無線與行動多媒體網路Wireless and Mobile Communication Networks

國立中山大學 電機系

許蒼嶺 教授

Multimedia Traffic Characteristics

Traffic Types Traditional Data Traffic Image Voice, High-Quality Sound Full Motion Video Interactive Multimedia

Data Traffic

Interactive Transactions Banking, Airline Reservation, Insurance Processing 30 Transactions per sec for 1000 Terminals Very Stable and Predictable Traffic Patterns

File Transfer From a Few Kbytes to Hundred Million Bytes Uniform, Regular Transmission of Data at a Constant Rate

Interactive LAN Data Traffic Interactive Response May be Relatively Sensitive to Network Delay

Real Issues for Data Traffic Flow/Congestion Control Error Recovery or Re-Transmission

IMAGE Traffic

Major Difference from Data Traffic Very Large Block Transfer Response Time Between 1 - 5 sec

Transporting Image Compression Ratio of 4 to 1 on Average Network Load can be Reduced

On-Line Books with Illustrations Consecutive Display of Many Images with 1 - 2 sec Apart Unplanned Load on Network

Voice and High-Quality Sound

Voice Traffic PCM Encoding Gives 64 Kbps Acceptable End-to-End Delay is about 90 msec Variable-Rate Coding

– Half Duplex– About 60% of One-Way Voice Conversation Consists of Silence

High-Quality Sound CD Quality Stereo (5 Channels) Involves Very High Bit Rate Continuous (Such as Film Soundtrack) Delay Does not Matter as Much as Voice

Full-Motion Video

A Sequence of Pictures or Frames NTSC ： 525 Lines at 30 Frames per sec PAL ： 625 Lines at 25 Frames per sec

High Compression Ratio Very Little Different From the Frame Before MPEG-2 ： I, B, and P Frames

Timing Considerations Jitter May Affect Presentation Quality

Network Considerations Sufficient Bandwidth via Reservation Acceptable End-to-End Delay

SVC: Scalable Video Coding Three Types of SVC

Spatial SVC: based on resolution Temporal SVC: based on frame rates Quality SVC: based on PSNR (Peak SNR)

Granularity of SVC Fine Grain Medium Grain Coarse Grain

SVC: VCL and NAL

H.264/AVC and SVC

SVC 保留了 H.264/AVC 的 Network Abstraction Layer （ NAL ） NAL units 是 SVC bit stream 的基本結構 NAL 是影片經過 Encoding 後的一段一段資料封包 (Packet) ，此時

的 Packet 還沒有加上 MAC 、 IP 、 UDP 、 RTP 等網路協定的 Headers

NAL 可分成 VCL （ Video Coding Layer ） NAL-unit non-VCL NAL-unit

SVC Bit Stream 在 SVC bit stream 中，一開始出現的 NAL 是影片解碼資訊

SPS （ Sequence Parameter Set) PPS （ Picture Parameter Set)

最後才是影片內容（ Payload) 在每個 NAL 前都會有一個 NAL Header ，參照此 Header 可得知此

Payload 是屬於 Base layer 或 Enhancement layer 的那一層 NAL Header 的內容與欄位用法 :

NUT （ NAL unit type , 2 bits) ，用來指出目前 NAL 的種類。 DID （ Dependency id , 3 bits ），代表畫面大小 (Spatial ) 。 QID （ Quality id, 4 bits ），代表畫面品質 (Quality) 。 TID （ Temporal id, 3 bits ），代表播放速率 (Frame Rate) 。

RTP Packetization

RFC 3984 定義了三種 RTP Packetization Modes ： Single NAL unit Mode Non-interleaved Mode Interleaved Mode

Single NAL unit Mode 代表 RTP packet 中只包含一個 NAL unit 。 Non-interleaved Mode 代表 RTP packet 中的 NAL 必須依照撥放先後

之順序從發送端送出。 Interleaved Mode 則表示 RTP packet 中的 NAL 可不依照播放順序

送出。