What is MOST?
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The MOST (Media Oriented System Transport) bus is an attempt to bring many of the benefits of fiber optic data transmission to the Volkswagen market. Many automakers are now installing them in a variety of models, marking the ambition to achieve significant success. You may have always refused the temptation to install an OEM DVD player in a car. But when you drive on the highway, you can occasionally pay attention to the high-end cars that flashed around you. You will find that the children's attention is attracted by the backseat DVD, which may prompt you to make purchasing decisions. The rear-seat DVD screen is just the most common application of the MOST bus in the car. The fiber-optic bus can also process digital audio signals for entertainment systems and hands-free phones, as well as transmitting information system data such as GPS. For example, the Porsche Cayenne 4x4 and the BMW 7 Series are the intended targets for the application of such multimedia systems, and they do carry MOST. However, the current model specifications show that this technology is spreading with the car model, covering high-end models to mid-range models.
Today, MOST is not the only fiber channel that automakers have equipped in production cars. BMW also uses plastic optical fiber to connect the sensor and airbag system control modules to the 7 series, using the ByteFlight protocol.
Say goodbye to EMI
For design engineers, the benefits of using fiber in an automotive environment are obvious. This medium is capable of transmitting high bit rate data in a noisy environment inside the car, preventing the data stream from being subjected to electrical interference. It has a fast edge rate, but eliminates any risk of data interference that is unavoidable in the signals transmitted by the in-vehicle electronic bus, especially in electric vehicles and other systems where safety is critical.
MOST is the result of cooperation in the automotive industry and does not have formal standards. Its original concept began in the mid-1990s as a union between BMW, DaimlerChrysler, Harman/Becker (audio system manufacturer) and Oasis Silicon Systems. Shortly thereafter (1998), the participating parties established an autonomous entity, MOST Corporation, which controlled the definition of the bus. Oasis reserves the right to name MOST itself. The product certification process is handled by an independent testing organization, such as Ruetz Technologies. In addition to compliance testing, Ruetz also provides software and hardware analysis tools for the development of MOST bus systems, as well as training for MOST systems.
Although the potential advantages of fiber optics are already evident, telecommunications experience has further confirmed this, but the costs associated with telecommunications are certainly not suitable for automotive manufacturing. Therefore, the focus of MOST is to strive to reduce component costs.
MOST defines all seven layers of the standard OSI model and, as expected, is highly optimized for the automotive multimedia environment, specifying signal categories (such as CD stereo and DVD video) for the main traffic types to be processed.
Low cost plastic fiber
On the physical layer, the transmission medium itself is a PMMA (polymethyl methacrylate) fiber with a plastic protective cover and a 1 mm core. OEM suppliers can bundle a bundle of optical fibers into a cable like a wire. The fiber optic transmission uses a 650 nm (red) LED emitter (650 nm is the low loss "window" in the PMMA spectral response). The data is transmitted in 50 Mbaud, bi-phase encoding with a maximum data rate of 24.8 Mbps.
The definition of MOST is very common, allowing a variety of topologies, including stars and rings, and most automotive devices have a circular layout. There can be up to 64 nodes in a MOST network. Once the car is powered up, all MOST nodes in the network are activated, which is a major focus on low-power, power-down mode design, including the amount of power consumed by the system in this state and how it enters the state. The default state of the MOST node at power-on is Pass-through, where incoming data is passed directly from the receiver to the transmitter to keep the loop open.
The MOST data transfer uses a 512 b frame and a block of 16 frames (Figure 1). The frame repetition rate is 44.1 kHz (22.67 ms per frame), and each frame contains synchronization, asynchronous, and control data in addition to the preamble and other internal management bits. The bus is fully synchronized, and the designer can designate any device within the network as the master device, and all other nodes get their own clock from the master device. The network is completely plug-and-play, and there is a process of finding a device when power is turned on or when there is a connection change. The central registry of the connected device is maintained on the primary node.
The continuous improvement of product support levels demonstrates the development of this bus. The main part of implementing the MOST node includes the optoelectronic interface, the transmitter module and the receiver module. The main supplier in this market is Infineon, which recently introduced specifications for its ODIN MIT/MIR4 transceiver module. They are compliant with a defined 4-pin physical form factor and provide 650 nm receive/transmit functionality. The receiver module combines a photodiode with signal processing and activation functions on an IC that is now produced by Infineon, while the previous generation was acquired from Oasis Silicon Systems. Martin Weigert, director of plastic fiber marketing at Infineon, confirmed that the supply of these products faces the automotive industry's urgent demands for quality and reliability, as well as continued price pressures. Weigert explained that the price of a pair of transceiver modules is about 3 euros, and when the fiber technology is used for the first time in the automotive environment, the price is much higher. Infineon also manufactures transceivers for ByteFlight and manufactures SFH products for its short-haul industrial chain.
This generation of transceiver components rated the temperature above the 85 ° C industry standard upper limit, reaching 95 ° C, which also became a challenge, Agilent first introduced the AFBR1010/2010 device pair earlier this year. The 95°C temperature value slightly exceeds the ideal operating range of PMMA fiber materials, and this limitation may eventually lead to the replacement of plastic fibers by silicon core fibers.
