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4. Definitions and Abbreviations

4. Definitions and Abbreviations​

4.1. Definitions​

This document uses the definitions of [1]. The following terms, defined in [1], are summed up for convenience:

access unit: A set of NAL units always containing a primary coded picture. In addition to the primary coded picture, an access unit may also contain one or more redundant coded pictures or other NAL units not containing slices or slice data partitions of a coded picture. The decoding of an access unit always results in a decoded picture.

coded video sequence: A sequence of access units that consists, in decoding order, of an instantaneous decoding refresh (IDR) access unit followed by zero or more non-IDR access units including all subsequent access units up to but not including any subsequent IDR access unit.

IDR access unit: An access unit in which the primary coded picture is an IDR picture.

IDR picture: A coded picture containing only slices with I or SI slice types that causes a "reset" in the decoding process. After the decoding of an IDR picture, all following coded pictures in decoding order can be decoded without inter prediction from any picture decoded prior to the IDR picture.

primary coded picture: The coded representation of a picture to be used by the decoding process for a bitstream conforming to H.264. The primary coded picture contains all macroblocks of the picture.

redundant coded picture: A coded representation of a picture or a part of a picture. The content of a redundant coded picture shall not be used by the decoding process for a bitstream conforming to H.264. The content of a redundant coded picture may be used by the decoding process for a bitstream that contains errors or losses.

VCL NAL unit: A collective term used to refer to coded slice and coded data partition NAL units.

In addition, the following definitions apply:

decoding order number (DON): A field in the payload structure or a derived variable indicating NAL unit decoding order. Values of DON are in the range of 0 to 65535, inclusive. After reaching the maximum value, the value of DON wraps around to 0.

NAL unit decoding order: A NAL unit order that conforms to the constraints on NAL unit order given in Section 7.4.1.2 in [1].

NALU-time: The value that the RTP timestamp would have if the NAL unit would be transported in its own RTP packet.

transmission order: The order of packets in ascending RTP sequence number order (in modulo arithmetic). Within an aggregation packet, the NAL unit transmission order is the same as the order of appearance of NAL units in the packet.

media-aware network element (MANE): A network element, such as a middlebox or application layer gateway that is capable of parsing certain aspects of the RTP payload headers or the RTP payload and reacting to the contents.

Informative note: The concept of a MANE goes beyond normal routers or gateways in that a MANE has to be aware of the signaling (e.g., to learn about the payload type mappings of the media streams) and that it has to be trusted when working with Secure Real-time Transport Protocol (SRTP). The advantage of using MANEs is that they allow packets to be dropped according to the needs of the media coding. For example, if a MANE has to drop packets due to congestion on a certain link, it can identify and remove those packets whose elimination produces the least adverse effect on the user experience.

static macroblock: A certain amount of macroblocks in the video stream can be defined as static, as defined in Section 8.3.2.8 in [3]. Static macroblocks free up additional processing cycles for the handling of non-static macroblocks. Based on a given amount of video processing resources and a given resolution, a higher number of static macroblocks enables a correspondingly higher frame rate.

default sub-profile: The subset of coding tools, which may be all coding tools of one profile or the common subset of coding tools of more than one profile, indicated by the profile-level-id parameter.

default level: The level indicated by the profile-level-id parameter, which consists of three octets, profile_idc, profile-iop, and level_idc. The default level is indicated by level_idc in most cases, and, in some cases, additionally by profile-iop.

4.2. Abbreviations​

  • DON: Decoding Order Number
  • DONB: Decoding Order Number Base
  • DOND: Decoding Order Number Difference
  • FEC: Forward Error Correction
  • FU: Fragmentation Unit
  • IDR: Instantaneous Decoding Refresh
  • IEC: International Electrotechnical Commission
  • ISO: International Organization for Standardization
  • ITU-T: International Telecommunication Union, Telecommunication Standardization Sector
  • MANE: Media-Aware Network Element
  • MTAP: Multi-Time Aggregation Packet
  • MTAP16: MTAP with 16-bit timestamp offset
  • MTAP24: MTAP with 24-bit timestamp offset
  • NAL: Network Abstraction Layer
  • NALU: NAL Unit
  • SAR: Sample Aspect Ratio
  • SEI: Supplemental Enhancement Information
  • STAP: Single-Time Aggregation Packet
  • STAP-A: STAP type A
  • STAP-B: STAP type B
  • TS: Timestamp
  • VCL: Video Coding Layer
  • VUI: Video Usability Information
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