Today's multimedia networks have to meet a vast range of requirements. The network must provide mechanisms to transfer real-time control data, send packet data, and transmit audio and video data efficiently. A great number of components must be efficiently networked also from a topological point of view. The components, on their part, can function as data source as well as data sink. In a multimedia network, special attention must be given to the quality of audio and video applications. For automakers, extremely tough requirements additionally apply to the electromagnetic compatibility of the physical layer.
The de-facto standard, now established in all areas requiring networked infotainment components, is MOST (Media Oriented Systems Transport). The philosophy behind MOST in transferring audio and video data lies in reserving a dedicated channel with the necessary bandwidth. This is the only way to guarantee the highest quality of service (QoS).
Until now, this was available solely to transmission of synchronous data. Under MOST25, for example, an MPEG data stream was artificially "stuffed" with bits, or the stream was converted (transcoded) to a fixed bit rate. Sample rate converters are sometimes used for audio applications to adapt PCM audio data to the MOST frame rate. However, not all audio or video data streams can be simply synchronized to this MOST time base. To support transmission of this type of data in accordance with the MOST philosophy, MOST150 introduced isochronous transfer. Isochronous channels are handled in much the same way as synchronous channels in MOST: the required bandwidth is firmly reserved, which means channels for isochronous data are allocated, and the data is routed as needed.
MOST150 can handle three different isochronous mechanisms designed for different applications.
A/V Packetized Isochronous Streaming
This mechanism permits transfers of video data streams that arrive without any reference to the MOST time frame. The data is already consolidated in small data packets, which can optionally include a time stamp. MPEG data streams that are coded either with variable (VBR) or constant bit rates (CBR) are a typical example of this. A MOST transfer initially reserves the maximum isochronous bandwidth required. The data arriving at the network interface controller (MOST150 INIC) is transferred automatically by INIC to internal memory whose content is transferred cyclically via MOST. The network interface controller of a data sink can copy the isochronous data from the bus to its internal memory, and pass it via a suitable interface on to the external hardware - an MPEG decoder, for example. The whole handling of MPEG data is encapsulated completely in the MOST150 INIC, so that all the application has to do is reserve the required isochronous bandwidth and configure the corresponding connections.
