Image Processing Reference
In-Depth Information
The need to guarantee a deterministic response mandates using appropriate scheduling schemes,
which are frequently implemented in application domain-specific real-time operating systems or
frequently custom designed “bare-bone” real-time executives.
The networked embedded systems used in safety-critical applications such as Fly-by-Wire and
Steer-by-Wire require a high level of dependability to ensure that a system failure does not lead to
a state in which human life, property, or environment are endangered. The dependability issue is
critical for technology deployment; various solutions are discussed in this chapter in the context of
automotive applications.
As opposed to applications mandating hard real-time operation, such as the majority of industrial
automation controls or safety-critical automotive control applications, building automation control
systems, for instance, seldom have a need for hard real-time communication; the timing require-
ments are much more relaxed. he building automation systems tend to have a hierarchical network
structure and typically implement all seven layers of the ISO/OSI reference model []. In case of field
area networks employed in industrial automation, for instance, there is little need for the routing
functionality and end-to-end control. As a consequence, typically, only the layers (physical layer),
(data link layer, including implicitly the medium access control layer), and (application layer, which
covers also user layer) are used in those networks.
This diversity of requirements imposed by different application domains (soft/hard real-time,
safety critical, network topology, etc.) necessitated different solutions, and using different protocols
based on different operation principles. This has resulted in a plethora of networks developed for
different application domains.
Design methods for networked embedded systems fall into the general category of system-level
design. They include three aspects, namely, node design (covered extensively in Section I of the
topic), network architecture design, and timing analysis of the whole system. he network architec-
ture design involves a number of activities. One of them is selection of an appropriate communication
protocol and communication medium. A safety-critical application will employ a protocol based on
Time Division Multiple Access (TDMA) medium access control to ensure deterministic access to the
medium. For an application in building automation and control, the choice of the communication
medium may be the power line wires in the existing building or dedicated twisted pair wires in a
new construction. The topology of the network heavily depends on the application area. In indus-
trial automated systems, the prevalent topology is the bus. Building network may have a complex
topology with many logical domains. Configuration of the communication protocol, among other
things, involves allocation to the communication nodes priorities in the priority busses, or slots in
the TDMA-based protocols, for instance. he timing analysis aims at obtaining actual times for the
chosen architecture. hat involves task execution time measures such as worst-case execution time
(WCET), best-case execution time (BCET), and average execution time; response time of a task from
invocation to completion; end-to-end delay; and jitter, or variation in execution time of a task, for
instance. In the end, the whole system has to be schedulable to guarantee that deadlines of all dis-
tributed tasks communicating over the network will be met in all operational conditions the system is
anticipated to be subjected to. As an example, let us consider a simple control loop comprising a sens-
ing node with a single application task dedicated to sensing, an actuator node processing data received
from the sensing node, and generating control value delivered to an actuator over a dedicated link.
The composite time of data processing (WCET) and transmission (worst-case response time) has to
be shorter or equal to the maximum time allowed by the process dynamics under control. In case
of other nodes connected to the shared communication network and forming similar control loops,
a contention for the medium access may arise to be remedied for safety-critical and hard real-time
systems by adopting a fixed transmission schedule as in the case of the time-triggered TDMA-based
protocols, for instance. he schedulability analysis is to determine if the worst-case response time for
allthosecompositetasksformingcontrolloopsislessthenorequaltothedeadline.
