Understanding the OSI Model: The Basics


The OSI model is a conceptual framework that characterizes and standardizes the function of a communication system. Each layer receives the information from the layer above it and sends the information to the layer below, performing data transfer in a manner specified by the layer above it.

The OSI model is a way to characterize the ideal functioning of a computer network. In this blog post, we will discuss what a computer network is and how these different layers of the OSI model work, so you can consider how to improve your networks with it.

What is the OSI Model?

The OSI (Open Systems Interconnection) model describes how two or more computers talk to each other and what they exchange. The model is widely used to describe the set of data exchanged between computer networks. One benefit of the model is that it allows you to implement your network in a matter of minutes.

It is the set of guidelines that lay out how information is passed around the internet. It contains 7 different layers, each layer having its own particular role to play in passing information from one device to another. We will look at each layer individually.

Layer One – Physical

The first layer – called the Physical layer – is the lowest and the slowest layer. This layer is about cables, as well as the hardware interfaces that they connect to. The physical layer controls electronics on a physical level. Also, it handles the transportation of signals across a cable or through the air. Also, the physical layer handles the conversion data points into voltage changes or light pulses that represent binary values.

The physical layer also includes the means of transferring data from one data center to another. This is the basis for the internet as we know it today. The physical layer is the foundation on which the rest of the OSI model rests, but it also comes with many trade-offs that have to be considered.

Layer Two – Data Link

The data link layer – often referred to as the analog network layer – specifies the physical data link such as cables and fiber optic cables. It is responsible for logical addressing and for resolving any contention that might happen between two devices trying to transmit over a shared transmission medium.

The Data Link layer allows for packets to be handed off to the appropriate network node. Using error checking, this layer also ensures that frames received are intact and in the right order.

Layer Three – Network

Networks connect communication devices and computers together. A network comprises 3 elements:

  1. Physical Components
  2. Media
  3. Transportation

The network layer is responsible for receiving frames from the data link layer. It then delivers them to their intended destinations based on logical addresses, such as IP.

Layer Four – Transport

The fourth layer of the OSI model refers to the transmission of data between layers, hence suitably called the Transport layer. With this type of layer, we are talking about the data transfers between two computers or machines, rather than between two human beings.

The transport layer takes care of the required protocol and format to transmit data from one application to another. The major function of this layer is to break down the data into smaller units and transfer them across a network.

The transport layer in the OSI model provides a flow in a network to transport data in a reliable way. This is done in a buffer-driven method, with multiple buffers being dedicated to the same destination from the source.

If one is full, when another becomes available, it is filled, and so on. The transport layer ensures that either the data is delivered one time or that all data is delivered, though where both options are possible, it chooses what has been delayed the least.

Layer Five – Session

It is the fifth layer of the OSI model. A session is a way to connect two hosts. The Session layer comes fifth in the OSI model and it provides dialog control between computer applications.

The typical services provided in this layer include maintaining a dialog, user authentication, and logging off from a session. This layer allows multiple processes to share a common connection between applications. It also provides conversion of function codes and character strings.

Layer Six – Presentation

The Presentation layer in the OSI Model is responsible for data representation. It takes the OSI data from the Application layer and prepares it to be read by end-users so that it would appear in an understandable format.

The presentation layer includes the functionality to display the data as per users’ needs and requests. The presentation layer is higher in applications that support complex and formatted data transmission. Also, it is responsible for translating the data from an application to a format that is usable by other applications.

Layer Seven – Application

The Application layer is responsible for interoperating various application programs, ensuring that they receive reliable data transfer services. In the OSI model, the application layer corresponds to the Transport Layer, as it supports the end-to-end connection in the form of a virtual circuit.

This layer deals with applications and services that are independent of any particular protocol. Moreover, the protocols and services on this level are very diverse, ranging from FTP, SMTP, DNS, and NFS to Remote Shell Protocol (RSH) and TFTP.

TCP/IP Model vs OSI Model

Often, the OSI model is compared with the TCP/IP model. Important differences among the two models are:

  • TCP model follows a horizontal approach while the other one follows a vertical approach.
  • TCP model is both connection-oriented and connectionless. OSI model (the transport layer), however, is only connection-oriented.
  • OSI has 7 layers while TCP has only 4.


That sums up the OSI model. Understanding the same is very important to develop a robust understanding of how computers and devices communicate over a network. We hope that this article helps you to understand the OSI model in an easy way.

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