OSI Model: An Overview

The OSI model is a network framework that breaks down network communication functions into seven distinct layers to convey information across distinct network systems. Understanding this model can deepen your knowledge of network communication and organization. 

Explore the basic concepts of the OSI model, how the layers work together, and how you can build the necessary skills to utilize this framework. Plus, you can check out the TCP/IP model, its layers, and how it compares to the OSI model so you can choose the right framework for you.

What is the open systems interconnection (OSI) model?

The OSI model is a standardized framework that divides network communication and computing system functions into seven layers. This enables disparate technologies to communicate accurately via standard communication protocols. A reference model, the OSI model guides developers to understand a clear framework for the functions of networking and telecommunication systems.

Each of the seven layers in the OSI model provides a set of functions that work together to define a system of network protocols that can be understood and troubleshooted. By layering the network, you break down the design into smaller pieces, which in turn reduces complexity and allows each layer to change independently. This model allows for interoperability between different types of systems and networks. 

What are the layers of the OSI model?

The OSI model includes seven layers: physical, data link, network, transport, session, presentation, and application. Each layer completes specific tasks and protocols. Take a closer look at the functions of each layer below.

1. Physical layer

The physical layer is the first layer of the OSI model. Using physical hardware like electrical, procedural, or mechanical interfaces, this layer transmits data physically through cables and channels. The physical medium converts raw data bits into electronic signals before transmitting them. It determines how devices are physically laid out and connected to one another.

2. Data link layer

The data link layer is the second layer of the OSI model, and it creates links between device connection points for data transfer. This layer is responsible for detecting data errors, correcting them, managing the data flow between devices, as well as organizing data bits into frames that can be sent to the network layer.

3. Network layer

The third layer of the OSI model is the network layer, which divides, routes, and reassembles data packets to and from networks to deliver data to the proper networks. Routing, addressing, and forwarding data packets are all components of the network layer, as well as controlling data flow. 

4. Transport layer

The transport layer is the fourth layer of the OSI model, providing end-to-end delivery of data. This layer is responsible for adding the port number to the data and providing identification for applications and client-side entities, as well as confirming that the packets arrive intact. If the data is incomplete, the transport layer plays a critical role in error control, along with maintaining the order of data.

5. Session layer

The session layer is the fifth layer of the OSI model, and it establishes, maintains, and ends application connections. The session layer synchronizes the exchange of data between different systems, allowing devices to transmit signals between one another. 

6. Presentation layer

The presentation layer is the sixth layer of the OSI model. As the name suggests, this layer converts data into a format suited to the application that receives it. This often involves translation, compression, conversion, encryption, and decryption. 

7. Application layer

The application layer is the seventh layer of the OSI model, and it creates an interface between the user and the rest of the network services. This layer is also responsible for network-specific services, including email, file transfers, web browsing, and accessing other network resources. Depending on the type of application used on this layer, you can choose different protocol options, such as HTTP, SMTP, and FTP.

Together, these seven layers make it possible for one application to communicate with another on a different device using standardized communication protocols. Each layer only interfaces with the adjacent layers, and this chain of layers linked by protocols allows for complex data communications through a universal language.

OSI model vs. TCP/IP model

Like the OSI model, the TCP/IP model is a computer networking framework that defines and breaks down network communications into layers. The TCP/IP model, also known as the transmission control protocol/internet protocol model or internet protocol suite, is a networking framework that is more common in practical applications than the OSI model. However, both models have defined standards and logical structures that can pinpoint breakdowns in network communication. 

The TCP/IP model has four to five layers, whereas the OSI model further breaks down functions into seven. In some cases, this can make troubleshooting more difficult with the TCP/IP model since errors may be less defined in their origin than in the expanded OSI model. 

When choosing which model is right, consider how each model functions and its advantages and disadvantages.

What is the TCP/IP model?

The TCP/IP model is a fundamental framework for network computing, and it ensures reliable communication, or data transfer, between different devices. The Defence Advanced Research Projects Agency (DARPA) created the TCP/IP model, which typically sets the industry standard. This model was specifically developed for the internet, and it exists to guarantee the accurate data transfer. It does this by dividing data into individual data packets, which are then conveyed to the destination device and recombined. Many professionals consider it the standard for internet communication. The TCP/IP model divides data transfer into a four- or five-layer procedure, which functions similarly to the seven-layer task division of the OSI model:

Four-layer model:

1. Application layer

2. Transport layer

3. Network layer

4. Physical layer

Five-layer model:

1. Application layer

2. Transport layer

3. Network access layer

4. Network interface layer

5. Hardware layer

Pros and cons of the OSI Model

When considering which model is right for you, it is important to weigh the advantages and disadvantages of each model. Explore some of the pros and cons of using the OSI model. 

OSI model benefits:

• Supports wired and wireless services

• More secure and reliable

• Layers are easily adaptable

• Supports interoperability

• Flexibility with protocols

OSI model disadvantages:

• Layers can’t work in parallel

• Doesn’t define protocol

• Duplication of services across layers

• Can be overly complex

Pros and cons of the TCP/IP model

Like the OSI model, the TCP/IP model has advantages and disadvantages. The following offers a brief breakdown of the pros and cons of the TCP/IP model. 

TCP/IP model benefits:

• Establishes a connection point between different computers

• Compatible with the internet

• Supports different routing protocols

• Can be independently operated

TCP/IP model disadvantages:

• Can be difficult to replace protocols

• Complicated to manage

• Less secure and reliable

• Intertwined interfaces, protocols, and services

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The OSI model uses seven layers of distinct operations to help support digital communication worldwide, while the TCP/IP model employs four or five layers of operations to convey data reliably and compatibly with the Internet. Understanding networking basics can help you make personal technology choices and explore professional opportunities. 

To start learning, consider The Bits and Bytes of Computer Networking course offered by Google and available on Coursera. If you are new to computer networking, you might want to check out Akamai’s Networking Fundamentals course to learn basic network theory and terminology. You can also expand your knowledge by earning a certificate, like the Google IT Automation with Python Professional Certificate available on Coursera.