4주차 목표

• H.264와 RTSP에 대한 조사 및 공부

회고

RTP

RTP란?

• Real Time Transport Protocol
• Server → Client one-way transmission
• Packet loss is possible because it is transmitted through UDP

• RTP header structure

  

  V (Version, 2 bits): Usually 2.

  P (Padding bit, 1 bit): Padding bit is added at the end of the header if P is 1.

  X (Extension, 1 bit): The presence of an extension header.

  1 indicates that the extension header is followed

  CC (CSRC count, 4 bits): The number of CSRCs.

  M (Marker bit, 1 bit): ???????????

  PT (Payload Type , 7 bits): The type of data contained in payload.

  Client check this and decide how to play.(Video, music file, etc)

  Sequence Number(16 bits): Number for guaranteeing order of packets.

  Increasing by 1 each time a packet is sent.

  Initial value is random

  Timestamp (32 bits): The timing when the packet is played.

  Put packets in a buffer of a certain size and run according to the timestamp.

  Used to sync.

  SSRC (SSRC ID, 32 bits): ID of the media stream source.

  CSRC (CSRC IDs, 32*CC bits): Id of sources utilized in the media.

RTSP

RTSP란?

• Real Time Steaming Protocol
• Application layer protocol for controlling real-time data transmission.
• Using port 554.
• It is text-based in the Request/Response method → similar with HTTP
• Both TCP and UDP are available, but typically TCP is used.
• It does not transmit streaming data itself. It is used for controlling streaming media servers.

• Stream Control example.

  

• It has 11 instruction.

  1. OPTION: Request a supported method from the server.

     (ex. Response: DESCRIBE, SETUP, PLAY, PAUSE, TEARDOWN, OPTIONS)

  2. DESCRIBE: Request media information. The response comes in the form of SDP.
  3. SETUP: Request a definition of media information delivery method(session number, etc)
  4. PLAY: Play the media.
  5. PAUSE: Pause the media.
  6. RECORD: Record the media.
  7. ANNOUNCE: Request real-time media description update.
  8. TEARDOWN: End media session.
  9. GET_PARAMETER: Request the parameter values.
  10. SET_PARAMETER: Set the parameter values.
  11. REDIRECT: Request the server that should move.

H.264

H.264란?

• Standard of Video Compression
• = AVC(Advanced Video Coding), MPEG-4 part 10

특징

• Based on block unit motion compensation.
• Very high data compression rate. The transmission speed is 60% faster than the MPEG-2.
  • MPEG-1: Maximum resolution is 352*288
  • MPEG-2: Maximum resolution is 1920*1152
  • MPEG-4: It has a similar image quality with MPEG-2 but bandwidth is low.
• But, encoding time is increased compared to before.

Profile & Level

• Profile: It allows users to use only certain technologies depending on the area of use.

  

  ex) Baseline: Low power, low resolution, low delay

      Main: High compression rate using B Picture
  

• Level: It allows detailed settings for each profile.

Stream 구조

• NAL (Network Abstraction Layer): NAL unit + Payloads,

  Since H.264 was developed from the beginning for network transmission, it is composed of NAL for efficient transmission.

• RBSP: Raw Byte Sequence Payload
• SPS (Sequence Parameter Set): Contain parameter data like Profile, level, resolution, etc.
• PPS (Picture Parameter Set): Similar with SPS, but contains more specific data.
• Coded Slice of an IDR (or non-IDR) Pic: Image data
  • I-slice: Frame that contains all of data.
  • P-slice: Frame with motion prediction data referring to the previous data.
  • B-slice: Frame with motion prediction data referring to the previous and next data.

H.264 vs H.265

• H.264 use 1616 pixel macro block, but H.265 use 6464 pixel macro block.

  → Higher encoding efficiency.

• Better intra frame prediction. H.264 allows only 9 motion direction, but H.265 allows 33 motion direction.
• But, H.265 is not supported everywhere.