Rapid and real-time – the new FlexRay bus systems need to meet high demands, not just in vehicles, but also in telecommunications. Based in Stuttgart, Germany, the Steinbeis Transfer Center ExpertCom has been investigating FlexRay and Ethernet bus systems with the Technical Academy of Esslingen, looking into similarities and differences. The results were presented and discussed at a two-day public event in December 2008.
The increasing complexity of electronic control systems in automobiles means powerful bus systems are needed for rapid, reliable communication. These bus systems connect electronic and mechatronic control units to a network of sensors and actuators in the vehicle. The two main competitors, now and in the foreseeable future, are Ethernet and FlexRay. The functions of both are structured hierarchically: one layer – the physical layer – processes device-related functions. Above this, the second layer – the MAC layer – handles functions controlling access to the bus.
Looking at the physical layer, Ethernet is characterized by its many optional and hardware-dependent sub-layers, meaning it can serve an extremely wide range of applications in communications and information technology. The FlexRay protocol is designed for applications in control engineering, so does not require this variety and thus does not support it. However, the physical layer of the FlexRay bus is available in both fiberglass and copper versions. These different transmission methods are supported by different bus drivers.
The FlexRay bus is a deterministic system, in which data is transmitted synchronously between the ECU and the sensors via time slices. In contrast, the Ethernet bus is eventdriven.
Comparing the FlexRay concept with an Ethernet LAN, similar requirements are handled on the MAC layer rather than the physical layer – such as collision-free Duplex connections, priority control and flexible bandwidth requirements. Full Duplex point-to-point connections are established via an additional switch in the network.
The MAC layer of FlexRay is a frame structure containing a header, reference data and a checksum trailer. The header contains frame information such as the frame type, number and length. The reference data (max. 254 bytes) does not contain any further FlexRay protocol data. The Ethernet MAC layer uses a similar frame system, but with sender and recipient addresses – in FlexRay, the recipient address is implicitly replaced by the time slice system for bidirectional data transmission.
Choosing between the two competing bus systems means taking the technical, operational and economic criteria of the project into account. For example, Ethernet is advantageous in areas of automotive and control engineering which combine control and communication applications. FlexRay is best used in areas of control engineering and mechatronics requiring high safety, reliability, and speed. A third possibility? Use the two bus systems together, allowing the advantages of each to benefit different applications.