Application Layer
Provides a means for the user to access information on the network through an application. This layer is the main interface for the user to interact with the application and therefore the network.
The application layer is the OSI layer closest to the end user, which means that both the OSI application layer and the user interact directly with the software application. This layer interacts with software applications that implement a communicating component. Such application programs fall outside the scope of the OSI model. Application layer functions typically include identifying communication partners, determining resource availability, and synchronizing communication. When identifying communication partners, the application layer determines the identity and availability of communication partners for an application with data to transmit. When determining resource availability, the application layer must decide whether sufficient network resources for the requested communication exist. In synchronizing communication, all communication between applications requires cooperation that is managed by the application layer.
The Application layer provides three basic services to applications, it makes sure the resources needed to carry out a session are present. It matches the application to the appropriate communication protocol or service. It synchronizes the tgransmission of data between the application and its protocol.
The Application layer is used to support the following services, File services - store, move control access to, and retrieve files, Print services - send data to local or network printers, Message services - transfer text, graphics, audio, and video over a network, Application services - process applications locally or through distributed processing, Database services - allow a local computer to access network services.
The Application layer services and protocols you should know are FTP (File Transfer Protocol), E-mail clients , Web Browsers , Telnet, SNMP (Simple Network Management Protocol), BBS (bulletin board system) services , EDI (Electronic Data Interchange) and other transaction services.
The Application layer supplies network services to end-user applications. Network services are typically protocols that work with user's data. For example, in a Web browser application, the Application layer protocol HTTP packages the data needed to send and receive Web page content as illustrated above. The Application layer provides data to (and obtains data from) the Presentation layer.
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