Specification of Reactive Hardware/Software Systems - Electronic ...

11.4 Essential Specification Modelling 361
Object
Class
Diagrams
Initial Requirements Description
Message Flow Diagrams
Unified Model
(POOSL)
Requirements Catalogue
Instance
Structure
Diagrams
Arch/
Impl.
Structure
Diagrams
Figure 11.17: Development of Models in the Course of Time
11.4.7.1 Design of a Channel Structure
An ISD is a rather abstract representation of a system. Figure 11.18 shows an ISD.
An ISD shows clusters, objects, terminators, channels and channel names. During the
development of a system model a designer usually designs Message Flow Diagrams
without immediately considering channel structures. This is necessary to get an initial
model. Subsequently however it is necessary to relate flows to channels. A part of
a channel structure can usually be derived almost automatically from message flows.
Another part must be designed carefully.
Every message flow between process objects, clusters or terminators needs to be mapped
onto a channel. A number of flows between a pair of instances (of process objects,clusters
or terminators) can be mapped in principle on one channel between that pair. Therefore
an automatic approach can be followed to assign channels between pairs of instances
such that all flows can be mapped. An automatic approach results in channels that
connect pairs of instances.
This basic approach of pair-wise channels is only the simple part of the ISD modelling.
ISDs are used to perform considered channel design, and to verify previous modelling
activities. We already stated that the structure that has been designed in the Architecture
Structure Diagrams must be consistently incorporated into Message Flow Diagrams.
ISDs let us verify this because both types of diagrams should be used as input. The
structure of ISDs is in general based on the basic structure of the Message Flow Diagrams
of a system. It must be verified whether this structure is consistent with the Architecture
Structure Diagrams of the system. Structures such as backbones or other shared channel
time of the development proces