System Design and V Model

System design mainly involves two parts : Architecture Design and Component Design. Which part of design should be should be performed first is dependent.

1 If design the architecture first, then the "Top Down" approach is being used.

2 If design the component first, then "Bottom Up" approach is being used.

In V-model, the System design is corresponding to system testing, that means system testing includes integration testing(testing Architecture and communication) and functional testing (testing component).
Note that, most time system testing may just test how the system works so that means it mainly just does functional testing. Note that, functional testing sometimes is also called accptence testing, but they are actually different, where acceptance testing should be undettaken by clients to test the function.

3 Architectural design represents the structure of data and program structure of software. 

4 Componeng -level design defines the data structures ,algorithms ,interface characteristics ,and communications  mechanisms allocated to each software component.

In v-model, there is a testing calle unit testing corresponding to modules design. Modules are classes in OO view. The relationship between component and module is that, a component contains a couple of modules.  But in the conventional software development, namely structural design and development, modules are componenets. Form this view point, we can see that the difference between the OO way and conventional way is lying on the Hierarchy they use. OO has more hierarchy than the conventioal one.

Unit test is testing the modules, sometimes it could be white-box testing.

How to evaluate components and their relationship(dependencies) ?  What are Cohension and coupling ?
Cohension measures "one single componet" that how well it is designed in terms of inside "operations".
Coupling measuers "some components" that how well their dependencies is designed in terms of the "operations" between them.