Guide to Automotive MOST Bus: Multimedia Transfers in Vehicles

Most of the critical transport in an automotive need not so much of bandwidth and can be accomplished with technologies like CAN, LIN etc. But one requirement in the modern vehicle is the infotainment systems and the surround view/reverse camera systems. Such systems need a lot of bandwidth and very less latency that cannot be achieved with the conventional vehicle networks. This called for a specialized bus and one of the key protocols that has revolutionized the way vehicles exchange data is the Media Oriented Systems Transport (MOST) bus. In this comprehensive guide, we will delve into the intricacies of the MOST bus and explore its applications in the automotive domain.

Understanding the Automotive MOST Bus

The Automotive MOST Bus is a high-speed multimedia network that enables seamless communication between various components within a vehicle. It serves as a backbone for transmitting audio, video, and control signals, facilitating the integration of infotainment, driver assistance, and other vehicle systems. The MOST bus plays a pivotal role in modern vehicle architectures and is crucial for automotive engineers and enthusiasts to understand.

The MOST bus was designed to carry high bandwidth multimedia data over the vehicle. It was initially specified in the 1990s through joint efforts from BMW, Becker (Harman), and OASIS Silicon (SMSC/Microchip). Over time, the consortium expanded to include more members, and today many OEMs use the Automotive MOST Bus to carry their multimedia data.

Mapping the MOST Bus to OSI Layers

To comprehend the workings of the MOST bus, it is essential to understand how it maps to the OSI layers. The OSI model is a framework for implementing network protocols and consists of seven layers, each with its own distinct functionality. The Automotive MOST Bus specification defines all seven layers of the Open Systems Interconnection (OSI) model, mapping them onto the underlying physical and logical components of the network. The OSI layer mapping provides a structured approach to understanding the different aspects of the MOST bus and its communication protocols.

The first layer, the Physical layer, deals with the transmission of raw bit streams over physical media. In the context of the MOST bus, this layer encompasses the optical and electrical physical connections used for data transfer. The second layer, the Data Link layer, is responsible for error detection and correction, ensuring reliable transmission of data packets.

MOST Variants and Interfaces

The MOST bus comes in different variants, each tailored to meet specific requirements of automotive applications. The three main variants are MOST25, MOST50, and MOST150, each offering different data transfer rates and physical layer options.

The MOST25 variant, as the name suggests, supports a maximum data transfer rate of 25 Mbps and only utilizes optical physical layer connections. The second generation MOST50 variant offers a higher data transfer rate of 50 Mbps and supports both electrical and optical communication. The third generation MOST150 variant boasts a maximum data transfer rate of 150 Mbps and provides enhanced network security and additional features like Ethernet bridging.

Here is a comparison of the key features of the three MOST variants:

The frame format and signaling speed vary between these MOST interfaces, making them directly incompatible. The MOST150 is the most widely used protocol in automotive multimedia networks today.

Architecture of a MOST Network

A typical MOST network implements a ring topology, where all the devices, also known as nodes, are connected in a circular fashion with the end node connected back to the starting node. This ring topology allows for efficient and reliable data transmission within the network.

The MOST bus can support up to 64 nodes, and while a ring topology is the most common, it is also possible to realize a star topology, although it is not widely used. To ensure high availability and fault tolerance, the MOST bus also utilizes a double-ring topology. In this configuration, if there is a breakage between two nodes in a ring, the ring will close and communication will be rerouted through another ring.

Media Oriented Systems Transport Device

The internal architecture of the Media Oriented Systems Transport Device is essential to understand the functioning of the MOST bus. The device consists of various components that handle different aspects of data transmission.

The MOST Network Interface Controller (NIC) is responsible for the management of the data link layer in MOST25 protocols, while the Intelligent Network Interface Controller (INIC) performs the same function for MOST50 and MOST150 protocols. These hardware interfaces provide control ports and source ports for higher-level layers to communicate with the network.

The control port handles the transfer of control information and packet data, while the source port handles the streaming multimedia data. The higher-level network layer is implemented using middleware libraries as the network service layer, and the application layer is implemented through function blocks. Additionally, the physical layer has a bypass mechanism to allow the passage of incoming data when the Media Oriented Systems Transport device is booting up or not yet ready to process data.

Physical Layer Connections: Optical and Electrical

The Automotive MOST Bus relies on both optical and electrical physical layer connections for data transmission. The use of optical fibers ensures high-speed and noise-free communication, while electrical connections provide flexibility and cost-effectiveness.

The Optical Physical Layer (OPL) comprises optical transceivers and fiber optic cables. The optical transceivers convert electrical signals into optical signals and vice versa, enabling seamless transmission over the fiber optic cables. The use of plastic optical fibers (POF) with a 1mm core is common in MOST networks due to their low cost, resistance to electromagnetic interference (EMI), and ability to transmit data without signal degradation.

In addition to the optical interface, the MOST50 and MOST150 protocols also support an Electrical Physical Layer (EPL). The EPL utilizes unshielded twisted pair (UTP) cables or shielded twisted pair (STP) cables for communication. The electrical transceivers convert electrical signals into differential signals, reducing susceptibility to electromagnetic interference. Receivers in the MOST bus employ equalizers to compensate for higher frequency attenuation, ensuring reliable data transmission.

Media Oriented Systems Transport Frame Format

The Media Oriented Systems Transport bus uses a specific frame format for transmitting data and control signals between network nodes. Understanding this frame format is essential for analyzing and interpreting the data exchanged on the bus.

The frame format of the MOST25 protocol consists of a 1-byte header, with 4 bits used for synchronization purposes. The remaining bits in the header indicate the number of quadlets available for streaming data in the packet. Control data and frame control information follow the header, with the size of each control message being 32 bytes.

The MOST50 and MOST150 protocols have different frame formats, but they share similarities with the MOST25 format. The header in the MOST50 format contains 11 bytes, including sub-fields such as administration, lock flag, and control data. The MOST150 header is similar to the MOST50 header but includes an additional byte for preamble information.

Automotive MOST Network Services

Nodes in a MOST network have various functional blocks implemented above the data link layer. One essential block is the NetBlock, responsible for the general administration of the device, including positions, group/logical addresses, and more.

Each node in the network is addressed based on its position. The Media Oriented Systems Transport specification defines the allowed address ranges in a 16-bit format. The node acting as the Timing Master is assigned position 0, and subsequent nodes are assigned incremental positions based on their connection order. The node address is determined by adding a value of 0x400 to the position.

MOST also allows for group addressing, where devices of the same class, such as amplifiers or infotainment units, can be assigned the same group address and controlled with a single frame. Special addresses are assigned for blocking-broadcast and non-blocking broadcast, ensuring efficient data transmission within the network.

Conclusion

With the clearly defined architecture, the Media Oriented Systems Transport (MOST) bus has emerged as a game-changer in the automotive industry, enabling seamless multimedia communication between various components within a vehicle. In this comprehensive guide, we have explored the fundamentals of the Automotive MOST Bus, including its OSI layer mapping, variants and interfaces, network architecture, physical layer connections, frame format, and network services.

The MOST bus plays a crucial role in modern vehicle architectures, facilitating the integration of infotainment, driver assistance, and other vehicle systems. Its high-speed multimedia network ensures reliable data transmission and enhances the overall user experience in the automotive domain. As the automotive industry continues to evolve, the Automotive MOST Bus will remain a key technology in accelerating data exchange in vehicles.

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