Figure 1 shows the functional components of a PSS client. Figure 2 gives an overview of the protocol stack used in a PSS client and also shows a more detailed view of the packet based network interface. The functional components can be divided into control, scene description, media codecs and the transport of media and control data. The control related elements are session establishment, capability exchange and session control (see clause 5).

• Session establishment refers to methods to invoke a PSS session from a browser or directly by entering an URL in the terminal's user interface.
• Capability exchange enables choice or adaptation of media streams depending on different terminal capabilities.
• Session control deals with the set-up of the individual media streams between a PSS client and one or several PSS servers. It also enables control of the individual media streams by the user. It may involve VCR-like presentation control functions like start, pause, fast forward and stop of a media presentation.

The scene description consists of spatial layout and a description of the temporal relation between different media that is included in the media presentation. The first gives the layout of different media components on the screen and the latter controls the synchronisation of the different media (see clause 8). The PSS includes media codecs for video, still images, vector graphics, bitmap graphics, text, timed text, natural and synthetic audio, and speech (see clause 7). Transport of media and control data consists of the encapsulation of the coded media and control data in a transport protocol (see clause 6). This is shown in figure 1 as the "packet based network interface" and displayed in more detail in the protocol stack of figure 2.

The delivery of media over HTTP provides an alternative delivery mechanism to the RTSP/RTP based media delivery. HTTP Progressive download and Dynamic Adaptive Streaming over HTTP (3GP-DASH) are described in TS 26.247.