Complementary solution MOST and Ethernet using IP addressing

When MOST was introduced in 1998 it filled a gap: other automotive bus systems like CAN couldn‘t fulfil the data rate requirements of the emerging multimedia systems. And on the other hand existing networking technologies did not meet the tough automotive requirements. The optical data trans-mission technology of MOST solved this issue.

Among the car manufacturers BMW, Daimler and Audi in particular have been driving the development of the MOST standard. Also the tier 1 supplier Continental has been using MOST25 for almost ten years in its head units, radios and connectivity units.

With applications in a hundred car models MOST is already mature. The optical data transmission using a ring topology proved to be powerful and robust against EMI. Furthermore the used POFs are lighter than copper wires. The reason that some OEMs are still rejecting optical fiber may be the difficulties when installing it in the car body. Space limitations and narrow cable passages make it hard to meet the maximum bending radius that can be applied to POFs. Also, installing an optical network in a car needs experienced workers, which are skilled in handling the sensitive interconnection technology.

Today, twelve years after the foundation of the MOST Cooperation and ten years after the first use of MOST in the 2001 BMW 7 series the world is starting to change. With the rapid progression of Ethernet, e.g. in the fields of industrial automation or aerospace, an alternative solution to MOST is evolving that could be seen as future complementary option (figure 1).

BMW is the first OEM to walk down this path, using unshielded electric Ethernet cabling and a data rate of 100 Mbit/s for a pilot application to be on the road in 2013. This car is supposed to posses a driver assistance system utilizing four cameras to provide a bird‘s eye view of the car and its surroundings. Up to now these functions were implemented using shielded cabling.

For the addressing of the individual devices IP addresses are used - in the normal Ethernet way. This method proved of value not only in industrial automation but, using the AFDX protocol, also in the aerospace industry. Ethernet has a 30 year history with permanent enhancements and an enormous community worldwide.

It has overcome many of its previous limitations and has become a mass product. Functions, like routing between WiFi an Ethernet, are specifically easy to implement. Therefore it has to be rated very positive, that MOST150 offers an Ethernet compatible packet data channel that eases the communication with the consumer electronics world (figure 2).

Ethernet inside the car is a hot topic: the study SEIS (Safety in Embedded IP-based Systems), which is funded by the German ministry for education and research and conducted by the Innovationsallianz Automobilelektronik (innovation alliance for automotive electronics) is driving in-car communication using IP addressing via Ethernet. Although final results of this study, in which Continental is involved, will not be available before mid-2012, a trend is visible. Ethernet might make sense commercially, as the cost of unshielded copper cabling is lower than that of POFs. A random case of comparing MOST25 to Ethernet showed a cost margin up to 20 percent for the connection of all devices. However, this is just a spotlight, not a universal statement.

The only drawback for automotive Ethernet could be that there are only a few companies acting in this area; for most of the established Ethernet technology companies the automotive market is unknown territory. But there is a roadmap for automotive qualification of Ethernet components and first products are already working on a prototype level.

For the automotive industry it‘s a benefit to have two powerful and established communication standards at hand, which have slightly different strengths.  

J. Noebauer, Continental