|  | 1.1 | The OSI Model
| 1.1.1 | Layered network model
New business practices are driving changes in enterprise networks. Employees at corporate headquarters and in worldwide field offices, as well as telecommuters in home offices, need immediate access to data, regardless of whether the data is on centralized or departmental servers. Enterprises such as corporations, agencies, schools, or other organizations that tie together their data communication, computing, and file servers need: interconnected LANs that provide access to computers or file servers in other locations higher bandwidth onto the LANs to satisfy the needs of the end users support technologies that can be relayed for WAN service To improve communication with partners, employees, and customers, enterprises are implementing new applications such as electronic commerce, videoconferencing, voice over IP, and distance learning. Businesses are merging their voice, video, and data networks into global enterprise networks as shown in Figure that are critical to the organization's business success. Enterprise networks are designed and built to support current and future applications. To accommodate increasing requirements for bandwidth, scalability, and reliability, vendors and standards bodies introduce new protocols and technologies at a rapid rate. Network designers are challenged to develop state-of-the-art networks even though what is considered state-of-the-art changes on a monthly, if not weekly basis. By dividing and organizing the networking tasks into separate layers/functions, new applications can be handled without problems. The OSI reference model organizes network functions into seven categories, called layers. Data flows from upper-level user applications to lower-level bits that are then transmitted through network media. The task of most wide area network managers is to configure the three lowest layers. Peer-to-peer functions use encapsulation and de-encapsulation as the interface for the layers. As shown in the Figure there are seven layers in the OSI reference model, each of which has separate distinct functions. The Transmission Control Protocol/Internet Protocol (TCP/IP) models' functions fit into five layers. This separation of networking functions is called layering. Regardless of the number of layers, however, the reasons for the division of network functions include the following: to divide the interrelated aspects of network operations into less complex elements to define standard interfaces for plug-and-play compatibility and multivendor integration to enable engineers to focus their design and development efforts on a particular layer's functions to promote symmetry of the different internetwork modular functions for the purpose of interoperability to prevent changes in one area from significantly affecting other areas, so that each area can evolve more quickly to divide the complex operations of internetworking into discrete, more easily learned operational subsets