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Application Layer

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Note: Many topics at this site are reduced versions of the text in "The Encyclopedia of Networking and Telecommunications." Search results will not be as extensive as a search of the book's CD-ROM.

The application layer is the top layer of the OSI (Open Systems Interconnection) model. The OSI model guides software developers and hardware vendors in the design of network communications products. When two systems need to communicate, they must use the same network protocols. The OSI models divide protocols in seven layers, with the lowest layer defining the physical connection of equipment and electrical signaling. The highest layer defines how an application running on one system can communicate with an application on another system. Middle layers define protocols that set up communication sessions, keep sessions alive, provide reliable delivery, and perform error checking to ensure that information is transmitted correctly. See "OSI (Open Systems Interconnection) Model" for more information on the complete OSI stack.

The application layer is the top layer in the OSI protocol stack. Applications that provide network features reside at this layer and access underlying communication protocols. Examples include file access and transfer over the network, resource sharing, and print services. The OSI model specifies that applications must provide their own layer 7 protocols. The OSI FTAM (File Transfer Access and Management) utility and the X.400 electronic mail standard provide services at the OSI application layer.

In the Internet world, the application layer resides directly on top of the TCP/IP protocol stack. In this model, the presentation layer and session layer of the OSI protocol stack are used. The application layer talks directly with the transport layer (TCP and UDP). Common Internet applications in the application layer include Telnet, FTP (File Transfer Protocol), NFS (Network File System), SMTP (Simple Mail Transport Protocol), and DNS (Domain Name System).

Copyright (c) 2001 Tom Sheldon and Big Sur Multimedia.
All rights reserved under Pan American and International copyright conventions.