In order to describe a SPIF in an XML document, a appropriate schema needs to be defined which will describe the structure of the XML document.
There are three possible options for developing such a schema
Additions to the ASN.1 standards have provided XML Encoding Rules (XER) which allow ASN.1 objects to be encoded as XML
Further information in XER, including the relevant standards, is available here.
So for example, by taking the X.841 ASN.1 definition of a SPIF, an appropriate schema could be generated.
There is an existing schema definition for a SPIF. It is used by the Visual SPIF program, developed by Cynacom Solutions in 2000, to provide an import/export facility. The program includes the associated DTD1.
The definition is however incomplete and, being a DTD, doesn't provide as rigourous description of SPIF as an XSD.
1If using Internet Explorer, ignore the error message "Cannot have a DTD declaration outside of a DTD.", and just view the source.
A new schema could be developed that exploits the features of the XML Schema Definition language to provide a sophisticated definition of a SPIF. This could include explicit referential integrity checks that are not specifically defined in the ASN.1 definitions.
A new definition could also encompass both SDN.801 and X.841 definitions and additional elements that these standards do not cover (for example, privacy marks, X.411 category definitions).
Whilst each of the options have their merits, it was decided to develop a new schema with a view to encapsulating both SDN.801 and X.841 definitions and also to the use of the XML schema directly, rather than as a textual representation of a SPIF.