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Fast Infoset (FI) is a specification that describes a means for delivering the data in an extensible markup language (XML) document in a binary-encoded format. The encoded XML output is referred to as a fast infoset document and is considerably smaller than its full XML representation. Though similar in concept to other document compression techniques, the encoding scheme used not only compresses a XML document, but also improves the processing of the XML data as well.
The FI specification was developed jointly by the International Telecoms Union (ITU-T) and the International Organization for Standardization (ISO) to build off of the World Wide Web Consortium's (W3C) XML information set recommendation. A W3C infoset is a means for defining the data that can be contained in an XML document, but it doesn't have to be. According to the W3C recommendation, an infoset must meet the proper syntax requirements for what's known as well-formed XML and must also adhere to proper attribute and element names as identified in an XML namespace. The original XML document, however, does not need to be valid before being encoded into a fast infoset document. This means that the infoset isn't required to be contained within an XML document and may be generated by other means.
The encoding method used in fast infoset documents is known of as Abstract Syntax Notation One (ASN.1). The use of the ASN.1 encoding language as a base for the Fast Infoset protocol allows for the document to retain all of the self-describing aspects of an XML document, as well as a few additional features. Strings that are regularly used throughout the document are indexed and replaced by a number. This includes the XML names as well as their attributes. Any binary data, such as images or video, can also be included directly within the encoding without having to go through an additional encoding scheme.
As a serialization technique, Fast Infoset differs somewhat from other methods. One example is the efficient XML interchange (EXI) technique, which is the W3C's recommended binary XML format. While an EXI document may provide a more compact representation of the XML infoset, it requires that the XML schema, the full description of the original XML document, is known at both ends of the transmission so that the encoding and decoding of the document can occur. A FI encoding may also use advanced features which also make use of the XML's schema for additional compression.
The fast infoset encoding technique is used in a number of telecommunications areas. It is most commonly found on the Internet, to conserve bandwidth during transfer, improve performance on the server for delivering XML data, and alleviate stress on the client applications on the receiving end. This is particularly useful in mobile Internet platforms, where devices may have severely reduced processing power and transmission throughput.