1. Streaming digital video over wireless link
PROGRESS project EES.5653
18-Sep-18
Sergei N. Kozlov,
TU/e Informatica, System Architecture and Networking
PROGRESS project EES.5653

2. Agenda General issues with streaming over a wireless link
Streaming MPEG video - link layer approach
Briefly - transport layer approach
Future work
18-Sep-18
Sergei N. Kozlov,
TU/e Informatica, System Architecture and Networking
PROGRESS project EES.5653

3. Video over wireless link
Typical scenario: video transmission from set-top box to the (mutiple) screens
wireless
18-Sep-18
Sergei N. Kozlov,
TU/e Informatica, System Architecture and Networking
PROGRESS project EES.5653

4. Simplified sender/receiver communication
video source
video sink
sender buffer
receiver buffer
- Latency or playback delay.
- What the jitter is.
- Why do we need buffers?
The bigger the buffers – the higher the latency, the less the jitter. If we want lower latency (par example conferencing), we want smaller buffers.
(In case of RT video – conferencing) The bigger the buffers – the less probability of data loss
- So, we have a dillema – we need to find how to minimize latency and still provide data delivery.
- Explain why it’s not meaningful to use the sender buffer bigger than the receiver buffer.
- Explain that the size of the receiver buffer is oftern limited and that for sake of lower latency we may want smaller buffers
- With smaller buffers more data will get lost
That’s why we need to consider sending first of all what is most important. If we need to drop data, then we drop pieces which influence our quality leas
wireless interface (sender)
wireless inerface (receiver)
18-Sep-18
Sergei N. Kozlov,
TU/e Informatica, System Architecture and Networking
PROGRESS project EES.5653

5. Video over wireless link
Wireless link properties:
High jitter vs high losses
because of interferences on physical level
jitter comes up to 0.25s with default settings on CISCO 350 WiFi card
Low troughput (< 5Mbps for b)
can be higher with a/g, but never as high as wire/optical
Variable throughput
Typical perceived quality issues:
Artefacts
“Hick-ups”
18-Sep-18
Sergei N. Kozlov,
TU/e Informatica, System Architecture and Networking
PROGRESS project EES.5653

6. IP (Internet Protocol)
Layers we look at
OSI stack:
application layer
MPEG en-/decoder
MPEG-packetizer
transport layer
TCP, UDP/RTP
network layer
IP (Internet Protocol)
packet scheduler
link layer
driver
physical layer
WLAN
18-Sep-18
Sergei N. Kozlov,
TU/e Informatica, System Architecture and Networking
PROGRESS project EES.5653

7. Link layer approach Sender Application/ encoder TCP/IP stack
MAC-level retries
video stream
driver/scheduler buffer
IP packets
TCP/IP stack
Explain here what the MAC retries means, how it influences the fullness of the scheduler buffer, how the jitter occurs.
Retries parameter lets us specify the trade-off between reliability of the wireless link and it’s throughput
18-Sep-18
Sergei N. Kozlov,
TU/e Informatica, System Architecture and Networking
PROGRESS project EES.5653

8. I BBPBBPBB(I) MPEG encoding GOP (group-of-pictures):
Frame types: I, P, B
Typical GOP structure and dependences:
I BBPBBPBB(I)
So, losing B frames is less undesirable than P frames. Same with P and I accordingly.
Importance of a frame decreases in following order: I, P, B
18-Sep-18
Sergei N. Kozlov,
TU/e Informatica, System Architecture and Networking
PROGRESS project EES.5653

9. Importance of I and P frames
Missing of I/P frames causes a lot of artefacts:
A complete stream only missing B frames has no artefacts
in worst case you only get then 8.3 fps instead of 25
18-Sep-18
Sergei N. Kozlov,
TU/e Informatica, System Architecture and Networking
PROGRESS project EES.5653

10. Example of an mpeg stream
5Mpbs stream (LOTR)
B-frames make up more than a half of the whole bitrate (i.e. >2.5Mbps)
18-Sep-18
Sergei N. Kozlov,
TU/e Informatica, System Architecture and Networking
PROGRESS project EES.5653

11. Scheduling of frames (I)
Packetizing
Assign prioritites to packets according to frame types:
P(B) = 1
P(P) = 2
P(I) = 3
Scheduling
Packets with higher priorities should get a better chance to be sent
Involved layers
application layer
transport layer
network layer
driver
link layer
IP (Internet Protocol)
TCP, UDP/RTP
MPEG en-/decoder
packet scheduler
physical layer
WLAN
MPEG-packetizer
18-Sep-18
Sergei N. Kozlov,
TU/e Informatica, System Architecture and Networking
PROGRESS project EES.5653

12. Scheduling of frames (II)
Scheduling algorithm
incoming frame
buffered frame
sent frame Fi Fb Fs
if P(Fi) > P(Fs) and P(Fb) > P(Fs)
Fs := Fb, Fb := Fi (discard Fs)
else
if P(Fi) >= P(Fb)
Fb := Fi (discard Fb)
Fi := 0 (discard Fi)
18-Sep-18
Sergei N. Kozlov,
TU/e Informatica, System Architecture and Networking
PROGRESS project EES.5653

13. Advantages of link layer approach
We only need to modify the sending part
it will work with any terminals supporting RTP reception
it can be used for broadcasting
it is very reactive against fast network fluctuations
18-Sep-18
Sergei N. Kozlov,
TU/e Informatica, System Architecture and Networking
PROGRESS project EES.5653

14. Transport layer approach
If...
we don’t have direct access to the wireless interface
we don’t want to modify it
we want more reliability, than UDP/RTP does
or we want to combine link layer and transport layer approaches
Then we would like to look at the tranport layer
Our main interest: Transport Control Protocol (TCP)
Can we make it suitable for real-time?
Can we make it suitable for wireless?
18-Sep-18
Sergei N. Kozlov,
TU/e Informatica, System Architecture and Networking
PROGRESS project EES.5653

15. Further work Link-layer & transport-layer creating math. model
evaluation of the approach
gathering statistics
evaluation of perceived quality
18-Sep-18
Sergei N. Kozlov,
TU/e Informatica, System Architecture and Networking
PROGRESS project EES.5653

16. Questions? 18-Sep-18 Sergei N. Kozlov, s.n.kozlov@tue.nl
TU/e Informatica, System Architecture and Networking
PROGRESS project EES.5653