 extra overhead bits transmitted along with documents containing XML commands. Simple Object Access Protocol (SOAP) This enables communications between programs on different operating systems such as Windows and Linux by specifying how to encode, for example, an HTTP (Hypertext Transfer Protocol) header and an XML file. It eliminates the requirement to modify infrastructures to process HTTP and other communication transport protocols on networks. PROTOCOLS AND LAYERS When describing their products' capabilities, organizations often refer to the OSI layers. 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 profound 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 1-5 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. An understanding of the functionality at each layer of the OSI provides an understanding of the capability of particular protocols and equipment. Examples of the layers include the following. Layer 2: Switches This layer corresponds to capabilities in the Data Link Layer, functioning as the links to the network. However, Layer 2 devices cannot choose between multiple routes in a network. Layer 2 switches are placed in wiring closets in LANs and route messages to devices within their LAN segment. See Figure 1-6 for an illustration of how this works. Layer 3: Switches and Routers Layer 3 corresponds to the Network Layer. These devices can select an optimal route for each www.TechnicalPDF.