OSI Model: An Overview

Key takeaways

• The OSI model is a network framework that breaks down network communication functions into seven distinct layers to convey information across distinct network systems.

• The OSI model layers include the physical, data link, network, transport, session, presentation, and application.

• The purpose of the OSI model is to create standard communication protocols so that all developers have a detailed framework to refer to as they create applications for and maintain computer networking.

Breaking down the OSI model layers gives you a clearer understanding of each function, helping you troubleshoot issues more effectively. Explore the basic concepts of the OSI model below, how the layers work together, and how you can build the skills to use this framework.

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What is the purpose of the OSI model?

The purpose of the OSI model is to provide a framework of network layers for better troubleshooting practices. This enables disparate technologies to communicate accurately via standard communication protocols. It's important to note that the OSI model is a reference model, whereas the Transmission Control Protocol/Internet Protocol (TCP/IP) is the protocol suite that allows the internet to function.

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 OSI model layers?

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, and organizing data bits into frames for transmission 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.

Why is OSI not used?

Networking courses use the OSI model as a way of thinking through network layers; however, when it comes to the actual functioning of the internet, TCP/IP proved the better model to physically implement than the OSI protocol.

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 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.

Read more: What Is TCP/IP?

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 Defense 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 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

The OSI offers a standardized solution to bridge potential communication incompatibilities across networks, computer systems, and applications. As you decide which model is best for you, consider some of these pros and cons of the OSI model:

OSI model benefits

Some benefits of the OSI model include its reliability and interoperability:

• Supports wired and wireless services

• More secure and reliable

• Layers are easily adaptable

• Supports interoperability

• Flexibility with protocols

OSI model disadvantages

Some of the disadvantages of the OSI model include:

• 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

The benefits of the TCP/IP model are its usage of routing protocols and others, such as:

• Establishes a connection point between different computers

• Compatible with the internet

• Supports different routing protocols

• Can be independently operated

TCP/IP model disadvantages

Some disadvantages of the TCP/IP model include:

• Can be difficult to replace protocols

• Complicated to manage

• Less secure and less reliable

• Intertwined interfaces, protocols, and services

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