X Series. Recommendations drawn up by the ITU-TS to establish interfaces for data terminal equipment (DTE) and data circuit terminating equipment (DCE) and public data networks (PDN).

X.400. A message handling system standard that permits the electronic exchange of text as well as other electronic data like graphics and fax. The X.400 standard is an overview and consists of:

• X.402 Overall Architecture.
• X.403 Conformance Testing.
• X.407 Abstract Service Definition Conventions.
• X.408 Encoded Information Type Conversion Rules.
• X.411 Message Transfer System.
• X.413 Message Store.
• X.419 Protocol Specifications.
• X.420 Interpersonal Messaging System.

X.500 An open systems standard for worldwide electronic mail directory services. A directory standard that lets applications like e-mail access information which can either be central or distributed. The benefit of a directory is the ability to minimize the impact on the user of changes to a network. The standard is broken down under subsequent numbers:

• X.501 Models.
• X.509 Authentication Framework.
• X.511 Abstract Service Definition.
• X.518 Procedures for Distributed Operation.
• X.519 Protocol Specifications.
• X.520 Selected Attribute Types.
• X.521 Selected Object Types.

XML. Extensible Mark-up Language. XML is a meta language defined by the W3C (a series of rules that allows you to create other languages for different applications) and is used to create common information formats. While HTML describes a web page’s content in terms of how it is to be displayed, XML describes the content in terms of what data is being described. Since WML is fully compliant with the XML rules, WML is an XML application.