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

11.4 Essential Specification Modelling 349
We cannot give precise rules for the amount of detail in the architecture model. The architecture
model should in principle cope with system level design solely. The essential
modelling phase is followed by the extended modelling phase. This latter phase offers
enough opportunities for more detailed modelling. It is nevertheless not wise to postpone
the modelling of corollaries of implementation decisions that are taken definitively.
Taking them into account in an early phase leads to the required structure without the
need to perform iterations concerning structure.
Architecture design must be considered as an important informal way to prepare the
structure of a model, and in particular of Message Flow Diagrams and Instance Structure
Diagrams. The formalisation of the structure is in general performed in two ways.
Firstly, clusters that represent the architecture modules are introduced consistently in
Message Flow Diagrams. Secondly, channel structures of Instance Structure Diagrams
are designed consistently with ASDs. Properties of architecture modules are denoted
by boundaries on clusters in all object instance models with keywords such as HIDE,
IMPL and DISTR.
11.4.4.2 Specification of Timing Requirements
Timing requirements are specified in a textual form or as a table. The response time
for various sorts of functional behaviour can be defined for each module in the ASDs.
Alternatively, timing can be specified on the system boundary level only. Notations,
formalisation, simulation and timing verification are subjects for further research.
11.4.4.3 Keeping Architecture Decisions
Architecture design decisions are denoted in written text. Rejected alternatives and
motivations must be described briefly. The main point is that future developments
and improvements of the system can be assessed for their feasibility. Recall that we
already mentioned that a statement like ’no alternatives have been evaluated’ is also
very valuable information. Estimations on which the design is based should be given
as well as important assumptions, for instance about traffic densities on channels. Some
modules are introduced based on very volatile information such as availability, price
and market. This is also valuable information to be reassessed.
11.4.5 Modelling of Message Flow Diagrams (MFD)
11.4.5.1 Introduction
The creation of Message Flow Diagrams (MFD) is the main activity during the modelling
process of a system to be designed. It is the most time consuming activity. There are
many aspects that must be considered. MFDs must be discussed extensively with users
and various kinds of experts. In general, discussions are also very time consuming.
So, how can this method shorten the development time of a product? First a model is
developed to a certain degree of maturity before it is effectively used as communication