Layer 7 - Application Layer
In the context of the OSI reference model, the application layer (Layer 7) supports the communicating component of an application. The application layer is responsible for the following:
The application layer is the OSI layer closest to the end system. This determines whether sufficient resources exist for communication between systems. Without the application layer, there would be no network communication support. The application layer does not provide services to any other OSI layer. It does provide services to application processes lying outside the scope of the OSI model. Examples of such application processes include spreadsheet programs, word processing programs, and banking terminal programs. Additionally, the application layer provides a direct interface for the rest of the OSI model by using network applications (e.g. Browser, e-mail, FTP, Telnet), or an indirect interface by using standalone applications (e.g. word processors, spreadsheets, presentation managers) with a network redirector.
Most applications that work in a networked environment are classified as client-server applications. These applications, such as FTP, web browsers, and e-mail, all have two components, which allow them to function - the client side, and the server side. The client side is located on the local computer and is the requestor of the services. The server side is located on a remote computer and provides services in response to the clients requests.
A client-server application works by constantly repeating the following looped routine: client-request, server-response; client-request, server-response; etc. For example, a web browser accesses a web page by requesting a uniform resource locator (URL), or web address, on a remote web server. After it locates the URL, the web server that is identified by that URL responds to the request. Then, based on the information received from the web server, the client can request more information from the same web server, or can access another web page from a different web server.
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