The OSI model is a cornerstone of computer networking, breaking communication into clear, manageable layers. Understanding it helps IT students, tech professionals, and business leaders grasp how data flows across networks. This article introduces the basics and highlights why the OSI model remains essential for modern connectivity. Let’s dive in!

What is the OSI Model?

The open systems interconnection (OSI) model is a conceptual model created by the International Organization for Standardization which enables diverse communication systems to communicate using standard protocols. In plain English, the OSI provides a standard for different computer systems to be able to communicate with each other.

The OSI Model can be seen as a universal language for computer networking. It is based on the concept of splitting up a communication system into seven abstract layers, each one stacked upon the last.

Each layer of the OSI Model handles a specific job and communicates with the layers above and below itself.

Why does the OSI model matter?

Although the modern Internet does not strictly follow the OSI Model (it more closely follows the simpler Internet protocol suite), the OSI Model is still very useful for troubleshooting network problems. Whether it’s one person who can’t get their laptop on the Internet, or a website being down for thousands of users, the OSI Model can help to break down the problem and isolate the source of the trouble. If the problem can be narrowed down to one specific layer of the model, a lot of unnecessary work can be avoided.

What are the 7 layers of the OSI Model?

The seven abstraction layers of the OSI model can be defined as follows, from top to bottom:

7. The application layer

This is the only layer that directly interacts with data from the user. Software applications like web browsers and email clients rely on the application layer to initiate communications. But it should be made clear that client software applications are not part of the application layer; rather the application layer is responsible for the protocols and data manipulation that the software relies on to present meaningful data to the user.

Application layer protocols include HTTP as well as SMTP (Simple Mail Transfer Protocol is one of the protocols that enables email communications).

6. The presentation layer

This layer is primarily responsible for preparing data so that it can be used by the application layer; in other words, layer 6 makes the data presentable for applications to consume. The presentation layer is responsible for translation, encryption, and compression of data.

Two communicating devices communicating may be using different encoding methods, so layer 6 is responsible for translating incoming data into a syntax that the application layer of the receiving device can understand.

If the devices are communicating over an encrypted connection, layer 6 is responsible for adding the encryption on the sender’s end as well as decoding the encryption on the receiver's end so that it can present the application layer with unencrypted, readable data.

Finally, the presentation layer is also responsible for compressing data it receives from the application layer before delivering it to layer 5. This helps improve the speed and efficiency of communication by minimizing the amount of data that will be transferred.

5. The session layer

This is the layer responsible for opening and closing communication between the two devices. The time between when the communication is opened and closed is known as the session. The session layer ensures that the session stays open long enough to transfer all the data being exchanged, and then promptly closes the session in order to avoid wasting resources.

The session layer also synchronizes data transfer with checkpoints. For example, if a 100 megabyte file is being transferred, the session layer could set a checkpoint every 5 megabytes. In the case of a disconnect or a crash after 52 megabytes have been transferred, the session could be resumed from the last checkpoint, meaning only 50 more megabytes of data need to be transferred. Without the checkpoints, the entire transfer would have to begin again from scratch.

4. The transport layer

Layer 4 is responsible for end-to-end communication between the two devices. This includes taking data from the session layer and breaking it up into chunks called segments before sending it to layer 3. The transport layer on the receiving device is responsible for reassembling the segments into data the session layer can consume.

The transport layer is also responsible for flow control and error control. Flow control determines an optimal speed of transmission to ensure that a sender with a fast connection does not overwhelm a receiver with a slow connection. The transport layer performs error control on the receiving end by ensuring that the data received is complete, and requesting a retransmission if it isn’t.

Transport layer protocols include the Transmission Control Protocol (TCP) and the User Datagram Protocol (UDP).

3. The network layer

The network layer is responsible for facilitating data transfer between two different networks. If the two devices communicating are on the same network, then the network layer is unnecessary. The network layer breaks up segments from the transport layer into smaller units, called packets, on the sender’s device, and reassembling these packets on the receiving device. The network layer also finds the best physical path for the data to reach its destination; this is known as routing.

Network layer protocols include IP, the Internet Control Message Protocol (ICMP), the Internet Group Message Protocol (IGMP), and the IPsec suite.

2. The data link layer

The data link layer is very similar to the network layer, except the data link layer facilitates data transfer between two devices on the same network. The data link layer takes packets from the network layer and breaks them into smaller pieces called frames. Like the network layer, the data link layer is also responsible for flow control and error control in intra-network communication (The transport layer only does flow control and error control for inter-network communications).

1. The physical layer

This layer includes the physical equipment involved in the data transfer, such as the cables and switches. This is also the layer where the data gets converted into a bit stream, which is a string of 1s and 0s. The physical layer of both devices must also agree on a signal convention so that the 1s can be distinguished from the 0s on both devices.

