sers browse the Internet and access information through web interfaces at enterprises. They are good examples of overhead bits. Extensible Markup Language (XML) This is another markup language based on elements surrounded by tags that identify fields requiring user input. The tag <name> is an example of a tagged label; it is not visible to users, but labels the field. Other variable labels might include quantity, Computing and Enabling Technologies address, age, and SO on. Firms can analyze responses provided by visitors to a site who fill out online surveys. Tagged responses can be sorted by fields such as geography or age. XML enables computers to automatically process responses collected online or in specialized applications such as purchasing and ordering functions in businesses. The protocol-related tags and labels identifying fields in XML create the many extra overhead bits transmitted along with documents containing XML commands. PROTOCOLS AND LAYERS When describing their products' capabilities, organizations often refer to the OSI lay- ers. In the 1970s, the International Organization for Standardization developed the Open Systems Interconnection (OSI) architecture, which defines how equipment from multiple vendors should interoperate. Architecture refers to the ways that devices in networks are connected to one another. Although not widely implemented because of its complexity, OSI has had a pro- found influence on telecommunications. The basic concept underpinning OSI is that of layering. Groups of functions are divided into seven layers, which can be changed and developed without having to change any other layer. (See Table 1-2 in the "Appendix" section at the end of this chapter for a complete list of layers.) LANs, public networks, and the Internet's TCP/IP suite of protocols are based on a layered architecture. If you have an understanding of the functionality at each layer of the OSI, you will better understand the capability of particular protocols and equipment. Examples of the layers include th