The OSI indication model is a covered, intangible illustration invented as a guideline about the network protocol design. The OSI model divides the process of networking to 7 layers, each one of them has single functionality_ and to which are assigned the_ detailed services and protocols.
-In this model, the information is passed one layer to the other next to it, it starts from the Application layer in the transmitting host, arranged down the hierarchy to the Physical layer, and then passing over the communications channel to the destination host,where the information proceeds back up the hierarchy,it ends at the Application_ layer. The figure describes the steps at this process.
The Application layer, Layer 7, is the top layer of both OSI and TCP/IP models. It is the layer which presents the interface linking the applications we use to be in touch with the fundamental network in which the messages are in transmission. The use the Application layer protocols for exchanging data between programs that is running in the source and target hosts. There Application layer protocols and new protocols are for all time being developed.
Although the TCP/IP protocol suite is developing previous to the explanation of the OSI model, the functionality of the TCP/IP application layer protocols fit roughly into the framework of the top three layers of the OSI model: Application, Presentation and Session layers.
Most TCP/IP application layer protocols were developed previous to the appearance of personal computers, and also the graphical user interfaces and multimedia objects. As a result, these protocols implement a little of the functionality that is specified in the OSI model Presentation and Session layers.
The 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 the 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). QuickTime is an Apple Computer specification for video and audio, and MPEG is a standard for video compression and coding.
Among the well-known graphic image formats are Graphics Interchange Format (GIF), Joint Photographic Experts Group (JPEG), and Tagged Image File Format (TIFF). GIF and JPEG are compression and coding standards for graphic images, and TIFF is a standard coding format for graphic images.
The Session Layer
As the name of the Session layer implies, functions at this layer create and maintain dialogs between source and destination applications. The Session layer handles the exchange of information to initiate dialogs, keep them active, and to restart sessions that are disrupted or idle for a long period of time.
Most applications, like web browsers or e-mail clients, incorporate functionality of the OSI layers 5, 6 and 7.
The TCP/IP Application layer protocols that define the format and control the information needed for more than one of the functions of internet communication are those that give for the transfer of user information. These TCP/IP protocols are:
Domain Name Service Protocol (DNS) used to solve the names of the internet to IP addresses.
Hypertext Transfer Protocol (HTTP) used to exchange files that design the web pages of the WWW.
Simple Mail Transfer Protocol (SMTP) used to exchange mail messages and attachments.
Telnet, a terminal emulation protocol, is used to provide remote access to servers and networking devices.
File Transfer Protocol (FTP) used for file exchange between systems.
The TCP/IP protocols are determined by Requests for Comments (RFCs). The RFCs as the regular for the TCP/IP suite are maintained by the Internet Engineering Task Force.
Application Layer Software
The application layer protocols with the functions associated with it enable the human network to interface the original data network. An application is begun when a web browser or a fast message window, and the program is existed into the device’s memory where it is executed. All programs that are executed, loaded on a device is referred to as a process.
We have inside the application layer two types of software programs that give access to the network: applications and services.
Network-Aware Applications
The programs used by us for the communication over the network, are the applications. Some end-user applications utilize application layer’s protocols and they can communicate with the lower layers of the protocol stack directly. As examples of these applications: E-mail clients and web browser.
Application layer Services
To use network resources, the assistance of application layer services may be needed by other programs, as an example, file transfer or network print spooling. These services are the programs that border with the network and organize the data for exchanging, while transparent to the user. Dissimilar network services are required by different forms of data, to be sure that it is arranged correctly for processing by the functions taking place at the very low layers of OSI model.
All applications and network services utilize protocols principles and data formats to be used. The data network wouldn’t have a general technique to arrange and direct data. It is crucial to become familiar with the original protocols that direct their function, to understand the role of diverse network services.
As mentioned previously, the Application layer uses protocols that are implemented within applications and services. While applications provide people with a way to create messages and application layer services establish an interface to the network, protocols provide the rules and formats that govern how data is treated. All three components may be used by a single executable program and may even use the same name. For example, when discussing "Telnet" we could be referring to the application, the service, or the protocol.
In the OSI model, applications that interact directly with people are considered to be at the top of the stack, as are the people themselves. Like all layers within the OSI model, the Application layer relies on the functions of the lower layers in order to complete the communication process. Within the Application layer, protocols specify what messages are exchanged between the source and destination hosts, the syntax of the control commands, the type and format of the data being transmitted, and the appropriate methods for error notification and recovery.
