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

126 Concepts for the Integration of Specification and Design
essential model may be replaced by a model of a complete communication protocol
in the extended model. This can mean additional messages that guard the safe communication
and possibly requests for repetition of corrupted messages, etcetera. This
view corresponds to the fact that architecture design defines structuring imposed by
the problem domain. This structure determines the need for communication channels,
but does in general not require the precise selection of a protocol. As soon as implementation
decisions can be taken, a protocol corresponding to the medium chosen for
the implementation of the channel is selected. So an implementation structure is not
necessarily an extension of architecture structure. It may be the same structure. The
difference may be solely that the implementation model extends the architecture model
with decisions about what precise technology is used to realise the communication.
5.5 Architecture and Implementation Structure Modelling
Architecture structure and implementation structure can have various forms depending on
the sort of system to be designed. We cannot give definitions that make a clear distinction
between architecture structure and implementation structure. The distinction between
structures depends strongly on:
1. the complexity of the system;
2. their heterogeneity:
– hardware only;
– software only;
– hardware and software;
– other mixes with other implementation techniques;
3. geographical distribution constraints;
4. the level of abstraction in the model.
The difference between architecture structure and implementation structure is mostly
determined by the difference between essence and extension. The structure models
must prevent iterations. Therefore they have to show the relevant structure on the
appropriate levels of abstraction. It will require mature judgement to do this efficiently.
We recommend to describe structures as free style graphical diagrams. Examples are
(block) schemes that represent:
decomposition of a system into modules and their interfaces;
conceptual solutions;
geographically separated modules;
layering;