MOST Infotainment Systems

MOST (Media Oriented System Transport) is a communication system with a flexible architecture designed specifically for automotive infotainment systems. This section gives an overview of MOST, a more in-depth description is given in “MOST - The Automotive Multimedia Network” (Grzemba 2008). The detailed specifications for the network and network interface controller used in this work are contained in MOST Specification Rev 2.4 (MOST Cooperation, 2005) and OS8104 MOST Network Transceiver Final Product Data Sheet (Oasis, 2003).

Growing infotainment system complexity, particularly multiple audio and video channels causing excessive amounts of wiring, lead to the need for a high bandwidth network. The key requirements for this network are that it should be capable of the distribution of multiple digital audio channels, have a high immunity to noise and be highly flexible and reconfigurable. As a result vehicle manufacturers and suppliers formed the MOST Cooperation to develop common standards for such a network. MOST was first applied to the BMW 7 Series in 2001 and is now used by BMW, Mercedes, VW/Audi, Toyota, Volvo, Porsche, Jaguar Land Rover and Hyundai in over 100 vehicle models. MOST is now available in three bandwidth variants, 25Mbits/sec (MOST25), 50Mbits/sec (MOST50) and 150Mbits/sec (MOST150), with the increase in bandwidth supporting video networking and high definition audio.

MOST is effectively a composite bus able to transmit three types of information with different requirements. The first is control data necessary for command signals, status signals and small packet data. This needs comparatively limited bandwidth, message arbitration and low latency. The second data type is synchronous data for continuous real-time transmission of streaming audio and video files. This needs large amounts of guaranteed bandwidth with minimal buffering. The third data type is asynchronous data for transmission of sporadic bursts of ‘large’ packet data e.g. contacts lists, navigation data or web pages. This needs intermittent access to a variable bandwidth channel.

To achieve a sufficiently high bandwidth, noise immune communication MOST typically uses an optical medium to transfer data with an eye-safe LED light-source linked by Plastic Optical Fiber (POF) in a ring topology. To accommodate different types of data the MOST data frame is split into three sections with an ability to dynamically re-apportion bandwidth between synchronous and asynchronous data. The MOST topology and frame

Figure 5.1 MOST topology and frame structure

The operation of the bus is controlled by the bus master which generates a frame that travels from node to node around the ring. Each node receives information and updates any of its own information with a 2 bit delay per node in transmission. Each frame is 512 bits so the bus master receives back the start of the frame while still sending the frame.

Each MOST node has a “Network Interface Controller” (NIC) which provides the low level interface to the bus via a Fiber Optic Transceiver (FOT). Each node must also implement a standard set of communication software called MOST NetServices which control communication from the application software to the network interface controller. This abstracts control to application level peer to peer communications via standard interface definitions known as Fblocks (short for function blocks). It is vital that the network interface controllers and NetServices implemented on all nodes are compatible.

For the robustness of MOST-based infotainment systems the initialisation is a key phase which is influenced by the status of the individual MOST nodes and the interactions between them on the control channel. MOST allows features to be dynamically registered, typically at start-up, through a process of each node registering its function blocks and then

compiling and distributing a registry of which functions each node contains. Following this the nodes can set up peer-to-peer interfaces. If new functions are registered or de-registered during or after the initialisation nodes are notified of this configuration change but this can be disruptive during initialisation.

Hence initialisation is the busiest period of MOST control channel communication, and becoming increasingly challenging to optimise as the number of customer features, and therefore number of MOST function blocks, increases. There are key customer requirements to satisfy, not only that the system starts up in a robust manner with continuity of correct service (i.e. reliability), but that certain infotainment and comfort features are available as soon as the vehicle is started (i.e. availability).

Types of service failures which could occur and must be mitigated in the system design include; excessive delay before audio sources are available, missing customer features, and delayed system startup. Traditionally, these types of failures are driven out during system practical testing and signoff. This can be a long resource consuming process, even with recent advancements in automated testing, and will only become more complex as feature numbers increase.

Infotainment systems must be designed to be highly modular and reconfigurable to allow varying levels of features to be offered and cater for variations between markets e.g. navigation systems and tuners. This presents an additional challenge in ensuring the system is robust in all configurations.