Grundlagen FlexRay - Institut für Automatisierungs- und ...

Grundlagen FlexRay BasicsV 1.1 11
• Reliability of transmission – Transmission of information should be reliable and
have deterministic timing, since the CSMA method used by the CAN bus can lead to
massive fluctuations in timing.
• Physical layer – the physical layer should be robust and simple.
• Extendibility – The network should be easily extendable. Single ECUs should be able
to be removed or added to the network..
• Support for time- and event-based communication – since – depending on the
application - both of these methods of communication may be suited better, both
methods should be supported.
CSMA:
TDMA:
Figure 1.1 Examples for time- (CSMA) and event-based (TDMA) communication
[Göhn10]
As a result of these requirements, the properties of the FlexRay system have been specified.
The key parameter of any communication system is the data rate. In respect to current and
future demand for bandwidth, a sufficiently high data rate of 10 Mbit/s per channel has been
selected.
A synchronous time base is provided, which ensures the deterministic character of the system.
Because of that, the event-based communication process using priorities (CSMA/CA), as used
in the CAN bus, had to be abandoned. Instead, a time-based method (TDMA) is used for data
transmission, see Figure 1.1. By using scheduling, an accurate time plan is possible, while
still allowing for a jitter of 0.5 to 10 µS (1 to 3µS typical)..
Because the communication process is organized in cycles, providing each ECU with defined
slots, a deterministic delay for messages can be guaranteed.
The focus during the development was to make the system flexible and extendable. As a
result, it is possible to choose which messages should be transmitted redundant, and the
system can be optimized for reliability or performance, using static or dynamic bandwidth
allocation.
As many as 60 parameters – e.g. message length or cycle length - in the network design allow
an optimization of the network for the specific application or task.
Bäurle 12.10.2012