- 1. Introduction
- 2. FlexRay Communication
- 3. FlexRay Bus Access
- 4. FlexRay Framing
- 5. FlexRay Synchronization
- 6. Learning Objectives Test
Motivation for FlexRay
Safety-critical driver assistance functions with electronic interfaces to the chassis place extremely stringent requirements on the reliability, safety and real-time capability of the communication system. What is needed is a communication system with the property of composability, whose core property is to guarantee deterministic and fault tolerant data communication independent of bus load.
CAN (Controller Area Network), the communication technology that has become established in the automotive field, cannot fulfill this challenging set of requirements, since CAN is based on an event-driven communication approach, which means that every bus node of a communication system must be able to access the common communication medium at any time. The use of technologies for resolving collisions leads to a communication flow that cannot be determined until runtime. Event-driven communication systems enable quick reaction to asynchronous processes, but they are non-deterministic.
Because there is no strict schedule in an event-driven communication system, adding and removing bus nodes affects the communication flow. Strictly speaking, such changes make it necessary to comprehensively revalidate the entire system. Event-driven communication systems do not exhibit the property of composability.
CAN communication technology cannot fulfill the high requirements for fault tolerance due to its lack of redundant structures and mechanisms and can also only deliver a maximum data rate of 500 kbit/s in series production. Therefore some automotive OEMs were already experimenting with fault tolerant, time-triggered communication technologies, which allowed very high data rates, in the 1990s.
Nonetheless, the studies and experience gained at automotive OEMs did not yield any communication technologies that could meet all of the requirements for production implementation of future, safety-critical systems in motor vehicles. That is why BMW and DaimlerChrysler in 1999 agreed to work together to advance the specification and development of a future, uniform, time-triggered and fault tolerant communication technology. This cooperative effort resulted in the first rough requirements specification for FlexRay.