Understanding the Application Layer of the OSI Model

The application layer is the top layer of the open systems interconnection (OSI) model, and it acts as a communication interface between application programs and users. Whether you have a professional or personal interest in network models, understanding the application layer and how it interfaces with the end user can help you better understand and optimize digital communications. 

Below, you can explore what the application layer is, different protocols, and how you can learn to build your applications.

What is the application layer?

The application layer is the top layer of the Open Systems Interconnection (OSI) model, providing an interface between you and a web application. This layer connects software applications and user experiences, allowing you to access network services and data, such as when you check your email or browse the web. The application layer also performs critical processes, such as data conversion, management, and encryption, to communicate between applications.

The OSI model 

The OSI model is a conceptual framework made up of seven distinct layers of computer networks. It provides a standardized way to explain how systems operations work step-by-step, communicating and functioning with other networks and related devices. 

The seven layers are as follows:

1. Physical layer: This layer defines interface, connection, and hardware requirements, including cabling, modems, repeaters, and adaptors. 

2. Data link layer: This layer divides the raw data into data frames, corrects errors, formats the data, and identifies devices on the network. It also involves media access control (MAC), a network data transfer policy that transmits data between computers using standard cable networks.

3. Network layer: This layer provides routing, addressing, and decongesting of data transfers for effective transmission across different networks to its destination device. 

4. Transport layer: This layer provides process-to-process communication, including segmenting data, correcting errors, and controlling the data flow through managing shared links, variability of delay, and contention for resources. This layer is essential for consistent data transfer and ensuring data packets reach their intended destination.

5. Session layer: This layer is vital for establishing, managing, and ending sessions on different network devices. It is also responsible for maintaining the network’s connection and security.

6. Presentation layer: This layer formats, encrypts, and compresses data presented to user displays and the application.

7. Application layer: This layer provides an access point for applications to communicate and access network services, facilitating user interactions. Applications such as email and web browsing must run.

Read more: What Are Computer Information Systems? Definition, Degree, and Careers

Why the application layer is important

The application layer is essential because it allows you to send, manipulate, and retrieve data using the network. It also allows for communication within the network, enabling you to communicate with different users, businesses, and organizations through email, file transfers, and video chats. The application layer provides critical functions like remote internet hosting and website browsing.

Who uses the application layer?

Many individuals, businesses, and organizations use the application layer to access online services like email, social media, and web searching. For example, you use the application layer whenever you have a video call or conduct online research. 

In a professional environment, you can use the application layer in several careers:

Software developer 

Average annual base salary: $100,828 [1]

Job outlook (projected growth from 2022 to 2032): 25 percent [2]

A software developer develops software architecture layers so that a web application can perform the necessary functions, such as detecting errors or sending a message. As a software developer, you will be responsible for using the correct protocols and design to develop these layers, including the application layer, so they function as intended.

Read more: What Does a Software Developer Do? Career Overview + Outlook

Software engineer

Average annual base salary: ​​$115,377 [3]

Job outlook (projected growth from 2022 to 2032): 25 percent [2]

Software engineers often design web-based programs that interact with the application layer. As a software engineer, you must integrate software that is compatible with the application layer's functions.

Read more: What Does a Software Engineer Do?

Information technology (IT) engineers

Average annual base salary: $103,471 [4]

Job outlook (projected growth from 2022 to 2032): 8 percent [5]

IT engineers who understand the OSI layers, including the application layer, can easily look for errors when software isn’t working correctly for a user. In this job, you may also create protocols, systems, and architecture to secure information in databases, ensuring safe access for users on the network.

Functions of the application layer

The application layer facilitates many types of end-user interactions with digital systems. Essential functions of the application layer include:

• Ensuring data transfer integrity and identifying receiving devices for transfer.

• Sending and receiving information, such as databases and files

• Providing communication services, such as email and live chats

• Allowing remote hosting

• Protocol and data syntax determination

Application layer types

Several application layer protocols exist that allow devices to connect. Protocols help complete user demands by using a network’s program languages. Depending on the task, you may choose to use one of the following types of application layer protocols: 

• File transfer protocol (FTP): You use FTP to share and transfer files between users and networks.

• Simple mail transfer protocol (SMTP): You use SMTP to send and receive email messages. Applications such as Gmail and Outlook use this protocol.

• Hypertext transfer protocol (HTTP): You use HTTP to transfer data across the web, including media and web data, by defining commands to request and receive web content.

• Domain name system (DNS): You use DNS to translate domain names to IP addresses that can load internet content.

• Teletype network (TELNET): You use TELNET to connect with remote devices and systems and relay terminal-to-terminal communication.

Learn more with Coursera.

The application layer is the topmost layer of the OSI model, interfacing between applications and devices connected to a network. Learn more about the application layer and network technologies with online courses offered on Coursera. For beginners, consider building a strong foundation with courses like the University of Michigan’s Internet History, Technology, and Security, which explores internet architecture and modern technological challenges. 

For more technical knowledge, you can take exciting courses such as the University of Maryland’s Cybersecurity For Everyone, where you can watch lectures on topics such as data movement, applications, and basic security protocols. Additionally, try the Network Systems Foundations course from the Univeristy of Colorado Boulder to learn about the different layers in the OSI model, including the application layer.