Over the next five to 10 years, traditionally mechanical vehicle systems, such as braking and steering, will become electrical systems connected to a high-performance CPU via a high-speed, fault-tolerant communications bus. Features such as brake-by-wire, steer-by-wire and electronic valve control will set up the ultimate driving experience through an integrated driver-assisted vehicle. To provide the communication between these systems-especially where safety is concerned-a high-speed, fault-tolerant, time-triggered protocol is required. FlexRay is a new network communication system specifically for the next generation of automotive applications.The FlexRay Consortium has been established to rally the industry around one standard for the by-wire systems. BMW, DaimlerChrysler, Motorola and Philips Semiconductors have united to develop and establish the FlexRay standard. Standards promote consistency, ease of development and use, reliability, competition, efficiency and lower costs.
FlexRay will enable vehicles to advance to 100 percent by-wire systems, eliminating dependency on mechanical backups. The by-wire applications will demand high-speed bus systems that are both deterministic, fault-tolerant and capable of supporting distributed control systems. In addition, the technology is designed to meet key automotive requirements like dependability, availability, flexibility and a high data rate to complement the major in-vehicle networking standards such as CAN, LIN and MOST .
The communication system serves as more than a communications protocol. It also encompasses a specifically designed high-speed transceiver and the definition of hardware and software interfaces between various components of a FlexRay node. The FlexRay protocol defines the format and function of the communication process within a networked automotive system.
Unlike current automotive protocols-MOST, CAN, LIN and J1850-it meets the requirements for dependable systems and plays an important role in the communication between the vehicle's electronic systems.
Need for speed
In a vehicle today, the body and comfort modules are connected to a controller-area network (CAN) bus, which is complemented by the local-interconnect network (LIN) for control of peripheral devices. The telematics and multimedia connectivity requires high-speed interconnects with synchronous streaming data formats for video transmission that the MOST protocol delivers. In many cases, high-speed vehicle control systems such as power train and transmission controls are networked today using private CAN and J1850 networks.
With the increasing amount of data communication between the electronic control units, it is important to achieve a high data rate. FlexRay is initially targeted for a data rate of approximately 10 Mbits/second; however, the design of the protocol allows much higher ones.
It is a scalable communication system that allows synchronous and asynchronous data transmission. The synchronous data transmission enables time-triggered communication, to meet the requirement of dependable systems; the asynchronous transmission, based on the fundamentals of the Byteflight protocol, allows each node to utilize the full bandwidth for event-driven communications.
FlexRay's synchronous data transmission is deterministic with a minimum message latency and message jitter guaranteed. This is an advanced feature compared with CAN and its priority arbitration scheme. The CAN arbitration scheme delays lower priority messages in favor of high-priority ones. Therefore, the delay for any message, except the highest-priority message, is not defined beforehand. FlexRay guarantees predefined transmission times for each message in its synchronous part without any interference with other messages.
FlexRay supports redundancy and fault-tolerant distributed clock synchronization for a global time base, thus keeping the schedule of all network nodes within a tight, predefined precision window. In the synchronous part, each electronic control unit is assigned a number of fixed time slots, which guarantee the required bandwidth and time slot is available for each message. This assignment ensures that all messages get through without competing for bandwidth or an arbitration scheme.
An independent bus guardian contributes towards error containment on the physical layer making sure that no data collisions can occur even in the event of a fault in the communication controller. With by-wire applications, it is vital that all application information in a distributed system is linked by a communications protocol allowing the application function to be carried out in a known time frame.
The system has been designed to support both optical and electrical physical layers to enable the user to implement a wiring scheme that best suits his or her needs. Optical data transmission has the advantage of not being sensitive to electromagnetic interference coupling that can cause significant communication disturbance.
Unfortunately, no single communications network will suffice to meet all the cost and performance requirements in an automobile of the future. Instead, a reasonable number of network protocols (which may include CAN, LIN, MOST, etc.) will be supported by leading automotive OEMs. The networked vehicle is the vehicle of the future.