The International Standards Organization (ISO) has defined a protocol test language called TTCN (Tree and Tabular Combined Notation) to specify abstract test suites for Open Systems Interconnection (OSI) protocols. TTCN combines a tree notation for dynamic behaviour description with a tabular representation of various language constructs. TTCN allows tabular constraints to enforce values on the Abstract Service Primitive (ASP) or Protocol Data Unit (PDU) parameters. For application layer protocols, Abstract Syntax Notation One (ASN.1) constraints are used. Dynamic behaviour description in TTCN is shown to address many important aspects of conformance testing such as modularity support in terms of test cases, steps and default behaviour tables and sophisticated timer management. TTCN has a machine processable form called TTCN-MP that defines all the TTCN syntax using BNF. Semantics of the tests specified in TTCN is operationally defined rendering TTCN almost a formal notation. © 1992.