dual access units within elementary streams. The synchronization layer manages the identification of access units and manages the time stamping. Independent of the media type, this layer allows identification of the type of access unit (e.g., video or audio frames, scene description commands) in elementary streams, and the recovery of the media object's or scene description's time base. It also enables synchronization among these. Video Communications: The Whole Picture The MPEG-4 ISO/IEC 14496 volume consists of four basic functions: Delivery Multimedia Integration Framework (DMIF) DMIF is a session protocol for the management of multimedia streaming over generic delivery technologies. It is similar to FTP except that, where FTP returns data, DMIF returns pointers toward (streamed) data. DMIF is both a framework and a protocol. The functionality provided by DMIF is expressed by an interface called DMIF-Application Interface (DAI), and translated into protocol messages that may differ based on any given network on which they operate. MPEG-4 specifies an interface to the TransMux (Transport Multiplexing) models, the layer that offers transport services for matching the requested QoS. Any suitable existing transport protocol stack (e.g., RTP in UDP/IP, AAL5 in ATM, or Transport Stream in MPEG-2) over a suitable link layer may become a specific TransMux instance. The synchronized delivery of streaming data may require the use of different QoS schemes as it traverses multiple public and private networks. MPEG defines the delivery-layer FlexMux multiplexing tool to allow grouping of Elementary Streams (ES) with low multiplexing overhead. It may be used to group ESs with similar QoS requirements, to reduce the number of network connections, or to reduce end-to-end delay. The FlexMux layer may be empty if the underlying TransMux instance provides adequate functionality. The DMIF framework enables control functions such as the ability to identify access units, to transport timestamps and clock reference information, and 