Introduction to Data Communications
32b. Layer 7 - Application Layer

A PC setup as a network workstation has a software "Network Redirector" (actual name depends on the network - we will use a generic term) placed between the CPU and DOS as in Fig 2. The Network Redirector is a TSR (Terminate and Stay Resident) program which presents the network hard-drive as another local hard-drive ("G:" in this example) to the CPU. Any CPU requests are intercepted by the "Network Redirector". The Network Redirector checks to see if a local drive is requested or a network drive. If a local drive is requested, the request is passed on to DOS. If a network drive is requested, the request is passed on to the network operating system (NOS).

Electronic mail (E-Mail), client-server databases, games played over the network, print and file servers, remote logons and network management programs or any "network aware" application are aware of the network redirector and can communicate directly with other "network applications" on the network. The "Network Aware Applications" and the "Network Redirector" make up Layer 7 - the Application layer of the OSI Model as shown in Fig 3.