MOST—High-speed data bus for automotive electronic infotainment systems

Although there are many kinds of transmission and control interfaces for automotive electronics, such as LIN Bus, CAN Bus, FlexRay, etc., the transmission rate performance of these transmission and control interfaces cannot meet the needs of carrier and transmission of multimedia information for vehicles. 20kbps, CAN Bus is only 1Mbps, FlexRay is generally only 10Mbps, dual-line combination can reach 20Mbps, these are not enough to deliver real-time multimedia information.

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However, with the development of in-car entertainment systems and the advancement of transmission and control technology (such as reversing rear video), automotive electronics are increasingly required to use multimedia transmission. The most suitable transmission interface for this aspect is MOST (Media Oriented). Systems Transport, the media-oriented system shipped, followed by Bluetooth, which is prevalent today. However, Bluetooth can't completely replace MOST's position in multimedia transmission in the car.

First of all, MOST is a solid line transmission, and it is a fiber-optic line transmission, and it can be a plastic optical fiber (more cost-effective). The use of optical fiber can increase the information transmission, and the future transmission enhancement potential is also higher, and it is also more stable. Transmission (because there is no ground loop and no electromagnetic interference), these are beyond the wireless transmission of Bluetooth.

Secondly, Bluetooth's transmission performance is not enough. Even Bluetooth 2.0 with 3 times the Enhanced Data Rate (EDR) is only 3Mbps, which is lower than CAN Bus, and Bluetooth only allows 8 in a single Piconet network. The devices are interconnected and the number of node devices is also significantly insufficient.

Therefore, MOST will still be the best multimedia communication network in automotive electronics. Bluetooth can be used as a backup aid to deliver simple audio (such as voice announcements, entertainment sound effects) or GPS navigation information. Audio and video transmissions that require more stringent requirements still require the use of a MOST control network.

In view of this, this article will give more explanation and introduction to the MOST control network, and hope to bring some benefits to the designers who are interested in electronic communication control.

In 2003, MOST's transmission and control network technology was gradually expanded. At least 10 kinds of European production cars adopted MOST technology, including Porsche, Audi, Pengchi, BMW, Sweden's Regal, Sic Bo, and Italy's Fiat. , Langia, France's Citroen, Peugeot and so on.

In addition to the core main standard specification, the MOST interface allows for the replacement between “wire” and “fiber”. In addition, there are application profiles that support MOST transmission, such as Tuner tuner, CD player, amplifier. (also called: expansion machine) and so on.

MOST development overview

The development of the MOST control network (also known as: communication interface) can be traced back to 1997. The technical concept of MOST came from an informal cooperation initiated by MOST Cooperation at that time. In 1998, the company was based on previous cooperation. Combine 17 international carmakers (Carmaker) with more than 50 key component suppliers (Key Component Supplier) to jointly develop MOST control technology.

MOST is in full compliance with the ISO/OSI 7-layer data communication protocol reference model, and MOST adopts a ring topology on the network cable connection. However, in more demanding traffic control applications, MOST also allows for the change of stars. Shape (also known as radial) or double-ring connection configuration, in addition to each MOST control network allows up to 64 devices (nodes) to connect.

MOST also supports Plug and Play (PnP) mechanism, which can directly add or remove devices during MOST control network operation, increasing the convenience of expansion, maintenance and use.

Furthermore, there is a Timing Master node in the MOST control network, which will continuously send the data frame into the ring wiring, which is equivalent to playing a data gate (The gate for data) )character of. In addition, in the transmitted frame, the preamble portion or the packet header portion of the previous segment will be repeatedly transmitted to maintain synchronization with the Timing Slave in leisure. .

In addition, the total data transmission rate of MOST is 24.8 Mbps. This is to combine the audio and video streaming data with the packet transmission and control data. It can also be divided into 60 transmission channels and 15 channels in the bandwidth of 24.8 Mbps. MPEG-1 video encoding channels, which can be configured, planned and deployed by the control designer.

It is worth mentioning that MOST's efforts to save cost are not only on the line material, but also the fine method of using plastic fiber. The transmission aspect is also because of the synchronous mode, it is not necessary to set the “transceiver buffer” and carry out “ The sampling rate conversion, which also contributes to the cost savings.

MOST distribution scale (since MOST has a heavy European flavor, the proportion of American cars is less than 9%, while European cars are the most used, up to 58%, and Asia-Pacific cars are somewhere in between. , for 33%.)

MOST mechanism overview

Next, we will further explain the details of the transmission and control of MOST. To be honest, the transmission technology of MOST is similar to the Public Switched Telephone Network (PSTN), which has a data channel and a control channel. Design definition, the control channel is used to set how to use and transmit data channels.

Once the setting is completed, the data will continue to flow from the sending location to the receiving location. There is no need for further packet processing procedures in the process. The operating mechanism is designed so that it is most suitable for real-time audio and video streaming.

Of course, not all information is transmitted in real time, such as Internet browsing, GPS navigation information, etc. The transmission characteristics of these data are irregular short-term bursts. This type of transmission does not require the aforementioned synchronization mechanism. Asynchronous transceivers can be used with less timeliness. In fact, MOST control networks also support this type of delivery. More simply, MOST supports both time-proven, synchronous streaming and non-time-sensitive, asynchronous data. transmission.

In the MOST control, there are actually three types of transmission: 1. Control control information transmission; 2. Packet non-aging packet data transmission; 3. Real-Time Information real-time streaming data transmission (ie Refers to: audio, video).

If we specify more, in fact, the synchronous audio and video transmission allows a transmission rate of up to 24 Mbps, while the control transmission and packet data transmission are actually recognized as asynchronous transmission, and the asynchronous transmission can be up to 14.4 Mbps.

Since asynchronous data transfer and control transfer coextrusion "asynchronous bandwidth", then when the packet data throughput increases sharply, it will not affect the transmission and transmission speed of control information. In fact, don't worry about this. MOST control design has already taken this into consideration. In any case, the bandwidth of 700kbps or more will be reserved for control transfer, and the bandwidth will be allocated to the data transmission, while the control transmission will be carried out. Part is also called a Sub-Channel.

Several features of MOST control technology. Including: 1. Up to 64 nodes can be connected, 2. There can be up to 8 interface joints (connectors) between the two nodes, 3. Up to 10 meters in length, 4. Most simple (simple) Connection type.

MOST's strong backing

At the end of this article, let's take a look at some representative supporters of MOST technology.

The first part is the car manufacturer (automaker). The supporters in this area are mainly Germany's BMW (BMW), the US's DaimlerChrysler (DC, which is now owned by the German BENZ Pengchi Group), and the United States. Ford, General Motors (GM), etc.

Followed by the Tier-1 vehicle electronics supplier, in this respect, there are Bosch in Germany, Delco in the US, and Denso in Japan (TOYOTA) Department), Germany's Infineon (Infineon) and so on.

In addition, the manufacturers of audio and video equipment for cars can not be ignored, including the British Linn Products (precision audio design company), the United States Motorola Motorola, the Netherlands Philips Philips, Japan's Sony Sony Sony and other companies have expressed support for MOST technology .

The transmission rate of the existing MOST is 24.8 Mbps, and MOST will work toward higher speeds of 50 Mbps and 150 Mbps in the future. The picture shows the current concept of wire-type MOST transmission. It uses two sets of unshielded twisted pair cables to achieve high-speed transmission of 50Mbps. One group focuses on transmission, the other group focuses on reception, and each group has one passive simulation. The filter is connected, and more importantly, this method does not require any new process.

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