Image Processing Reference
In-Depth Information
to arrive, and interprets the arrival of the message as a life sign at the membership point of the sender.
From the arrival of the expected messages at two consecutive membership points, it can be concluded
that the sender was operational during the interval delimited by these membership points.
14.5 Time-Triggered Communication Protocols
This section gives an overview of four important representatives of time-triggered communication
protocols: TTP/C, TTP/A, TTCAN, and TT Ethernet.
14.5.1 TTP/C
TTP/C is part of the TTA, a system architecture for the design and implementation of depend-
able distributed real-time systems [BK]. TTP/C provides four services: a predictable fault-tolerant
time-triggered transport service, clock synchronization, error containment, and a membership
service.
The basic building block of the TTA is a node computer, which is a self-contained composite hard-
ware/software subsystem (system component). A cluster is a set of nodes that are interconnected
by two redundant communication channels. For the interconnection of nodes, the TTA distin-
guishes between two physical interconnection topologies, namely, a TTA-bus and a TTA-star (see
Figure .).
A TTA-bus consists of replicated passive busses. Every node is connected to two independent
guardians, which use the a priori knowledge about the points in time of communication activities to
prevent communication outside a node's slot. In the TTA-star topology, the interconnection of nodes
occurs via two replicated central guardians. he star topology has the advantage of a higher level of
independence, as guardians are located at a physical distance from nodes. Furthermore, guardians
reshape signals and support additional monitoring services [ASBT].
The TTP/C protocol uses TDMA to control the media access to the two independent communi-
cation channels. Time is partitioned into slots, which are statically assigned to nodes. A sequence
of slots that allows each node to send a message forms a TDMA round. A sequence of predefined
TDMA rounds is called a cluster cycle. he cluster cycle also defines the periodicity of the message
transmissions. Information about the points in time of all message transmissions during a cluster
cyclearecontainedinthemessageschedule.
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FIGURE .
TTP/C network with star topology (left) and bus topology (right).
