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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?

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.

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.

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

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.

While there are many IT-related stacks for IP communications, the automotive industry has developed its own communication stacks.

They include many features specific to automotive applications, such as system management and control functions and gateways to other automotive networks, such as CAN. These automotive stacks are not geared to run over standard Ethernet, but they are already available for MOST networks.

Combining Ethernet and MOST technology on a single data link and physical layer speeds up the development effort required, as it is no longer necessary to "reinvent the wheel" to manage automotive information and entertainment systems.

The whole automotive network management infrastructure currently in place can be leveraged when adding Ethernet capabilities to the vehicle. Complete tool chains geared for automotive development and manufacturing systems already exist and would require little change to add Ethernet capabilities to the current set of MOST network functions.

The MOST technology streaming channels do not require separate processing stacks. Data can just be pumped down into the network, as shown in figure 4.

Figure 4: Streaming Channels do not require processing, so data can flow freely.

This results in very low-latency transmissions that are well suited for ADAS functions. End-to-end delays, including compression and decompression, are just a few milliseconds.

Finally, control information needs to be sent across the network with deterministic delays and low latency. This control information is also used to set up the streaming communications previously described.

The MOST INICs provide for control channels that are used for this purpose. The function blocks for INICs and MOST NetBlock are built into the hardware, so the network can come up as an independent entity without having to wait for the individual devices to boot up and present the application function blocks.

Those application function blocks can register themselves after each device finishes its start-up procedure, and they can tell the network what functions they provide. Figure 5 illustrates this concept.

Figure 5: Control Channel Reference Model.

All the stacks presented in figures 2-5 can run together in the latest generation of MOST150 vehicles. From a technical perspective, there's no need to argue about what kind of infrastructure the vehicle needs. Both packet and stream transmissions can be accommodated, and designers can take advantage of the best solutions to their problems.

Conclusion
The car makers that run the MOST Cooperation have taken a close look at the various networking technologies available today.

They have identified the advantages that various networks offer, and they have come up with a solution to take the best from each one. MOST technology can multiplex several different data transport mechanisms.

The Ethernet channel of MOST150 was developed to be able to use IP communications within and outside the vehicle while reaping the benefits of the efficient synchronous, streaming mechanisms of MOST technology that are already in place. It includes all the software layers needed for the car industry, and it does not require new automotive network management stacks.

MOST150 allows designers to take advantage of the decade-plus in automotive networking experience accumulated by the MOST Cooperation members, as well as the hundreds of millions of nodes already on the road.

The silicon and physical infrastructure required to implement these systems has been well proven in automotive applications, and the automotive specifications needed to implement automotive networking systems are all available today.

The tool chains needed for every stage of the automotive production cycle, from design and development, through manufacturing and service in the field, are already in place.

About the author
Henry Muyshondt is senior manager of automotive information systems division at Microchip Technology Inc.

To download the PDF version of this article, click here.


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