This Recommendation | International Standard specifies the system layer of the coding. It was developed principally to support the combination of the video and audio coding methods defined in Parts 2 and 3 of ISO/IEC 13818. The system layer supports six basic functions: 1) the synchronization of multiple compressed streams on decoding; 2) the interleaving of multiple compressed streams into a single stream; 3) the initialization of buffering for decoding start up; 4) continuous buffer management; 5) time identification; 6) multiplexing and signalling of various components in a system stream. A Rec. ITU-T H.222.0 | ISO/IEC 13818-1 multiplexed bit stream is either a transport stream or a program stream. Both streams are constructed from PES packets and packets containing other necessary information. Both stream types support multiplexing of video and audio compressed streams from one program with a common time base. The transport stream additionally supports the multiplexing of video and audio compressed streams from multiple programs with independent time bases. For almost error-free environments the program stream is generally more appropriate@ supporting software processing of program information. The transport stream is more suitable for use in environments where errors are likely. A Rec. ITU-T H.222.0 | ISO/IEC 13818-1 multiplexed bit stream@ whether a transport stream or a program stream@ is constructed in two layers: the outermost layer is the system layer@ and the innermost is the compression layer. The system layer provides the functions necessary for using one or more compressed data streams in a system. The video and audio parts of this Specification define the compression coding layer for audio and video data. Coding of other types of data is not defined by this Specification@ but is supported by the system layer provided that the other types of data adhere to the constraints defined in 2.7.