Selective Retransmission of MPEG Video Streams over IP

Networks

Árpád Huszák, Sándor Imre

Budapest University of Technology and Economics Department of Telecommunications

Mobile Communications and Computing Laboratory Phoenix project

Motivation Multimedia streaming is becoming increasingly

popular in IP networks New wireless technologies

3G, 4G mobile networks WiMAX WLAN

Two major issues arise in spite of the popularity of multimedia applications in mobile networks limited bandwidth restricts high bit rate video

transmission wireless transmission by its nature may introduce

higher rate of errors during transmission

Introduction Multimedia applications in mobile networks

Transmission over unreliable wireless channels The frequent errors should cause deterioration of

the quality of multimedia streams Delay and packet loss caused by handovers

New method to increase the quality of audio/video streams in mobile environment Differentiation of packets within a media stream Selective retransmission of important packets

Selective retransmission Differentiation of packets within an MPEG media stream MPEG frames

(I) Intra frame coded, key-frame (max. priority) (P) Predicted (B) Bidirectional

Error in a key-frame (I) propagates to all other frames till the next key-frame It is advisable to handle these frames on a different way Retransmission of the damaged part of the key-frame will

significantly raise the quality of the video stream

Selective retransmission Transport protocol for selective retransmission

Unreliable (no automatic retransmission) Packet numbers Acknowledgements Information on lost packets Information about the network (RTT) Congestion control (TCP-like, TFRC)

DCCP RFC4340, March 2006 think of DCCP as TCP minus bytestream semantics and

reliability or as UDP plus congestion control, handshakes, and

acknowledgements

DCCP

Datagram Congestion

Control Protocol

Selective retransmission scheme Packet identifying

Temporal storage of dedicated packets in buffers Delete only when acknowledgement arrives

Each DCCP packet has an individual sequence number that makes possible to detect packet losses

Buffer 2

Buffer 1

Channel(pbit)

Packetizer-packet length-header (IP,DCCP)

Depaketizer

MPEG fragmentatio

n(I,P,B)

Checksum calculation

(retransmission request)

MPEG

MPEG

Examination The packet drop probability

The retransmission probability is calculated as follows

In case of interactive media the RTT must be analyzed to decide whether the total delay is acceptable. In our solution the delay is

( )( )1 (1 )

PS Hsize I H

size GoP

packetloss bitp p

( )( )

( )(1 (1 ) )

( )

PS Hsize I H

size GoP

retransmission bit

size Ip p

size GoP

3

2delay transmission retransmissiont t t RTT

Examination

The retransmission probability in the function of size(I)/size(GoP) ratio according to the

actual channel bit-error probability

Retransmission probability in function of bit-error probability

Examination Comparison of UDP, UDPLite and DCCP Retransmission

Algorithm

There are N frames in a GoP, therefore single I-frame bit correction leads to the improvement of N bits increasing the MPEG quality significantly

Testbed

DCCP Selective Retransmission testbed Linux 2.6.14 Client-Server side applications Adjustable packet loss ratio

Results

Visible difference in MPEG video quality Our algorithm radically reduces the bit corruption

ratio and achieves significant increase of quality

Original Without Sel. Retr. With Sel. Retr.

Results MPEG - peak-to-peak signal-to-noise ratio (PSNR) [dB]

3-4 dB gain was achieved with our Selective Retransmission method

0

5

10

15

20

25

30

35

40

1 36 71 106 141 176 211 246 281 316 351 386 421 456 491 526 561 596 631 666

frames

PS

NR

[d

B]

with Selective Retransmission

without Selective Retransmission

Future plans The additional delay caused by our method

should be examined Time and bandwidth limitations should be

examined The algorithm can be extended to allow

multiple retransmissions realizing TCP-like reliable transmission

The evaluations were done on MPEG streams, but the selective retransmission algorithm is capable for other data type transmissions where the data can be differentiated

Conclusion

In this presentation a new selective retransmission algorithm was presented and studied

The results obtained show that the algorithm radically reduces the bit corruption ratio achieves significant increase of quality

especially in wireless networks with high bit-error ratio

Thank you for your attention!