Abstract: A method and a test tool 110 are provided for verifying the functionality of a software based unit 100 which is provided with an interface 105 for its external communication. Pre-recorded data is used for the reproduction of a test case and for the verification of a unit subject to the test case. The pre-recorded data includes pre-recorded input data 125 and pre-recorded output data 126. The pre-recorded input data is applied to an interface of the unit and the pre-recorded output data is compared with the data transmitted from the unit in response to the applied pre-recorded input data. If the data transmitted from the unit is in correspondence with the pre-recorded output data, the functionality of the unit in accordance with the specific test case has been verified.