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Electronic Design

Get The MOST Out Of Your Automotive Communications

The MOST network protocol for cars offers support for music, video, telecom, and anything else engineers can imagine.

T he clamor for more digital connectivity in vehicles has car designers scrambling to implement systems that efficiently distribute audio, video, and other content. These requirements have led to the design of a future-proof system and networking architecture that can cope with the different development time frames in the consumer and the automotive worlds.

While existing implementations focused on audio, Media Oriented Systems Transport (MOST) now provides distributed network protocols for multimedia high-definition (HD) audio/video networking. MOST also supports Digital Transmission Content Protection (DTCP), required by content owners for the secure movement of video over distributed networks.

MOST offers more than the physical connection between devices. It also provides the software infrastructure to manage the complexity of multiple devices communicating with each other. As telephones, navigation systems, portable media devices, and infotainment systems are integrated to provide a rich entertainment experience, they need to communicate so they don’t overwhelm the user with the details of moving audio and video to multiple stations in the car.

Via MOST, designers can tame this complexity by moving all audio, video, and necessary control signals over a single cable, using either plastic optical fiber (POF) or unshielded twisted-pair (UTP) wires.

MOST Technology is the result of the collaboration among members of the MOST Cooperation, which consists of 16 carmakers and more than 75 suppliers working to establish and refine a common standard for the evolving requirements of automotive multimedia networking. Through this work, MOST has become the de facto standard in the automotive industry for transporting high-bandwidth audio, video, and control information between various vehicle subsystems.

Its quality of service (QoS) makes it a prime transport for applications that stream content to provide consumers with high-quality information, video, and sound. MOST is used in over 58 vehicle models from more than 16 vehicle brands from around the world. The technology started in Europe but has now expanded into Asia, with Toyota, Hyundai, Kia, and SsangYong recently introducing several models.

The traditional way of connecting analog signals between various components and using controllerarea networking (CAN) to control communications isn’t viable in the long run. That’s because too many devices would have to be connected with each other. If several of the connections involved surroundsound, for example, each link between devices would need six or more wires just for the audio signals alone.

Car designers can significantly reduce the complexity of the wiring harness by using MOST, which uses a ring structure (Fig. 1). Figure 2 shows an actual example of one vehicle manufacturer going from a traditional analog-based system to using MOST.

MOST helps car companies connect to the consumer world. It allows the network backbone in the car to comply with the robustness and reliability requirements of the automotive industry and provides a pipeline for moving audio and video. The long design cycles of a car make it difficult to quickly adapt to the latest consumer trends, though.

With the standardized interfaces of MOST, car companies can maintain their infotainment backbone on their own time schedules and only need to develop a single customized gateway device to connect to the latest consumer electronics. It would even be possible to make such a gateway an accessory that could be upgraded over time. Other consumer- and computer- oriented technologies, such as Ethernet and USB, are relevant to the car.

Ethernet’s wide proliferation, high bandwidth, and the optimized communication of bursts or packets of information make it an excellent connectivity solution between the outside world and the automobile. The protocol can connect an external Ethernet-based infrastructure to a vehicle and move large amounts of diagnostic information between the two, such as downloading software into the vehicle when the car is in a repair bay.

Many vehicles rely on embedded Ethernet products. The non-PCI architecture is well suited for the automobile since it obviates the need for a full personal- computer infrastructure. Instead, it provides simple interfaces to the typical microcontrollers used inside the car while taking advantage of the vast computing power that exists outside of the vehicle.

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While Ethernet is an efficient data-transport technology that relies on packet switching, MOST provides for efficient audio and video streaming by using a circuit switched architecture. It establishes a direct channel between a source of streaming data and one or more users of that data. By combining Ethernet and MOST, vehicle makers can use the best functionality that each technology has to offer.

USB has become the interface of choice for many consumer electronics devices. When a consumer brings an MP3 or video player, memory card, digital camera, or even a cell phone into the car, its connection is likely to be over USB. Within the automobile, USB provides the connection to the consumer world. However, standard consumer USB cables introduce significant electromagnetic emissions to the vehicle.

