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ASN.1 (Abstract Syntax Notation One)

ASN.1 (Abstract Syntax Notation One) is a standard way to describe a message (a unit of application data) that can be sent or received in a network. ASN.1 is divided into two parts: (1) the rules of syntax for describing the contents of a message in terms of data type and content sequence or structure and (2) how you actually encode each data item in a message. ASN.1 is defined in two ISO standards for applications intended for the Open Systems Interconnection (OSI) framework:

  • ISO 8824/ITU X.208 specifies the syntax (for example, which data item comes first in the message and what its data type is)
  • ISO 8825/ITU X.209 specifies the basic encoding rules for ASN.1 (for example, how to state how long a data item is)

Here's an example of a message definition specified with ASN.1 notation:

Report ::= SEQUENCE { author OCTET STRING, title OCTET STRING, body OCTET STRING, biblio Bibliography }

In this very simple example, "Report" is the name of this type of message. SEQUENCE indicates that the message is a sequence of data items. The first four data items have the data type of OCTET STRING, meaning each is a string of eight-bit bytes (the term OCTET was used rather than BYTE because it can't be assumed that all computers will have eight bits in a byte). The bibliography data item is another definition named "Bibliography" that is used within this one. It might look like this:

Bibliography ::= SEQUENCE { author OCTET STRING title OCTET STRING publisher OCTET STRING year OCTET STRING }

Other data types that can be specified include: INTEGER, BOOLEAN, REAL, and BIT STRING. An ENUMERATED data type is one that takes one of several possible values. Data items can be specified as OPTIONAL (not necessarily present).

This was last updated in November 2010

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