This Recommendation defines how Abstract Syntax Notation One (ASN.1) modules can be used in combination with Specification and Description Language (SDL). This text replaces the semantic mappings from ASN.1 to SDL defined in ITU-T Rec. Z.105 (1999). The use of ASN.1 notation embedded in SDL previously defined in ITU-T Rec. Z.105 (1995) is not defined by this Recommendation. The main area of application of this Recommendation is the specification of telecommunication systems. The combined use of SDL and ASN.1 permits a coherent way to specify the structure and behaviour of telecommunication systems, together with data, messages and encoding of messages that these systems use.