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Media Oriented Systems Transport (MOST)

The MOST specification not only defines the Physical Layer and Data Link Layer; it covers all seven layers of the ISO/OSI Reference Model for data communication. Uniform interfaces simplify implementation of the MOST protocol in multimedia devices. For the application developer MOST serves primarily as a protocol definition. The user gets a uniform object-oriented interface (Application Programmer Interface - API) which allows device functionality to be utilized.

Communication functionality is provided by MOST System Services: Low-Level System Services, Basic-Layer System Services and the Application Socket. Low-Level System Services (Layer 2 services) are implemented in a MOST transceiver and are based on the Physical Layer. Wrapped between the MOST transceiver and the API (Layer 7) are the MOST NetServices, consisting of the Basic-Layer System Services (Layers 3,4,5) and the Application Socket (Layer 6), the MOST protocol.

A MOST network, usually laid out as a ring, may include up to 64 MOST devices. Due to its plug&play functionality it is not very difficult to either add or remove a MOST device. Given suitable topological conditions stars may also be laid out - and double rings may be used for safety critical applications.

In a MOST network one MOST device handles the role of the Timing Master which continuously feeds MOST frames into the ring. The preamble sent out at the beginning of the transmission of a MOST frame allows the Timing Slaves to synchronize. The Timing Slaves can continuously resynchronize themselves to the biphase coding that underlies the synchronous transmission.

In total the bandwidth available for the transmission of streaming data (synchronous data transmission) and packet data (asynchronous data transmission) is approx. 23 MBaud. This is subdivided into 60 physical channels which the user can select and configure. MOST provides numerous services and mechanisms for the allocation (and deallocation) of physical channels.

MOST supports up to 15 uncompressed stereo audio channels in CD quality or up to 15 MPEG1 channels for audio-video transmission. However it is still not possible to transmit high-resolution uncompressed video data streams with MOST.

Simultaneously MOST offers a channel for transmitting control messages. A bandwidth of 768 kBaud is available for this purpose, or in other words: Almost 3000 control messages can be transmitted per second. These control messages can be used to configure MOST devices as well as the synchronous and asynchronous data transmission.

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