Objective This Recommendation defines how Abstract Syntax Notation One (ASN.1) modules can be used in combination with Specification and Description Language (SDL). The intention is that the structure and the behaviour of systems are described with SDL, while parameters of exchanged messages are described with ASN.1. This Recommendation defines a mapping of ASN.1 constructs to already existing SDL constructs and contains only a small extension to Recommendation Z.100 to allow ASN.1 modules to be used. Coverage This Recommendation presents a semantic definition for the combination of SDL and ASN.1 modules. A mapping of the ASN.1 data defined in a module to the corresponding SDL constructs defined in Recommendation Z.100 [1] is given, including the operators that can be applied to the ASN.1 data. The ASN.1 data items can then be used within SDL (using SDL notation). The use of ASN.1 notation embedded in SDL is defined in Recommendation Z.107 [2]. Application 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. NOTE - Specification in this Recommendation includes definition of requirements in a standard, Recommendation, or procurement document, and description of an implementation. A specification conforms to this Recommendation if and only if it conforms to the syntactic and semantic grammar rules for the formal technical language defined by the Recommendation (which includes the referenced ASN.1 and SDL languages). Conformance implies that every possibly dynamic interpretation of the specification conforms to the language rules. A specification that uses extensions of the language does not conform. A tool does not fully support the language if it rejects some constructs of the language or that has a static or dynamic interpretation of a specification in the language that does not conform to language semantics. Status/stability This Recommendation replaces the semantic mappings from ASN.1 to SDL defined in Recommendation Z.105 (1995). The use of ASN.1 notation embedded in SDL previously defined in Recommendation Z.105 (1995) is not defined by this Recommendation. Changes to Recommendations X.680 [3], X.681 [4], X.682 [5] and X.683 [6] or Z.100 [1] may require modifications to this Recommendation. This Recommendation is the complete reference manual describing the combination of SDL and ASN.1 modules.