For software testing using integration, the functionality of a software is divided into small independent modules. In this method, testing is carried out starting from the lowermost sub modules and gradually moves upwards towards the main module.
A prominent feature of the bottom up approach is the use of small temporary programmes called DRIVERS to simulate the higher sub modules or the main module. Let us suppose that a software functionality needs to be tested with one main module 1 having sub modules 2&3 using the bottom up approach. The main module can be replaced by a driver and the testing is carried out on sub modules 2 and 3. The same strategy is utilised for other branches and drivers are used to simulate modules place higher in the hierarchy while testing is carried out on the lower sub modules. This approach proceeds upwards from the lowest of sub modules to the chief most module.