The Difference Between the OSI Model and the TCP/IP Model

Data communication is a process or act in which we can send or receive data. Understanding the fundamental structures of networking is crucial for anyone working with computer systems and communication. For data communication two models are available, the OSI (Open Systems Interconnection) Model, and the TCP/IP (Transmission Control Protocol/Internet Protocol) Model.

These models work as frameworks for organizing and understanding how data moves from one device to another across networks. While both models aim to achieve similar goals, they differ in their approach, layer organization, and practical application within computer networking. We will discuss these two models in this article and also see the differences between the two models.

The OSI Model

As we already explained, OSI stands for Open Systems Interconnection. It has 7 layers: the physical layer, the data link layer, the network layer, the transport layer, the session layer, the presentation layer, and the application layer. Each layer performs its task independently. It was developed in 1984 by the International Organization for Standardization (ISO).

Data Flow in the OSI Model

The data flow in the OSI (Open Systems Interconnection) model describes how data is transmitted from one device to another through the seven layers of the OSI model. This process involves encapsulation and decapsulation at each layer to ensure proper data transmission and reception.

The data flow in the OSI model involves encapsulating data at each layer on the sender side, transmitting it over the network, and decapsulating it at each layer on the receiver side to ensure the data reaches its intended destination correctly and reliably.

Advantages

• Both connection-oriented services and connectionless services are supported.
• It’s quite flexible.
• All the layers work independently.

Disadvantages

• Setting up a model is a challenging task.
• It sometimes becomes difficult to fit a new protocol into this model.
• It is only used as a reference model.

The TCP/IP Model

TCP/IP stands for Transmission Control Protocol/Internet Protocol. It has 4 layers called the Physical layer, Network layer, Transport layer, and Application layer.  It also can be used as a communications protocol in a private computer network. It was designed by Vint Cerf and Bob Kahn in the 1970s.

Advantages

• Many routing protocols are supported.
• It is highly scalable and uses a client-server architecture.
• It is lightweight.

Disadvantages

• Little difficult to set up.
• Delivery of packets is not guaranteed by the transport layer.
• Vulnerable to a synchronization attack.

Similarities Between the OSI Model and TCP/IP Model 

OSI and TCP/IP both are logical models. One of the main similarities between the OSI and TCP/IP models is that they both describe how information is transmitted between two devices across a network. Both models define a set of layers. Each layer performs a specific set of functions to enable the transmission of data.

Another similarity between the two models is that they both use the concept of encapsulation, in which data is packaged into a series of headers and trailers that contain information about the data being transmitted and how it should be handled by the network.

Differences Between the OSI Model and TCP/IP Model

The OSI (Open Systems Interconnection) Model and the TCP/IP (Transmission Control Protocol/Internet Protocol) Model are two frameworks used to understand how data moves through networks. While they both help in organizing network communication, they have distinct structures and purposes. Understanding these differences is essential for anyone learning about or working with computer networks.

OSI vs TCP/IP in a Nutshell

Full Form

• OSI Model: OSI stands for Open Systems Interconnection
• TCP/IP Model: TCP/IP stands for Transmission Control Protocol/Internet Protocol

Layers

• OSI Model: It has 7 layers
• TCP/IP Model: It has 4 layers

Usage

• OSI Model: It is low in usage
• TCP/IP Model: It is mostly used

Approach

• OSI Model: It is vertically approached
• TCP/IP Model: It is horizontally approached

Delivery

• OSI Model: Delivery of the package is guaranteed in the OSI Model
• TCP/IP Model: Delivery of the package is not guaranteed in the TCP/IP Model

Replacement

• OSI Model: Replacement of tools and changes can easily be done in this model
• TCP/IP Model: Replacing the tools is not easy as it is in the OSI Model

Reliability

• OSI Model: It is less reliable than the TCP/IP Model
• TCP/IP Model: It is more reliable than the OSI Model

Protocol Example

• OSI Model: Not tied to specific protocols, but examples include HTTP (Application), SSL/TLS (Presentation), TCP (Transport), IP (Network), Ethernet (Data Link)
• TCP/IP Model: HTTP, FTP, TCP, UDP, IP, Ethernet

Error Handling

• OSI Model: Built into Data Link and Transport layers
• TCP/IP Model: Built into protocols like TCP

Connection Orientation

• OSI Model: Both connection-oriented (TCP) and connectionless (UDP) protocols are covered at the Transport layer
• TCP/IP Model: TCP (connection-oriented), UDP (connectionless)

Conclusion

In conclusion, while both the OSI Model and TCP/IP Model are essential for understanding network communication, they differ in their structure and practical application. The OSI Model provides a theoretical framework with seven layers, emphasizing clear separation of functions, while the TCP/IP Model, with its four layers, reflects the protocols used on the internet today. Each model offers unique insights into how data is transmitted across networks, catering to different aspects of network design, management, and troubleshooting.