The fast growth in complexity of embedded and software enabled systems requires for automated testing strategies to achieve a high system quality. This raise of complexity is often caused by the distribution of functionality over multiple control units and their connection via a network. We define an extended symbolic transition system (ESTS) and their compositional semantics to reflect these new requirements imposed on the test generation methods. The introduced ESTS incorporates timed behavior by transition execution times and delay transitions. Their timeout can be defined either by a constant value or an attribute valuation. Moreover we introduce a communication scheme used to specify the compositional behavior and define a conformance relation based on alternating simulation. Furthermore we use the conformance relation as the basis for a simple random test generation technique to verify the applicability of the presented approach. This formal framework builds the foundation of our UML test case generator.