Bottom Up Approach:
The bottom up approach testing approach is carried out with the help of the test driver. The test driver calls a component to be tested.
The modules in the lower level of call hierarchy are tested independently. Then the next level modules are tested that call previously tested modules. This is done repeatedly until all the modules are included in the testing.
Bottom up testing stops when the top module in the call hierarchy is tested with all lower call hierarchy modules.
Top Down Approach:
The top down approach is carried out with the help of the test stub. Test stub are the dummy programs. The Stub is called from the software component to be tested.
Tests top layer of controlling subsystem first. Then combine modules that are called by the tested modules and test resulted collection of modules.
Do this until all the modules are combined and tested.