Recommendation ITU-T X.694 | ISO/IEC 8825-5 defines rules for mapping an XSD Schema (a schema conforming to the W3C XML Schema specification) to an Abstract Syntax Notation One (ASN.1) schema in order to use ASN.1 encoding rules such as the Basic Encoding Rules (BER), the Distinguished Encoding Rules (DER), the Packed Encoding Rules (PER) or the XML Encoding Rules (XER) for the transfer of information defined by the XSD Schema.

The use of this Recommendation | International Standard with the ASN.1 Extended XML Encoding Rules (EXTENDED-XER) provides the same Extensible Markup Language (XML) representation of values as that defined by the original XSD Schema, but also provides the ability to encode the specified XML with an efficient binary representation (binary XML). An XML document can be converted to binary XML (for storage or transfer) using the ASN.1 generated by this mapping, and the resulting binary can be converted back to the same XML document for further XML processing.

Two versions of the mapping are defined. Version 1 of the mapping was published in 2004, and a Corrigendum was subsequently issued renaming the types DATE-TIME and DURATION in Annex A (in order to avoid conflict with the DATE-TIME and DURATION types defined in Rec. ITU-T X.680 | ISO/IEC 8824-1). The version 2 mapping is more efficient in two areas: the ASN.1 time types are used rather than VisibleString for mappings of dates and times; the FastInfoset specification (Rec. ITU-T X.891 | ISO/IEC 24824-1) is used for the mapping of XSD wild-cards. Both these changes to the mapping provide much more compact binary encodings for the XML specified by the XSD.

NOTE − The specification of the version 1 mapping (with applicable corrections) will be maintained in the next edition of this Recommendation | International Standard, but it is expected that subsequent editions will document only the version 2 mapping.

Application of the ASN.1 extended XML Encoding Rules to both versions of the mapping will produce the same XML (which is the same as that specified by the XSD). However, application of other ASN.1 encoding rules to the version 1 mapping results in a verbose character-based encoding of date and time types and of XSD wild-cards, whilst application of the version 2 mapping results in a more compact binary encoding using ASN.1 time types and the FastInfoset specification.