page_title: Network Abstraction Layer
section_title: Access Units
hierarchical_section_title: Network Abstraction Layer / Access Units
caption_attribution_description: English: Structure of a NAL Access Unit
context_page_description: The Network Abstraction Layer is a part of the H.264/AVC and HEVC video coding standards. The main goal of the NAL is the provision of a "network-friendly" video representation addressing "conversational" and "non conversational" applications. NAL has achieved a significant improvement in application flexibility relative to prior video coding standards.
context_section_description: A set of NAL units in a specified form is referred to as an access unit. The decoding of each access unit results in one decoded picture.
  Each access unit contains a set of VCL NAL units that together compose a primary coded picture. It may also be prefixed with an access unit delimiter to aid in locating the start of the access unit. Some supplemental enhancement information containing data such as picture timing information may also precede the primary coded picture.
  The primary coded picture consists of a set of VCL NAL units consisting of slices or  slice data partitions that represent the samples of the video picture.
  Following the primary coded picture may be some additional VCL NAL units that contain redundant representations of areas of the same video picture. These are referred to as redundant coded pictures, and are available for use by a decoder in recovering from loss or corruption of the data in the primary coded pictures. Decoders are not required to decode redundant coded pictures if they are present.
  Finally, if the coded picture is the last picture of a coded video sequence (a sequence of pictures that is independently decodable and uses only one sequence parameter set), an end of sequence NAL unit may be present to indicate the end of the sequence; and if the coded picture is the last coded picture in the entire NAL unit stream, an end of stream NAL unit may be present to indicate that the stream is ending.
caption: Structure of a NAL Access Unit
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