Another company that is preparing to enter the MOST transceiver market is Melexis, part number MLX75600/02, which also meets the 95°C temperature requirement (Figure 2). Peit De Pauw, optical product bus manager at Melexis, confirmed that the target price of the device is between €3 and €3.5, and that the company has acquired a lot of expertise in the automotive sector through its own sensor products before entering the market. . De Pauw sees this as the advantage of Melexis over other companies, as other companies only have optical knowledge in the telecommunications field.
All optical/electrical and electrical/optical modules use a connector housing structure that accepts low cost fiber optic terminals. Many suppliers, including FCI and Furukawa, offer connectors that mix electrical connections with fiber pairs in various combinations.
Interface controller
Another aspect of the transceiver module, the basic component of the MOST node is the Network Controller IC, which comes from Oasis Silicon Systems, which is now a member of the SMSC. Oasis offers the OS 8104, which integrates a network interface, data routing engine, master and management interface, and a complete set of software and corresponding development tools. The 81050 adds a higher level of intelligence to enhance MOST functionality, while the 8805 is a system-level integrated device that includes an ADC and DAC for direct input and output of audio signal streams.
Major microprocessor and DSP vendors are also increasingly supporting this bus. Analog Devices uses its Blackfin DSP, which is configured for car video display and hands-free cell phone connectivity, to provide connectivity to the car network via MOST. Texas Instruments has just announced an OMAP processor optimized for automotive infotainment products. Texas Instruments licensed from SMSC to integrate the MediaLB bus, the on-chip and inter-chip communication bus, which can transmit locally demultiplexed MOST network data. According to Texas Instruments, this timeless design can be used for any future MOST upgrade. The on-chip digital audio routing engine (DSP core and MediaLB interface) handles and directs audio data between audio ports, which Texas Instruments claims is equivalent to 100 MHz DSP bandwidth.
Fujitsu is an early intervener of MOST-enabled processors. It announced in early 2004 that it has partnered with Oasis to establish a research and development company, mycable, to develop a reference design for multimedia automotive systems. Mycable focuses on embedded multimedia design, while Fujitsu's MPEG chip is used for video decoding in reference designs.
Application is more than car
According to MOST's vision, in addition to the harsh automotive environment, this bus can be used in a variety of applications. One possible application is to use bus transfer for the home environment, but this is unlikely from current trends. Gartner Dataquest has pointed out in its own analysis that homes with multimedia will need both high-speed data transmission media, both wired and wireless. When Oasis specifically provisions the bus for compatible content (audio, video), MOST may become a candidate for wired media. But Gartner analyst Paul O''Donovan said it is unlikely that there will be greater penetration into the family. He believes that even if the high-speed version of MOST is adopted in advance, "the possibility is not great, because the principle of no new wiring is needed: we hope that most of the households are power lines and other laid-up lines, which are transmitted between the home and the car. Content is not the transmission of the player, this is the guideline."
However, manufacturers must address access to automotive MOST networks for one reason, namely for system upgrades. In many cars, standard rectangular slots for radios, CDs, etc. are gone. Designers put radios, CDs, phones, GPS, and other systems in the back of the dashboard to place somewhere in the hardware, rather than in a dedicated In the box. Vendors such as Analog Devices have come up with the ultimate concept of turning each of these features into a software thread running on the Blackfin DSP. The benefit of this is not only that the radio is no longer easily stolen, but in fact even the alarm software can be bundled with the same processor.
Now go to the website of any car enthusiast and you will find a lot of anxiety that already exists on this issue. If you do not have a slot to install your own upgraded radio or other in-car equipment; or the car software does not recognize the upgraded device, or the upgraded device features do not function properly (or not at all). A worse situation may also occur because the replacement processor controls the electronic software in the car in addition to the entertainment functions, which may cause climate control to stop working when the audio device is replaced. As the number of cars equipped with MOST continues to increase, the possibility of such interface problems increases.
The Consumer Electronics Association of the United States is working with MOST Corporation to define a consumer electronics gateway function that can be used to connect to these upgrades. At the time of this publication, a draft recommendation for the gateway specification has been published.
Further development includes a revision of MOST that increases its base speed from 50 Mbps to 150 Mbps to meet the bandwidth requirements of multiple video streams and interface with the recommended in-vehicle wireless network. The fiber standard for ByteFlight seems to be further developed. ByteFlight's structural approach is far from the multimedia-oriented MOST, which currently has a 10Mbps rate. Because it is used in very secure systems, ByteFlight is completely deterministic and has failover capabilities. Infineon's Wiegert claims that ByteFlight requires products to be fully traceable because data storage is uncertain. These conditions also lead the FlexRay bus to seriously consider the use of optical transmission solutions.
Infineon's Weigert noted that industry system designers are increasingly interested in MOST. For designs that do not require more than 64 nodes (although bus extenders are available), MOST provides a complete, low-cost way to use fiber optics for data acquisition or network control in noisy environments. Although MOST is designed for in-vehicle wiring with a typical length of a few meters, the limits can be covered for longer distances. Typical plastic fibers have attenuation rates below 0.2 dB/m, while a pair of transceivers (such as Agilent's AFBR1010/2010) have a light budget of 14 dB from the output (optical) end of the transmitter to the receiver input. Considering that low-cost, high-volume commercially available connectors can cause some small transmission loss, and the loss of the first few meters in the fiber is higher than (relatively) the loss of the longer distance, so the length of tens of meters should be reasonable.
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