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One of the fundamental problems confronting users communicating with different systems is the efficient transfer of data in such a way that the data received is the same data transmitted. In the OSI model, the representation of data types and structures to facilitate this transfer is a function of the Application Layer; the encoding of the data into a specific sequence of bits for transfer is attributed to the Presentation Layer. This separation of functions enables the Application Layer to deal only with the content and structure of the data, leaving the choice of representation to the Presentation Layer. Consistent with that separation, we introduce an abstract notation for data values and structures in this chapter, prior to examining the Application Layer services in the next group of chapters. The notation introduced is called ASN.1 (Abstract Syntax Notation One). We defer discussion of the standards for converting ASN.1 described data, the Basic Encoding Rules (BER), until Presentation Layer Chapter.
We begin this chapter by identifying the purpose of ASN.1 and illustrating why it is used by means of an example. We introduce the basic unit of ASN.1, the module, and include significant aspects of ASN.1 syntax, primarily through examples. Additional examples will appear in later chapters. You will discover that learning ASN.1 is similar to learning the syntax of a high-level programming language.
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Computer Networks and Open Systems
An Application Development Perspective
Lillian N. Cassel
Richard H. Austing
Jones & Bartlett Publisher
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Real World ASN.1 and XML Solutions