Instead of using cables to a central location, USB ports can be located where consumers will connect their devices while sending content from those devices over the MOST network backbone. MOST enhances the single host/multiple device architecture of USB by providing the distributed control architecture (multiple controllers and slaves) and simple mechanisms for allocating the entertainment content that’s stored in various consumer products.

Here again, the connection to the external world is through a ubiquitous consumer interface. But the transport of audio, video, and control within the vehicles is over the stable MOST infotainment backbone.

Wireless technologies such as Bluetooth and Wi-Fi are also used to connect nomadic devices to MOST. They can’t completely replace wired solutions due to the need to charge the batteries of these portable components, but they’re complementary to the systems in the car. These technologies allow for seamless transfer of information without having to be tethered to the car.

Many nomadic devices use a standard mini-USB connector. Cell phones, media players, GPS receivers, and other devices are adopting it, even if they don’t need to communicate data, because it provides a common way to charge the batteries in these devices. MOST, Ethernet, and USB provide the next generation of automotive interfaces that enable feature-rich and easy-to-use information and entertainment systems. Ethernet and USB are well understood in the market.

A MOST network is very easy to use due to simple connections. Plug-and-play functionality permits the network to identify the characteristics and features of new devices that are added to it. Virtual network management functions include channel allocation, system monitoring, addressing, and power management. The synergy with the consumer and PC industries is possible with consistent PC streaming and because it operates with or without a PC.

The most efficient and cost-effective way to continue automotive innovations in all of these areas is to develop the devices independently and then connect them together via a MOST gateway using standard hardware and software interfaces.

The clear trend is to enable the automotive system to attach the required features instead of providing every possible upcoming interface. With the gateway, MOST will offer a way to successfully decouple the automotive development cycles from the consumer electronics cycles.

The latest MOST Specification is at Rev. 3.0. It’s a complete overhaul of the specification structure, offering several new features. While the specification is independent of speed grade, it can already work with the newly defined MOST150 physical layer. Designers now can use a higher bandwidth of 150 Mbits/s, an isochronous transport mechanism to support extensive video applications, and an Ethernet channel for efficient transport of IP-based packet data.

MOST provides the specification for audio and video signals to be transported with high bandwidth efficiency and without any overhead for addressing, collision detection/recovery, or broadcast. This way, it offers capacity that packet-switched networks can only achieve with much higher gross bandwidth. Consequently, multiple HD video streams and multichannel surroundsound with premium QoS can be transmitted while simultaneously moving high loads of packet data around.

The latest version adds Ethernet and isochronous channels to the well-known synchronous, packet, and control data channels of previous specification versions. The Ethernet channel can transport unmodified Ethernet. This permits software stacks and applications from the consumer and IT domain, where innovation is much quicker, to be seamlessly migrated into the car. TCP/IP stacks or protocols utilizing TCP/IP can communicate via MOST without any modification.

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As a result, the new generation of MOST provides the automotive-ready physical layer for Ethernet in the car. In addition, MOST Specification Rev. 3.0 offers an isochronous channel to support streams that aren’t synchronized to the MOST frame rate. A typical usecase is the transport of MPEG streams over a MOST network, since MPEG streams generally use variable bit rates. This new MOST feature enables extensive video applications.

MOST Specification Rev. 3.0 also adds significant enhancements to the control channel. By doubling the bandwidth of that available with MOST25, the channel can control devices in real time.

With the integration of DVD audio and DVD video into digital networks, content protection becomes a requirement. DVD content on a digital network must be DTCP-protected (Fig. 3). HD DVD and Blu-ray content over MOST is also supported. That’s because Advanced Access Content System (AACS) specifications allow for DTCP-protected digital outputs.

DTCP requires source and sink devices to authenticate each other, and there’s a need to encrypt multimedia streaming data before sending it over a digital network. A sink device, then, must be able to decrypt protected digital content. DTCP on MOST also supports point-to-multipoint connections. The single phases consist of authentication (32-bit public device key), key exchange (Elliptic Curve DH), and encryption and decryption (M6-56Bit, AES-128Bit).

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