ABSTRACT
Networking at engineering level is a complicated task. It involves software, firmware, chip level engineering, hardware and even electric pulses. To ease network engineering, the whole networking concept is divided into multiple layers. Each layer is involved in some particular task and is independent of all other layers. But as a whole the almost all networking task depends on all of these layers. Layers share data between them and they depend on each other only to take input and give output. In layered architecture of Network Models, one whole network process is divided into small tasks. Each small task is then assigned to a particular layer which works dedicatedly to process the task only. Every layer does only specific work. I have chooses application layer from the OSI model because the world experiences the Internet through the use of the World Wide Web, e-mail, and file-sharing programs. These applications, as well as others, provide the human interface to the underlying network, allowing you to send and receive information with relative ease. Most of the applications are intuitive; they can be accessed and used without the need to know how they work. As we continue to study the world of networking, it becomes more important to know how an application is able to format, transmit, and interpret messages that are sent and received across the network.

INTRODUCTION
Application layer is the highest most layers in OSI and TCP/IP layered model and this layer exists in both layered Models because of its significance which is interacting with user and user applications. This layer is for applications which are involved in communication system. A user may or may not directly interacts with these applications. Application layer is where the actual communication is initiated and reflects. Because this layer is on the top of the layer stack it does not serve any other layers. Application layer takes the help of Transport and all layers below it to communicate or transfer its data to the remote host.When an application layer protocol wants to communicate with its peer application layer protocol on remote hosts it hands over the data or information to the Transport layer. The transport layer does the rest of the things with help of all layers below it.

There’s an ambiguity in understanding Application Layer and its protocol. Not every user application can be put into Application Layer. Only application which interacts with the communication system. For example, a designing software or text-editor cannot be considered as application layer programs. On the other hand when we use a Web Browser, which is actually using HTTP (Hyper Text Transfer Protocol) to interact with the network. So in this case, HTTP is Application Layer protocol. Another example is File Transfer Protocol, which helps a user to transfer a text based or binary file across the network. A user can use this protocol in either GUI based software like FileZilla or CuteFTP and the same user can use FTP in Command Line mode. So it is not important what software we use, it the protocol which is considered at Application Layer used by that software. DNS is a protocol which helps user application protocols like HTTP to accomplish its work.

BACKGROUND
In the OSI model, information is passed from one layer to the next, starting at the application layer on the transmitting host and proceeding down the hierarchy to the physical layer, then passing over the communications channel to the destination host, where the information proceeds back up the hierarchy, ending at the application layer. The Figure depicts the steps in this process. The following explains the six steps:
1. People create the communication.
2. The application layer prepares human communication for transmission over the data network.
3. Software and hardware convert communication to a digital format.
4. Application layer services initiate the data transfer.
5. Each layer plays its role. The OSI layers encapsulate data down the stack. Encapsulated data travels across the media to the destination. OSI layers at the destination unencapsulate the data up the stack.
6. The application layer receives data from the network and prepares it for human use.

The application layer is the top layer of both the OSI and TCP/IP models. Its provides the interface between the applications we use to communicate and the underlying network over which our messages are transmitted. Application layer protocols are used to exchange data between programs running on the source and destination hosts. There are many application layer protocols, and new protocols are always being developed. Although the TCP/IP protocol suite was developed prior to the definition of the OSI model, the functionality of the TCP/IP application layer protocols fits roughly into the framework of the top three layers of the OSI model: application, presentation, and session. Most applications, such as web browsers or e-mail clients, incorporate functionality of the OSI Layers 5, 6, and 7. A comparison of the OSI and TCP/IP model is shown in Figure 3
Most TCP/IP application layer protocols were developed before the emergence of personal computers,GUIs, and multimedia objects. As a result, these protocols implement little of the functionality that is specified in the OSI model presentation and session layers.

Presentation Layer

The presentation layer has three primary functions:
■ Coding and conversion of application layer data to ensure that data from the source device can be interpreted by the appropriate application on the destination device.
■ Compression of the data in a manner that can be decompressed by the destination device
■ Encryption of the data for transmission and decryption of data upon receipt by the destination
Presentation layer implementations are not typically associated with a particular protocol stack. The standards for video and graphics are examples. Some well-known standards for video include QuickTime and Motion Picture Experts Group (MPEG).

Session Layer
Functions at the session layer create and maintain dialogs between source and destination applications. The session layer handles the exchange of information to initiate dialogs and keep them active, and to restart sessions that are disrupted or idle for a long period of time.

TCP/IP Application Layer Protocols
The most widely known TCP/IP application layer protocols are those that provide the exchange of user information. These protocols specify the format and control information necessary for many of the common Internet communication functions. Among these TCP/IP protocols are the following: * Domain Name System (DNS) is used to resolve Internet names to IP addresses. * Hypertext Transfer Protocol (HTTP) is used to transfer files that make up the web pages of the World Wide Web. * Simple Mail Transfer Protocol (SMTP) is used for the transfer of mail messages and attachments. * Telnet, a terminal emulation protocol, is used to provide remote access to servers and networking devices. * File Transfer Protocol (FTP) is used for interactive file transfer between systems.