XML, RSS, Atom, JSON and VoIP
For more information on any of the following technologies, visit the corresponding Resource Centers at http://www.deitel.com/resourcecenters.html (see Fig. 2 in the Preface for a complete list of Deitel Resource Centers).
XML (Extensible Markup Language, Chapter 14), developed in 1996 by the World Wide Web Consortium (W3C), is a markup language that allows you to label data based on its meaning. XML describes data in a way that is meaningful to both humans and computers.
XML documents are text files with a .xml extension; they can be created in text editors. These documents can reference a Document Type Definition (DTD) or a schema, which defines the structure for the document. This allows the information in the document to be verified by validating parsers, meaning they will check to make sure that no elements are missing (e.g., a last-name element in a document listing full names) and that the elements occur in the proper order. This makes XML data more reliable than data prepared with some other data-describing options. XML also can be used to create customized markup languages (e.g., XHTML for web content, CML for chemistry, MathML for mathematical content and formulas, and XBRL for financial data)—these are referred to as XML vocab-ularies. XHTML is described in Chapter 4, Introduction to XHTML. Chapter 14, XML and RSS, presents several examples that use MathML to render mathematical expressions.
RSS and Atom
Sites that offer RSS (Chapter 14) and Atom feeds can maintain an “open connection” with their readers. Users no longer have to regularly visit sites for updates—by subscribing to a site’s feed, users receive updates as new information is posted to the site. The difference between RSS and Atom is subtle and unimportant to most users—many tools support both formats. Versions of RSS (an XML-based web content syndication format) have ex-isted since the late 1990s; Atom dates to 2003.
Most major web browsers support RSS and Atom feeds, and many aggregators (or feed readers) are available to help users organize their subscriptions. Feedburner (acquired by Google) is used by many blogs to provide their readers with new posts by e-mail. This service also helps bloggers get a better idea of the size of their audience (by allowing them to see the number of subscribers).
Voice over Internet Protocol (VoIP) is the technology used to make free or inexpensive phone calls over the Internet. Some businesses and individuals have switched completely to VoIP and eliminated traditional phone lines to cut costs. There are many VoIP services,such as Vonage, Packet8 or Lingo; Skype is the most popular. Acquired by eBay to inte-grate buyer and seller voice communication into auctions,137 Skype offers free and fee-based services (such as calling non-Skype phones). VoIP is an enabling technology that can be layered into Web 2.0 companies and websites.
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