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Automotive communication backbone: MOST vs Ethernet

Posted: 15 Oct 2014 ?? ?Print Version ?Bookmark and Share

Keywords:Automotive? electronic systems? Advanced Driver Assistance System? ADAS? Media Oriented Systems Transport?

The Ethernet PHY itself may be inexpensive, but you need a capable processor to implement the required software stack. MOST technology, in contrast, uses Intelligent Network Interface Controllers (INICs), to automatically route audio, video, and control data directly to related hardware interfaces (I2S, Transport Stream Interface-TSI, I2C?, etc.) without any processor intervention. While Ethernet provides a single mechanism to move all packets, MOST technology multiplexes several separate data streams, effectively providing hardware assistance for the distribution of information once it arrives at its destination.

MOST technology even allows for the remote control of INICs, so that no additional processing power is required at the remote node for simple applications. For example, an active speaker can consist of just an INIC and a digital-to-analogue converter (DAC) with a power amplifier. There is no need to run a communications stack, or for a processor to be involved in assembling an audio stream, before conversion to an analogue signal to drive the speakers.

On the other hand, as more and more data is, by default, already packetised and flowing between consumer applications that use the IP communications, sending such data over synchronous or isochronous MOST network channels requires additional processing to convert the data into different formats and then back to the original.

Some of this processing could be wasteful when the applications already expect to process information in its native format. If you have a PC browsing the web over an LTE connection, it would be simplest if the data reached the PC in its IP format.

The benefit of the MOST network is that it can directly route the data to various applications that are expecting it in Ethernet format while eliminating the need for the host processor to be involved when sending continuously streaming information, such as audio or video streams, to rendering devices.

In an Ethernet network, every single packet has to be processed by the target device to examine its contents and decide what must be done with it. IP communications only route data to a target address, but once it is received, higher-level software on the host processor must decide what to do with the data.

For example, if various audio streams are coming in and must be mixed together (say, a chime from an error function, a music stream, and some verbal message from the navigation system), using Ethernet, each stream would interrupt the host processor, which would then have to unpack the audio bits and prepare them as a single stream to send to a DSP that could mix them together.

Then the resulting stream would have to be packetised again, so it could be sent to another processor in an amplifier, which would then also have to unpack the stream before it could be sent to an A/D converter. In a MOST system, each sub-system could just place its audio stream on a synchronous channel that could go directly to an I2S port on the receiving INIC and from there to a DSP that could mix the data and output it directly to an ADC. Should the resulting stream need to be sent to another device, it could go directly on a streaming channel and then on to an ADC without having to involve a host processor at all.

The bottom line for MOST technology is that it is very efficient for transporting the streaming data that is usually required by infotainment and driver-assistance systems.

Figure 2: ISO Open Systems Interconnect Reference Model for Ethernet.

Putting it all together
MOST150, the latest generation of the MOST standard, has a dedicated Ethernet channel within its frame. There is no need to force all data into a particular format to fit a single packet or streaming transmission protocol.

This channel can take a standard Ethernet packet, without any special processing by the higher levels of the Ethernet network management stacks, and send it over the MOST network. MOST150 INICs even have Ethernet-style MAC addresses, so the Ethernet packets can be extracted at the right location and passed on to other standard Ethernet devices.

You eliminate the need for central switch hubs and additional hardware in the system. Streaming data, such as audio and video programs, can then be sent in parallel using streaming channels to attain better efficiency in using the available bandwidth.

In fact, even if an application called for just IP-based transmission, a MOST150 network could allocate 100% of its bandwidth just to the Ethernet channel. Thus, a proven automotive physical layer is already available for Ethernet transmissions in the car.

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