3GPP2 Evolution Workshop - 2005Multimedia Codecs and Protocols

3GPP2 TSG-C SWG1.2

Multimedia in PS Domain

•Transitioning from CS to PS – Shared channels as opposed to dedicated channels– User expectation: Similar to that with dedicated

channels– Application Level Framing (ALF) in IP

– Requires special considerations for wireless hops

•Codec and transport related challenges– IP fragmentation– Jitter in packet arrival– Codecs’ view of QoS:

– Greater need for error resiliency and concealment– …

TSG-C SWG1.2 “Multimedia Services” ToR

• Evaluation and adoption of – Visual Codecs (Video, Image, Graphics),– Audio Codecs (synthetic and natural audio)– Audio/visual synchronization methods

• Evaluation and adoption of complementary protocols– RTSP/SDP, RTP/RTCP, to support real-time, near real-time

and interactive multimedia services • Development of fileformats to support

– over the air transmission for streaming, pseudo-streaming, downloading

– associated methods for content protection

IP Terminology

• ADU – Application Data Unit– Independently decodable data; minimum unit of error recovery– e.g. a vocoder frame, a video slice, …

• SDU – Service Data Unit– One or more ADUs, including required headers for transmission

• MTU – Maximum (size of a) Transmission Unit, in octets– e.g. 1500 octets on an Ethernet hop, 576 octets on PPP, …

• Fragmentation and Re-assembly – 1 SDU n PDU mapping, when SDU exceeds MTU on a given hop

• PDU – Protocol Data Unit– In this context, we consider physical layer frames– payload size in one physical layer frame

– Not including MAC, CRC and tail bits overhead

Observation: Significant fragmentation over cdma2000® hops

IP Fragmentation

“Goodput” above RTP

• From RFC2736, Best Current Practice for RTP payload design,– … relying on IP fragmentation is a bad design strategy as it significantly

increases the effective loss rate of a network and decreases goodput– … if one fragment is lost, the remaining fragments (which have used up

bottleneck bandwidth) will then need to be discarded by the receiver• Pick ADU/SDU sizes not to exceed PDU sizes• Do not concatenate SDUs (use stuffing bits)

Link Layer Packets: PDU i ... PDU n

Application Packets: SDU 2 SDU 1 ...

What is optimal ADU size?

• One size does not fit all– Except for Constant bitrate (CBR) Channels and CBR codecs

• Most 3G networks provide multiple PDU sizes to support “Bandwidth on Demand”

• Entropy in multimedia content (video, speech, Audio, …) is not constant over a TTI

– Variable bitrate (VBR) encoding: improved compression efficiency• Examples

– cdma2000 vocoders– Channel: RS-1 and RS-2 support 4 PDU sizes in addition to dtx– Source: cdma2000 vocoders generate 4 ADU sizes, based on source characteristics– SO60: ADU = PDU; CS like efficiency in PS!

– ITU and MPEG Video codecs– VBR rate control constrained to generate ADUs of known size – Improved error resiliency when ADUs are matched to PDUs

• Significant statistical multiplexing gains when codecs are optimized to lower layers

• Challenges: Optimize 3G services, without cross-layer violations and maintain interoperability with other networks

Jitter in Packet Arrival

Jitter in PS networks

• Variance in the ADU inter-arrival time– SDUs queued behind cross-traffic in routers– Routing changes in end-to-end path

• De-jitter buffer – a buffer to restore constant inter-packet timing prior to decoding– Increases playout delay

20ms De-jitter bufferDe-jitter buffer DecoderDecoder

Even delivery of Even delivery of voice framesvoice frames

20ms of voice20ms of voice per frameper frame

20ms

Variable Delay, in-order delivery not Variable Delay, in-order delivery not guaranteedguaranteed

Example: Jitter in packet arrival times

End2end Packet Delay

0

50

100

150

200

250

300

350

400

1 56 111 166 221 276 331 386 441 496 551 606 661 716 771 826 881 936 991 1046 1101

Frame Count

Del

ay (m

s)

Example: Packet Loss due to Delay bound

0

0.5

1

1.5

2

2.5

1 168 335 502 669 836 1003 1170 1337 1504 1671 1838 2005 2172 2339 2506 2673 2840AT Index

PER

Total PERDelayed PER

Smart Decoders

• Reduce playout delay with variable rate rendering– Speech: Signal processing tricks such as “Time Warping”– Video: Audio visual synchronization (lip-synch)

• Improved concealment techniques to mitigate packet loss– Interpolation in time and/or frequency domains

• Challenges– Develop improved objective metrics and subjective evaluation methodologies– Study and characterize the effects of such enhancements on user experience

GoodGoodErasureErasureGoodGood

Playback Time linePlayback Time line

CircuitCircuit

VoIPVoIP

RTP Packet Loss Simulator

• SWG1.2 Simulation methodology to characterize multimedia content in cdma2000 wireless IP

Video/Audio Codec

Cdma2000/WCDMA

IP

UDP

RTP

Video/Audio Codec

Cdma2000/WCDMA

IP

UDP

RTP

MS MS or Content Server

RTP Packet Loss S/W

BitratePDU SizesError Masks

Proposed Video Objective MetricsForeman example

 Avg PSNR

[dB]STD PSNR

[dB] pDVD [%]

Bitrate [Kbps

]Clean 32.35 0.80 NA 64.69

1.5% FER : Scheme A 26.92 7.09 45.67 64.691.5% FER : Scheme B 28.98 6.11 35.67 66.601.5% FER : Scheme C 32.03 2.57 3.67 67.111.5% FER : Scheme D 31.57 3.51 8.33 67.86

•Percentage Degraded Video Duration (pDVD)•Standard deviation of PSNR (STD_PSNR)

PSNR Traces: Foreman

Objctive Metrics comparison: Foreman QCIF 300 frames

0

5

10

15

20

25

30

35

40

1 11 21 31 41 51 61 71 81 91 101 111 121 131 141 151 161 171 181 191 201 211 221 231 241 251 261 271 281 291

Frame #

PS

NR

[dB

]

Clean

1.5% FER A

1.5% FER B

1.5% FER C

1.5% FER D

Summary

• Optimize codecs and bearer protocols to lower layers for efficient IP services

– e.g. VBR codecs and VBR channels in 3GPP2 CS voice– Rate control and RTP packetization to maximize “goodput”– Interoperability with other IP networks (cross-layer issues)

• Develop smart codecs – Improved error resiliency – Better concealment techniques– Variable rate rendering

• Develop/adopt suitable objective metrics and subjective evaluation methodologies

– Characterize user experience in realistic 3GPP2 environments