In telecommunications and computer networking abstract syntax notation one (ASN.1) is a standard, flexible method that describes data structures for representing, encoding, transmitting, and decoding data. It provides a set of formal rules for describing the structure of objects independent of machine-specific encoding techniques and is a precise, formal notation that removes ambiguities.

ASN.1 is an ISO/ITU-T standard, originally defined in 1984 as part of CCITT X.409 '84. ASN.1 moved to its own standard, X.208, in 1988 due to wide applicability. The substantially revised 1995 version is covered by the X.680 series.

ASN.1 defines the abstract syntax of information but does not restrict the way the information is encoded. Various ASN.1 encoding rules provide the transfer syntax (a concrete representation) of the data values whose abstract syntax is described in ASN.1. The standard ASN.1 encoding rules include BER (Basic Encoding Rules - X.209), CER (Canonical Encoding Rules), DER (Distinguished Encoding Rules), PER (Packed Encoding Rules), and XER (XML Encoding Rules).

ASN.1 together with specific ASN.1 encoding rules facilitates the exchange of structured data especially between application programs over networks by describing data structures in a way that is independent of machine architecture and implementation language.

Application layer protocols such as X.400 electronic mail, X.500 directory services, H.323 (VoIP) and SNMP use ASN.1 to describe the PDUs they exchange. It is also extensively used in the Access and Non-Access Strata of UMTS.

The compact binary encoding rules (BER, CER, DER, PER, but not XER) are considered alternatives to the more modern XML (which is also used by several Internet protocols). However, the ASN.1 allows to describe the data semantics, not only the transfer encoding syntax, so it is a higher level language than XML.

The Sample Neufeld ASN.1 Compiler to C/C++ (snacc) is, as the name suggests, a compiler to turn ASN.1 notation into usable code. The Free ASN.1 compiler (asn1c (http://lionet.info/asn1c)) is another open source compiler.

Documents and references

Documents describing the ASN.1 notations: ITU-T Rec. X.680, ISO 8824-1; ITU-T Rec. X.681, ISO 8824-2; ITU-T Rec. X.682, ISO 8824-3; ITU-T Rec. X.683, ISO 8824-4

Documents describing the ASN.1 encoding rules: ITU-T Rec. X.690, ISO 8825-1; ITU-T Rec. X.691, ISO 8825-2; ITU-T Rec. X.693, ISO 8825-4.

M. Sample et al, "Implementing Efficient Encoders and Decoders for Network Data Representations", IEEE Infocom 93 Proc, v.3, pp. 1143-1153, Mar 1993. Available from Logica, UK.

References

• Federal Standard 1037C
• MIL-STD-188.

External links

• The ASN.1 Consortium (http://www.asn1.org/)

• ASN.1 Information site (http://asn1.elibel.tm.fr/)

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