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Change in automotive communication systems

The authors take a look on the transformational change of Classical CAN, CAN FD, CAN XL, Flexray, and Ethernet. CAN XL provides the basis for cooperation between IP technology and signal-based communication. It closes the gap between CAN FD and Ethernet.

(Source: Vector)

The complete article is published in the December issue of the CAN Newsletter magazine 2020. This is just an excerpt.

Just a few years after the market launch of CAN FD, a new CAN variant, CAN XL, is on the start – sometimes viewed with a little suspicion. In fact, CAN XL owes less to the marketing strategy of electronics suppliers than it does to the dynamic development in automotive electronics over the last few years. In particular, the advent of automotive Ethernet with IP technologies is changing some things fundamentally. Currently, service-oriented communication is establishing itself in the vehicle parallel to signal-based communication. In this context, CAN XL provides the basis for efficient cooperation between IP technology and classic, signal-based communication. With data transmission speeds of up to 10 Mbit/s, it closes the gap between CAN FD and 100- Mbit Ethernet (100BASE-T1).

Currently, development departments in the automotive industry are, for the most part, concentrating on the challenges posed by the transformation in mobility. The focus is on assistance systems (ADAS – advanced driver assistance system), autonomous driving, electric mobility, and continuous connectivity to the Internet or to the cloud. High-performance sensor systems such as radar, laser scanners, and video cameras in the vehicle are an indispensable prerequisite for autonomous driving. They generate volumes of data that were unknown in the automotive sector only a few years ago. The challenge is how to transmit and process this exploding data volume in real-time. To meet this challenge, the industry has introduced Automotive Ethernet for fast transmission of data, covering primarily the bandwidths of 100 Mbit/s to 1 000 Mbit/s (100BASE-T1, 1000BASE-T1) used initially in the ADAS area. At the lower end of Ethernet networking, development is currently focused on 10BASE-T1S, with a transmission speed of 10 Mbit/s.

Service-oriented communication goes hand in hand with Ethernet and IP technology. Applications need data and services. It does not matter who provides them. However, this does require a dynamic link connection between data sink (consumer) and data source (provider). The ability to transmit dynamic data structures is another major advantage of service-oriented communication. The volume of data to be transmitted, for example in the case of sensor data fusion applications, is generated only during the runtime of the application. Such data cannot be mapped statically; instead, the communication system must serialize the data dynamically.

Figure 1: CAN XL frame (current status of development); With data lengths of up to 2 048 byte, CAN XL also lays the groundwork for future transportation of Ethernet frames and for the use of IP communication (Source: Vector Informatik)

Classic automotive serial bus systems

In contrast, the classic automotive networks such as Classical CAN/CAN FD and Flexray employ signal-based communication technology. In most applications, CAN operates at a transmission rate of 500 kbit/s and is used in automotive areas such as engine management and body control. The capabilities of CAN, a pioneer in automotive networks, are extended upwards by CAN FD and Flexray, whose transmission rates range from 1 Mbit/s to 10 Mbit/s. These newer systems are predestined for time-critical applications in engine management, body control, and chassis control, where they are used, for example, in the brake system. Lastly, Most, which is responsible for infotainment applications, covers the 25 Mbit/s to 150 Mbit/s range.

Given the rise of automotive Ethernet and in view of the growing variety of communication systems, a consolidation appears reasonable to limit complexity and costs. Since the fields of application of Flexray and Most can also be sufficiently covered by Ethernet, these systems will likely be replaced in the medium term. This would leave CAN and Ethernet, with Ethernet now handling infotainment, ADAS, telematics and connectivity at 100 Mbit/s to 1000 Mbit/s. Classical CAN and CAN FD operate in the range of 0,5 Mbit/s to 5 Mbit/s and are responsible for engine management and body control. In the future, CAN XL or 10BASE-T1S could be used for chassis control systems, running at 10 Mbit/s.

Considering that about 90 % of all network nodes communicate at speeds of up to 10 Mbit/s, the 10 Mbit/s domain covers a wide field of applications. It extends from audio applications to radar and ultrasonic sensors all the way to chassis control. From the technical viewpoint, the first applications mentioned focus on the streaming and serializing of data as well as on the principle of service orientation. In contrast, for applications in chassis control, signal-oriented communication dominates. As indicated above, CAN XL and the Ethernet variant 10BASE-T1S are competing in this sector.

Figure 2: Network topologies of 10BASE-T1S and CAN XL; CAN XL enables other topologies to be used, with a star and long stubs (Source: Vector Informatik)

CAN XL – the latest and fastest CAN

CAN XL is a further development of Classical CAN and CAN FD and operates largely on the same principle. A CAN frame can be divided into arbitration and data phases. While CAN XL uses low transmission speeds of 500 kbit/s to 1 Mbit/s in the arbitration phase, the speed in the data phase is scalable over a wide range of 2 Mbit/s to 10 Mbit/s. This bit-rate-switching is now mandatory with CAN XL.

Once again, the access method used is CSMA/CR (car- rier sense multiple access/collision resolution), which re- solves competing write access through bit arbitration. In this way, CAN XL follows a strict priority concept that allows the more important frame to be transmitted with no delays. CAN XL now only supports 11-bit identifiers, and 29-bit identifiers are no longer in use. Furthermore, CAN XL features a high level of data transmission reliability. With Hamming Distance 6 for headers and frames as well as format checks, it actually outperforms Flexray and the capabilities of Ethernet CRCs.

However, for future applications, it is not just the increased speed of data transmission that is important. A key motivating factor for the development of CAN XL is also the scalable length of useful data, which can extend to as much as 1 byte to 2 048 byte. Where necessary, this enables future automotive communication systems to package Ethernet frames in CAN XL frames, and/or to use IP communication via CAN XL (figure 1).

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