(Tree and Tabular Combined Notation) A programming language endorsed by ISO that is used to write test suites for telecommunications systems. TTCN is used for "black box testing," which means that all interaction to the system is via messages, rather than by a user with a mouse. Test suites are built in a hierarchical and modular manner, allowing many programmers to be involved in the project. Numerous telecom vendors use TTCN for their products, and organizations, such as the ITU, IP/MPLS Forum and ETSI, provide TTCN conformance tests for new standards.