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The ASN.1 Language


Abstract Syntax Notation One is a language specifically designed for describing structured information that is conveyed across some inter-face or communication medium. ASN.1 is standardized internationally (ISO/IEC 8824) and it is a key ingredient of Open Systems Interconnection (OSI).

In the presentation layer of the OSI hierarchy, data values of quite complex types, such as character strings, intricate structures or arrays of values, need to be determined in a unique way without saying anything about the representation. ASN.1 is developed to fill this need.

ASN.1 is a generic notation for the specification of data types and values. The basic principle is to define a small number of simple types by defining their possible values, and give rules for combining these into increasingly complicated types. The original use of ASN.1 was in the information description of high-level protocols (FTAM, CMIP, MHS etc.), but today it is widely used in the telecommunications industry for protocols and applications.

Figure 7 : A sample ASN.1 type definition

ASN.1 Encoding and Transfer Syntax

ASN.1 requires a transfer syntax in order to pass data between two entities. Basic encoding rules (BER, ISO 8825) is a standardized transfer syntax of OSI. Others exist as well: canonical encoding rules (CER) for security applications, distinguished encoding rules (DER) for digital signatures, traditional C/C++, etc. Any transfer syntax can be used for ASN.1 descriptions.

ASN.1, SDL and TTCN - a Powerful Combination

TTCN includes ASN.1, i.e. ASN.1 is used for creating data descriptions and constraints in test suite specifications. Through the new standard of Z.105, ASN.1 is merged with SDL (Specification and Description Language) to create an extremely powerful language environment for specification of real-time, interactive and distributed systems.

Data descriptions made in ASN.1 can be used for both SDL and TTCN specifications, thus making a tight integration between implementation and test, and promoting re-use.

Figure 8 : ASN.1 specifications can be shared between
SDL and TTCN specifications


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