Image Processing Reference
In-Depth Information
with each node allocated a certain number of mini-slots, which do not have to be consecutive. he
mini-slots are of a fixed length, and much shorter than static slots. As the length of a mini-slot is
not sufficient to accommodate a frame (a mini-slot only defines a potential start time of a transmis-
sion in the dynamic segment), it has to be enlarged to accommodate transmission of a frame. his in
turn reduces the number of mini-slots in the reminder of the dynamic segment. A mini-slot remains
silent if there is nothing to transmit. he nodes allocated mini-slots toward the end of the dynamic
segment are less likely to get transmission time. his in turn enforces a priority scheme. he symbol
window is a time slot of fixed duration used for network management purposes. The network idle
time is a protocol-specific time window in which no traffic is scheduled on the communication chan-
nel. It is used by the communication controllers for the clock synchronization activity; in principle,
similar to the one described for TTP/C. If the dynamic segment and idle window are optional, the
idle time and minimal static segment are mandatory parts of a communication cycle; minimum two
static slots (degraded static segment) or four static slots for fault-tolerant clock synchronization are
required. With all that, FlexRay allows for three configurations: pure static; mixed, with both static
and dynamic—bandwidth ratio depends on the application; and pure dynamic, where all bandwidth
is allocated to the dynamic communication.
FlexRay supports a range of network topologies offering a maximum of scalability and a consider-
able flexibility in the arrangement of embedded electronic architectures in automotive applications.
hesupportedconigurationsincludebus,activestar,activecascadedstars,andactivestarswithbus
extension. FlexRay also uses the bus guardians in the same way as TTP/C.
The existing FlexRay communication controllers support communication bit rates of up to Mbps
on two channels. he transceiver component of the communication controller also provides a set of
automotive network-specific services. Two major services are alarm handling and wake-up control.
In addition to the alarm information received in a frame, an ECU also receives the alarm symbol from
thecommunicationcontroller.hisredundancycanbeusedtovalidatecriticalsignals;forinstance,
an air-bag fire command. he wake-up service is required where electronic components have a sleep
mode to reduce power consumption.
FlexRay is a joint effort of a consortium involving some of the leading car makers and technol-
ogy providers, to mention BMW, Bosch, Daimler, Freescale Semiconductor, General Motors, NXP
Semiconductors, Volkswagen as the core partners.
Time-Triggered Controller Area Networks (TTCAN) [], which can support a combination of
both time- and event-triggered transmissions, utilize physical and Data-Link Layer of the Controller
Area Network (CAN) protocol. As this protocol, as in the standard, does not provide necessary
dependability services, it is unlikely to play any role in fault-tolerant communication in automotive
applications.
TTP/C and FlexRay protocols belong to class D networks in the classification published by the
Society for Automotive Engineers [,]. Although the classification dates back to , it is still
a reasonable guideline for distinction of different protocols based on data transmission speed and
functions distributed over the network. he classification comprises of four classes. Class A includes
networks with a data rate less than Kbit/s. Some of the representative protocols are Local Intercon-
nect Network (LIN) [] and TTP/A []. Class A networks are employed largely to implement the
bodydomainfunctions.ClassBnetworksoperatewithintherangeoftoKbit/s.Someoftherep-
resentative protocols are J [], low-speed CAN [], and Vehicle Area Network (VAN) []. Class
C networks operate within the range of Kbit/s to Mbit/s. Examples of this class networks are
high-speed CAN [] and J []. Network in this class are used for the control of power train and
chassis domains. High-speed CAN, although used in the control of power train and chassis domains,
is not suitable for safety-critical applications as it lacks the necessary fault-tolerant services. Class
D networks (not formally defined as yet) includes networks with a data rate over Mb/s. Networks
to support the X-by-Wire solutions fall into this class, to include TTP/C and FlexRay. Also, Media
