& Francis Group, LLC Compliance and Interoperability Testing of SIP Devices allows definition of more appropriate ATS, by analyzing only the key features and aspects of the protocol. 8.2.1 State Machine Approach Since a protocol can be modeled as a state machine, to solve the problem of protocol testing many solutions based on a mathematical approach to select test cases, were proposed [11,12]. The goal is to obtain a minimum set of test sequences, constituting an ATS, that cover all machine states or tran- sitions to perform a thorough test of an implementation under test (IUT). Various ATS development methods have been designed for the case that the reference specification is given in the form of an FSM. The best-known meth- ods [12] are called transition tour [13], w-method [14], distinguishing sequence method [15], and unique-input-output (UIO) method [16]. These methods are applicable to software and hardware systems as long as the system behav- ior can be characterized as a reactive system that provides output responses depending on the received inputs and the dynamically evolving system state. In FSM-based testing [17], both the specification and the implementation can be modeled as an FSM. If the behavior of the implementation FSM is different from the specified behavior, the implementation contains a fault. The types of implementation faults are: (1) output faults (the output of a transition is wrong), and (2) transfer faults (the next state of a transition is wrong). Each test derivation method mentioned above provides the following fault cover- age guaranteed: if the implementation can be modeled by an FSM with at most m states (where m is larger or equal to n, the number of states of the specification), then an ATS can be derived by the method (for this given m) and the implementation will only pass this ATS if and only if it complies with the specification (that is, it does not contain any output nor transfer faults) [17]. The ATS derived from each of the above methods will detect any output error of