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Car systems: MOST, Ethernet or both at once?

Posted: 03 Oct 2014     Print Version  Bookmark and Share

Keywords:electronic system  car  Ethernet  MOST  connectivity 

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.

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 per cent of its bandwidth just to the Ethernet channel. Thus, a proven automotive physical layer is already available for Ethernet transmissions in the car.

With MOST150, you have a single physical layer with the advantages that both packet and streaming technologies bring to the vehicle.

The International Standard Organisation (ISO) has developed an Open Systems Interconnect (OSI) reference model for network communications. The Ethernet implementation of the OSI model is shown in Figure 2.

Ethernet reference model

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

To use the same model over MOST150, you simply change the lower two layers and replace them with MOST technology. You can then run additional MOST network layers in parallel. Figure 3 illustrates this modified OSI model implementation.

MOST and Ethernet reference model

Figure 3: Modified OSI Reference Model that includes MOST technology and Ethernet. Source: Microchip

As shown in Figure 3, all the higher layers remain unchanged. Only the data link and physical layers need to change for Ethernet communications; no changes to the actual consumer-facing applications are required. The MOST network packet channels have their own stack that can run in parallel and independently from the Ethernet stacks.


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