Image Processing Reference
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that need predictability and fault-tolerance (e.g., TTP/C [] or FlexRay [] networks) or serve as
gateways between subsystems (see the use of FlexRay at BMW in Ref. []).
It is common, in today's vehicles, that the electronic architecture includes four different types of
networks interconnected by gateways. For example, the Volvo XC [] embeds up to ECUs
interconnected by a LIN bus, a MOST bus, a low-speed CAN, and a high-speed CAN. In the near
future, it is possible that a bus dedicated to Occupant Safety Systems (e.g., airbag deployment, crash
sensing) such as the “Safe-by-Wire plus” [] will be added.
13.1.4 Event-Triggered versus TT
One of the main objectives of the design step of an in-vehicle embedded system is to ensure a proper
executionofthevehiclefunctions,withapredeinedlevelofsafety,inthenormalfunctioningmode
but also when some components fail (e.g., reboot of an ECU) or when the environment of the vehicle
creates perturbations (e.g., electromagnetic interferences [EMI] causing frames to be corrupted).
Networks play a central role in maintaining the embedded systems in a “safe” state as most critical
functions are now distributed and need to communicate. hus, the different communication systems
have to be analyzed in regard to this objective; in particular, messages transmitted on the bus must
meet their real-time constraints, which mainly consist of bounded response times and bounded jitters.
There are two main paradigms for communications in automotive systems: TT and event-
triggered. Event-triggered means that messages are transmitted to signal the occurrence of significant
events (e.g., a door has been closed). In this case, the system possesses the ability to take into account,
as quickly as possible, any asynchronous events such as an alarm. he communication protocol must
define a policy to grant access to the bus to avoid collisions; for instance, the strategy used in CAN
(see Section ...) is to assign a priority to each frame and to give the bus access to the highest
priority frame. Event-triggered communication is very efficient in terms of bandwidth usage as only
necessary messages are transmitted. Furthermore, the evolution of the system without redesigning
existing nodes is generally possible, which is important in the automotive industry where incremen-
tal design is a usual practice. However, verifying that temporal constraints are met is not obvious and
the detection of node failures is problematic.
When communications are TT, frames are transmitted at predetermined points in time, which is
well-suited for the periodic transmission of messages as it is required in distributed control loops.
Each frame is scheduled for transmission at one predefined interval of time, usually termed a slot,
and the schedule repeats itself indeinitely. his medium access strategy is referred to as time division
multiple access (TDMA). As the frame scheduling is statically defined, the temporal behavior is fully
predictable; thus, it is easy to check whether the timing constraints expressed on data exchanges are
met. Another interesting property of TT protocols is that missing messages are immediately iden-
tified; this can serve to detect, in a short and bounded amount of time, nodes that are presumably
no longer operational. The first negative aspect is the inefficiency in terms of network utilization
and response times with regard to the transmission of aperiodic messages (i.e., messages that are not
transmitted in a periodic manner). A second drawback of TT protocols is the lack of flexibility even if
different schedules (corresponding to different functioning modes of the application) can be defined
and switching from one mode to another is possible at run-time. Finally, the unplanned addition of
a new transmitting node on the network induces changes in the message schedule and, thus, necessi-
tates the update of all other nodes. TTP/C [] is a purely TT network but there are networks, such as
time-triggered controller area network (TTCAN) [], FTT-CAN [] and FlexRay, that can support
a combination of both TT and event-triggered transmissions. his capability to convey both types of
traffic fits in well with the automotive context since data for control loops as well as alarms and events
have to be transmitted.
Several comparisons have been done between event-triggered and TT approaches, the reader can
refer to Refs. [,,] for good starting points.
