Introduction to Data Communications
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Layer 7 - Application Layer

Fig. 1 Basic PC Logical Flowchart

A basic PC logical flowchart is shown in Fig. 1. The Keyboard and Application are shown as inputs to the CPU that would request access to the hard-drive. The Keyboard requests accesses to the hard-drive through user enquiries such as "DIR" commands and the Application through "File Openings" and "Saves". The CPU, through the Disk Operating System, sends/receives data from the local hard-drive ("C:" in this example).

Fig. 2 Simple Network Redirection

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.

Fig. 3 PC Workstation with Network Aware Software

The Application layer identifies the network aware service by assigning a unique number to it. The actual number and identifier will depend on the Network Operating System (NOS) used. For example, the suite of TCP/IP protocols calls the Application layer identifier a port or socket.

Assigning unique identifiers allows multiple network aware applications or services to be run on the same machine. This means that you can run your email client while surfing the web at the same time. The unique identifier will direct the network information to the appropriate network aware program.

Introduction to Data Communications
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