Each component at lower hierarchy is tested individually and then the components that rely upon these components are tested.
The order of Integration by Bottom-down approach will be:
4,2 5,2 6,3 7,3 2,1 3,1
+ Firstly, Test 4,5,6,7 individually using drivers. + Test 2 such that it calls 4 and 5 separately. If an error occurs we know that the problem is in one of the modules. + Test 1 such that it calls 3 and If an error occurs we know that the problem is in 3 or in the interface between 1 and 3
Though Top level components are the most important, yet tested last using this strategy. In Bottom-up approach, the Components 2 and 3 are replaced by drivers while testing components 4,5,6,7. They are generally more complex than stubs.