Ashley, NDS Ltd

12. Error Correcting Codes
May 2007
Error Correcting Codes
Video is latency and jitter sensitive
Retransmissions built in to are problematic
One option is to not retransmit
But packet losses cause lousy video quality
Adding Forward Error Correction information can deal with the packet losses
But increases the bitrate and adds (a fixed) latency
Server
Forward error correction encoding and data interleaving
De-interleaving and FEC decoding
Client
Wireless network
Alex Ashley, NDS Ltd

13. Error Correction Codes
May 2007
doc.: IEEE yy/xxxxr0
May 2007
Error Correction Codes
Error free without
re-transmissions
Alex Ashley, NDS Ltd
Alex Ashley, NDS Ltd

14. May 2007
Future FEC Work
May be desirable to use a hybrid system with FEC plus a limited amount of retransmission
Need to decide how decide when to use retransmissions
Fixed maximum number of retransmissions
Maximum amount of time between packet arrival at the wireless network and its successful transmission
Only request a retransmission if the FEC cannot correct the error
May be possible to improve FEC performance
Populate known data such as IP headers and repeated data such as MPEG PSI tables to reduce error rate before FEC
Alex Ashley, NDS Ltd

15. Current & Future Work Improved Inter-network coexistence
May 2007
Current & Future Work
Improved Inter-network coexistence
Co-operation between BSS (time sharing)
Better transmit power control (reduce BSS overlap)
Using FEC on an encrypted BSS
Using frames with failed FCS gives huge gain to FEC, but does not work if frame was encrypted after FEC was applied
Investigate use of multicast
Most current APs only use basic rate for multicast traffic
How to negotiate “best” rate (esp with all the modes in 11n!)
Are errors correlated (all STAs loose the same packet) or different per STA?
Alex Ashley, NDS Ltd

16. VTS PAR / 5 Criteria Need for the project Broad Market Potential
May 2007
VTS PAR / 5 Criteria
Need for the project
becomes over-crowed at fairly modest video traffic levels when every home operates simultaneously
Slightly below bitrates commonly used for SD broadcasts
Using 11n will probably solve this
But we also want to do HD video
Broad Market Potential
TV almost ubiquitous (e.g. >94% in many regions)
Technical Feasibility
We have found some MAC layer modifications that seem to help
Alex Ashley, NDS Ltd

17. May 2007
References
Alex Ashley, NDS Ltd