Image Processing Reference
In-Depth Information
14.4.1 Clock Synchronization
Due to clock drifts, the clock times in an ensemble of clocks will drift apart, if clocks are not periodi-
cally resynchronized. Clock synchronization is concerned with bringing the values of clocks in close
relation with respect to each other. A measure for the quality of synchronization is the precision. An
ensemble of clocks that are synchronized to each other with a specified precision offers a global time.
The global time of the ensemble is represented by the local time of each clock, which serves as an
approximation of the global time [KO].
Two important parameters of a global time base are the granularity and horizon. he granularity
determines the minimum interval between two adjacent ticks of the global time (also called macro-
granule), i.e., the smallest interval that can be measured with the global time. he horizon determines
the instant when the time will wrap around.
The reasonableness condition [Kop] for a global time base ensures that the synchronization error
isboundedtolessthanonemacrogranule.However,duetothesynchronizationanddigitalization
error, it is impossible to establish the temporal order of occurrences based on their timestamp, if
timestamps differ by only a single tick. A solution to this problem is the introduction of a sparse time
base [Kop].
14.4.2 Periodic Exchange of State Messages
A time-triggered communication system is designed for the periodic exchange of messages carrying
state information. hese messages are called “state messages.”
Information with state semantics contains the absolute value of a real-time entity (e.g., temperature
in the environment is
○
C). he self-contained nature and idempotence of state messages ease the
establishment of state synchronization, which does not depend on exactly-once processing guaran-
tees. As applications are often only interested in the most recent value of a real-time object, old state
values can be overwritten with newer state values. Hence, a time-triggered communication system
does not require message queues.
The periodic transmission of state messages is triggered by the progression of the global time
according to a TDMA scheme. TDMA statically divides the channel capacity into a number of slots
and assigns a unique slot to every node. he communication activities of every node are controlled
by a time-triggered communication schedule. The schedule specifies the temporal pattern of mes-
sages transmissions, i.e., at what points in time nodes send and receive messages. A sequence of
sending slots, which allows every node in an ensemble of
n
nodes to send exactly once, is called a
TDMA round. he sequence of the different TDMA rounds forms the cluster cycle and determines
the periodicity of the time-triggered communication.
The a priori knowledge about the times of message exchanges enables the communication system
to operate autonomously. he temporal control of communication activities is within the sphere of
control of the communication system. Hence, the correct temporal behavior of the communication
system is independent of temporal behavior of the application software in the host computer and can
be established in isolation.
14.4.3 Fault Isolation Mechanisms
A time-triggered communication protocol and the corresponding system architecture need to pro-
vide rules for partitioning a system into independent FCRs. In a system with active redundancy, the
dependencies among FCRs (i.e., correlation between failure probability of FCRs) have a significant
impact on the system reliability [OKS]. he independence of FCRs can be compromised by shared
physical resources (e.g., hardware, power supply, time base), by external faults (e.g., electromagnetic
interference, heat, shock, spatial proximity), and by design faults.
