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OSI Model

by SW Team

To better understand how the Internet works and how secure it is, you need to know about the OSI concept/model on which all security and network management systems on the Internet are starting to be based nowadays.

What is the OSI model?

The Open Systems Interconnection (OSI) model is a conceptual model created by the International Organisation for Standardisation, which allows various communication systems to be connected using standard protocols. In other words, OSI provides a standard for different computer systems to communicate with each other.

The OSI model can be seen as a universal language for connecting networks of equipment. It is based on the concept of dividing a communication system into seven abstract layers, each layer stacked on top of the previous one.


Why is the OSI model important?

The modern Internet does not strictly follow the OSI model, but it is still very useful for solving network problems. Whether a person can't get their laptop to connect to the Internet or a website is down for thousands of users, the OSI model can help to tease out the problem and isolate the possible source.

OSI-based network troubleshooting?

OSI allows us to know and interpret at any given moment where the network packet we want to analyse may be located, allowing us to elucidate the possible point or layer where this packet may have a problem.

If we use it as a map or guide for problem solving, OSI is always used with the intention of reducing and localising a network problem to a specific layer of the model, in order to avoid a lot of unnecessary work for the detection and resolution of the problem.

##What are the 7 layers of the OSI Model? The OSI model is made up of 7 layers. We show them in descending order:

7. Application layer

The application layer: content requested and returned in the required format.

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

Some of the most widely used protocols on the Internet that are managed by OSI layer 7 are:


Hypertext Transfer Protocol (HTTPS) is the fundamental communication protocol on the World Wide Web. Its main purpose is to allow the transfer of information, such as text, graphics, sound, video and other multimedia files, between a client (usually a web browser) and a server. This transfer of data is essential for loading and viewing web pages, as it forms the basis of most online interactions.

HTTPS unlike HTTP produces secure or encrypted communication through the use of SSL certificates.


File Transfer Protocol (FTP) is a network protocol for transferring files between systems connected to a TCP (Transmission Control Protocol) network, based on client-server architecture. From a client computer, you can connect to a server to download files from it or to send files to it, regardless of the operating system used on each computer.


Simple Mail Transfer Protocol (SMTP) is a network protocol used for the exchange of e-mail messages between computers or other devices. POP

The Post Office Protocol (POP) is used in local mail clients to retrieve e-mail messages stored on a remote server, called a POP server.


Internet Message Access Protocol (IMAP) allows access to messages stored on an Internet server. Using IMAP, e-mail can be accessed from any computer with an Internet connection.

This OSI layer is the conceptual basis for modern Application Firewalls or Layer 7 Firewalls.

6. Presentation layer

Presentation layer: encryption, compression, translation

This layer is primarily responsible for preparing data for use by the application layer; in other words, layer 6 makes data ready for consumption by applications. It is responsible for translation, encryption and compression of the data.

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

If the devices communicate over an encrypted connection, layer 6 is responsible for adding the encryption at the sender's end, as well as decoding the encryption at the receiver's end, so that decrypted and readable data can be presented to the application layer.

Then, the presentation layer is also responsible for compressing the data it receives from the application layer before it is sent to layer 5. This helps to improve the speed and efficiency of communication.

5. Session layer

The session layer: communication session

The session layer is responsible for opening and closing communications between two devices. The time that elapses between the opening of the communication and the closing of the communication is known as a session. The session layer ensures that the session remains open long enough to transfer all the data being exchanged, after which it will promptly close the session to avoid wasting resources.

4. Transport layer

The transport layer: segment, transport, reassembly

Layer 4 is responsible for end-to-end communications between two devices. This involves, before proceeding to execute the send to layer 3, taking data from the session layer and then fragmenting it into smaller pieces called segments. The transport layer of the receiving device is then responsible for reassembling these segments and constructing from them data that the session layer can consume.

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

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

This OSI layer is the conceptual basis used by network firewalls or Layer 4 firewalls to manage, allow or block packet traffic between networks.

3. Network layer

The network layer: packet creation, transport, packet assembly

The network layer is responsible for facilitating the transfer of data between two different networks. This layer divides the transport layer segments into smaller units, called packets, at the sender's device, and reassembles these packets at the receiver's device. The network layer also looks for the best physical route for data to take to reach its destination; this is known as routing. Network layer protocols include IP addressing, Internet Control Message Protocol (ICMP), Internet Group Message Protocol (IGMP) and IPsec packet.

2. Data link layer

The data link layer: creation of frames, frames sent between networks

The data link layer is very similar to the network layer, except that the data link layer facilitates the transfer of data between two devices within the same network. The data link layer takes packets from the network layer and divides them into smaller parts called frames. Like the network layer, this layer is also responsible for flow control and error control for intra-network communications (the transport layer only performs flow control and error control tasks for intra-network communications).

1. Physical layer

The physical layer: sender cable, bit stream, receiver cable

This layer includes the physical equipment involved in the transfer of data, such as network cables and switches. This is also the layer where data is converted into a sequence of bits, i.e. a string of ones and zeros. The physical layer of both devices must also agree on a signalling convention so that 1's can be distinguished from 0's on both devices.

Knowing the OSI model and its layers will be very useful to understand much more about security concepts and the tools that are used to ensure security in network or internet environments