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OSI Model : Application Layer Explained- I

OSI Model : Application Layer Explained- I 

The Application layer protocols' functionalities allow our network to communicate with the underlying data network. When we open a web browser or an instant messaging window, we launch an application, and the application is loaded into the device's memory and run. A process is any application that is currently running on a device. 

There are two types of processes that enable network access within the Application layer: apps and services. 

OSI Model : Application Layer Explained
Fig 1.1- OSI Model : Application Layer Explained

⚡ Network-Aware Applications ๐Ÿ“œ

Applications are the software programs used by people to communicate over the network. Some end-user applications are network-aware, meaning that they implement the application layer protocols and are able to communicate directly with the lower layers of the protocol stack. E-mail clients and web browsers are examples of these types of applications. 

⚡ Application layer Services ๐Ÿ“œ

Other programs may need the assistance of Application layer services to use network resources, like file transfer or network print spooling. Though transparent to the user, these services are the programs that interface with the network and prepare the data for transfer. Different types of data - whether it is text, graphics, or video - require different network services to ensure that it is properly prepared for processing by the functions occurring at the lower layers of OSI model. 

Each application or network service uses protocols which define the standards and data formats to be used. Without protocols, the data network would not have a common way to format and direct data. In order to understand the function of various network services, it is necessary to become familiar with the underlying protocols that govern their operation.

⚡ User Applications, services and Application Layer Protocols ๐Ÿ“œ

Application layer uses protocols that are implemented within applications and services. While applications provide people 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.

⚡ Application Layer Protocol Functions ๐Ÿ“œ

Application layer protocols are used by both the source and destination devices during a communication session. In order for the communications to be successful, the application layer protocols implemented on the source and destination host must match.

Protocols establish consistent rules for exchanging data between applications and services loaded on the participating devices. Protocols specify how data inside the messages is structured and the types of messages that are sent between source and destination. These messages can be requests for services, acknowledgments, data messages, status messages, or error messages. Protocols also define message dialogues, ensuring that a message being sent is met by the expected response and the correct services are invoked when data transfer occurs. 

Many different types of applications communicate across data networks. Therefore, Application layer services must implement multiple protocols to provide the desired range of communication experiences. Each protocol has a specific purpose and contains the characteristics required to meet that purpose. The right protocol details in each layer must be followed so that the functions at one layer interface properly with the services in the lower layer. 

Application Layer OSI

Applications and services may also use multiple protocols in the course of a single conversation. One protocol may specify how to establish the network connection and another describe the process for the data transfer when the message is passed to the next lower layer.