Introduction. Software Testing. Distributed Systems. Communication Protocols. Standardization Work on Testing Methodology. Interoperability Testing. The OSI Conformance Testing Methodology. OSI Conformance Testing Projects. The Field of the OSI Conformance Testing Methodology. General Concepts. Testing Documents. Abstract Test Methods. Abstract Test Suites. Test Realization. The Conformance Assessment Process. TTCN - The Static Parts. Conventional TTCN and Concurrent TTCN. TTCN.GR and TTCN.MP. TTCN Test Suites. Definitions and Declarations. Constraints. TTCN - Behavior Descriptions. Fundamentals of Test Case Behavior Description. Behavior Tables. Psuedo-Events. Event Behavior Lines. Constructs and Defaults. Verdicts. Global Aspects of Test Suite Execution. Questions of TTCN Style. PCO Problems. Test Documents for an Example Protocol. The EXPRO Protocol. EXPRO Test Suite Structure and Test Purposes. The EXPRO PICS. A PIXIT for EXATS. The EXPRO Abstract Test Suite EXATS. Annex A: The BNF Rules for TTCN. Annex B: An Overview of Test Tools. Annex C: List of Abbreviations. References. Index.