What are the different levels of testing? Explain in details.Ans:The different levels of testing are:• Unit Testing• Integration Testing• System TestingUnit Testing: The unit testing is done to test the individual module of the software.Test cases are designed to test the program logic, functionality, interfaces etc. in amodule. Since individual module is being tested, white box testing techniques areused here. Wherever required, stubs and drivers are also written to test the module,which increase the cost of the testing.Integration Testing: In this the module are systematically integrated and tested to findinterface problems, protocol design error, errors due to global values, input/ outputformat errors etc. Different strategies can be:• Bottom up Integration: In this type of testing the modules at the leaf level arefirst tested and then we move up in the hierarchy. Drivers are used in this type oftesting at different levels of hierarchy. Driver is a program which accepts the test casedata to be inputted and printed by the module to be tested.• Top Down Integration: In this case we start the top most module in thehierarchy and move down till the leaf modules are tested. Where ever requirednecessary stubs/ drivers are used. A stub is a program which simulates the modulecalled by the module to be tested.Big-Bang Testing: In this all the modules after unit testing are combined andtested in one go. The problem with this kind of testing is debugging of errors.• Sandwich Testing: This technique makes use of combination of top down andBottom up testing.System Testing: This testing focuses on validating the product with respect tosoftware specification Document. Techniques like function testing (using Black Boxtesting), Performance testing and acceptance testing (to be done by end user) is donehere to test the product.