What isTop-down approach ?

Top-Down approach:
1. We start testing from the main module with stubs for subordinate modules
2. Replace the stubs with sub modules
3. Replace one stub at a time with sub module
4. Test after each module is integrated
5. Do regression testing.
Advantages:
1. No drivers
2. We can test major functionalities as early as possible
3. Interface errors are discovered early
4. We have partially working model so we can demonstrate to the client
Disadvantages:
1. Stubs are needed
2. Errors in critical modules in low level are found lately