OSI Model In Networking Explained

The International Organization for Standardization introduced the Open Systems Interconnection (OSI) model in 1984. The primary aim of the OSI was to help different hardware manufacturers to standardize the interworking of diverse communication systems. Ultimately, the OSI model emerged as a solution to challenges related to computer networking standards.

The OSI reference model explains computing functions through standard protocols to support the interoperability of different communication systems. More specifically, the OSI model breaks down network communication into seven layers, also known as protocol layers or the protocol stack.

So, what are the seven layers of the OSI model? Keep reading to learn what they are and how they work.

OSI model’s seven layers

The OSI framework divides the communication between computer systems into seven abstraction layers. The layers of the OSI model are:

• Application
• Presentation
• Session
• Transport
• Network
• Data link
• Physical

This approach is top-down, meaning that we start from the highest layer and work our way down to the last layer. Let’s explore each of the OSI layers.

7 – Application layer

The application layer is the top layer of the OSI model; therefore, the closest to the end-user. In short, the application layer establishes communication between the end-user and the application they use. For example, web browsers or email clients.

Moreover, the application layer defines protocols for successful communication between the user and applications.

Some of the most notable application layer protocols include Hypertext Transfer Protocol (HTTP), Simple Mail Transfer Protocol (SMTP) and File Transfer Protocol (FTP). These protocols enable the software to send and receive information and present it to the user.

6 – Presentation layer

The presentation layer is the sixth layer in the OSI model. It is also known as the syntax layer or translation layer because it translates from the application to the network format.

This layer formats the data so that the application can understand it. This data representation process also involves encrypting and decrypting data.

Two devices may communicate over an encrypted network. In this case, the presentation layer encrypts the users’ incoming data received from the application and decrypts it to readable data at the receiving device. Essentially, encryption ensures the secure transmission of data.

Moreover, the presentation layer compresses the data received from the application layer to send it over to the session layer. The compression ensures more efficient communication by minimizing the size of the data transferred.

5 – Session layer

The fifth layer of the Open Systems Interconnection model is the session layer responsible for establishing a session between two devices.

A session is a time passing between the opening and closing of communication. The session layer ensures that this time is long enough to transmit data efficiently. Also, it closes the session on time to avoid wasting resources.

Moreover, this OSI layer performs data transfer synchronization to ensure that data flows smoothly. For example, if a system sends a lot of data at once, the session layer can add checkpoints and break the data up into smaller pieces.

Let’s say you need to send a 100-page file. The session layer could add a checkpoint after 5 or 10 pages. If the transfer is interrupted before the system sends the whole file, the process can resume from the last checkpoint. Consequently, the system doesn’t need to start the transfer from the beginning and, in turn, can save time.

4 – Transport layer

The transport layer makes end-to-end communication between two devices possible. In other words, it ensures that the message safely travels from the source to its destination.

The transport layer receives the data from the session layer and divides it into smaller pieces, known as segments. Then, the receiving device reassembles the data so that the session layer can read it.

Furthermore, the transport layer is responsible for flow control and error control.

Flow control optimizes the data transfer speed so that the device with a faster connection does not send data too quickly. Otherwise, the device with a slower connection might not be able to handle it. 

Error control, or error checking, ensures that the system sends all the data. If the data is incomplete, the transport layer requests the information again.

Examples of the transport layer protocols include the Transmission Control Protocol (TCP) and User Datagram Protocol (UDP), an alternative to TCP. To put it simply, TCP allows applications and computing devices to communicate. The protocol sends data packets – smaller chunks of information – across the internet and ensures successful data transmission.

3 – Network layer

The network layer simplifies the communication between two different networks. In other words, the network layer takes the segments from the transport layer, breaks them into smaller packets, and puts them together again at the receiving device.

Also, the network layer is responsible for routing. Routing is a process that handles the data transmission through the best possible physical path to connect devices on different networks efficiently. 

Routers are essential components of the network layer since they take the data to its destination in the most appropriate format. It means that routers may break the data into smaller pieces to ensure efficient transmission.

Most importantly, the network layer uses the Internet Protocol (IP), one of the most critical network layer protocols, to ensure smooth data delivery. IP is a standard protocol that helps transfer data packets across networks until they reach their final destination.

2 – Data link layer

Layer 2 – data link layer – is responsible for node-to-node data transfer. In other words, the data link layer transmits data between two directly connected nodes or two devices on the same network architecture.

Specifically, the data link layer accepts the data packets from the network layer and divides them into frames. Then, it sends the data to its destination.

The data link layer has two sublayers: media access control (MAC) and logical link control (LLC). The media access control encapsulates the frames to transmit them through the physical medium (wires and cables). If data transmission fails, LLC manages the retransmission of packets.

The Address Resolution Protocol (ARP) is the most important data link layer protocol because transmitting data is impossible without it. Why? ARP translates IP addresses to physical addresses (MAC addresses) to help systems communicate because these addresses differ in bit length (32 bits vs. 48 bits, respectively).

1 – Physical layer

The lowest layer of the OSI model is the physical layer with all the other layers of the OSI on top. As the name implies, the physical layer handles the equipment transmitting the data, such as cables and switches.

From the data perspective, the physical layer transmits raw data in bits (1s and 0s). That means that the layer takes bits from one device, encodes them into a physical signal, sends them over and decodes them on the other end. Without these signals, the physical layer would not transmit the data through the physical medium.

Consequently, the network architecture cannot exist without the physical layer, as this layer defines the hardware necessary to transmit bits over a network.

Why is the OSI model important?

This conceptual framework divides the communication system into seven layers to isolate the origin of the issue, be it cabling problems or communication failures. 

Most network problems appear at the physical, data link or network layers. Network operators or administrators may start from the bottom layer and proceed to the application layer to try to find the issue. They can also begin from the application layer and go down to the physical layer.

Experienced operators may even choose the divide-and-conquer approach and solve the issue faster if they suspect what the problem with the network might be. This allows saving time and ensures smooth communication.

Perhaps most importantly, the OSI model helps hardware manufacturers create devices that can successfully communicate over the network.

It’s worth mentioning that the OSI model is often compared to the TCP/IP model. Both use the layer structure and defined protocols. However, the OSI model is a basic reference model that describes and understands network functions. In contrast, the TCP/IP suite is more functional and also simpler. Unsurprisingly, the modern internet relies on TCP/IP over OSI.

Conclusion

Despite its age, the OSI reference model is still applicable today to standardize the networking system. Specifically, the OSI model divides the computing system into seven layers to simplify the troubleshooting of different network problems.

The OSI model can also assist equipment manufacturers in creating products that can communicate with any software. Smooth communication between computer networks, in turn, can increase the interoperability of different devices of the end-user.

Maybe, more comprehensive standards, like TCP/IP, will completely replace OSI with time. However, many network administrators are likely to continue applying the OSI model to make their computing systems more efficient in the meantime.