This Recommendation defines an informal test notation, called the Tree and Tabular Combined Notation (TTCN), for OSI conformance test suites, which is independent of test methods, layers and protocols, and which reflects the abstract testing methodology defined in ITU-T Recs X.290 and X.291. This edition incorporates corrections of defects received and is equivalent to ETSI TR 101 666, which is also known as TTCN2++.