What Is The OSI Model?

Date First Published: 25th March 2022

Topic: Computer Networking

Subtopic: Internet Protocols

Article Type: Computer Terms & Definitions

Difficulty: Advanced

Difficulty Level: 8/10

CONTENTS

Layers
History

Learn more about what the OSI model is in this article.

Short for Open Systems Interconnections model, the OSI model defines seven layers that devices use for communicating over a network. Communication between networks is divided into these layers, which include multiple standards and protocols. It was created by the Internal Organisation of Standardisation in 1984.

The internet does not strictly follow the OSI model. It follows the TCP/IP protocol, a simpler version of the ways that devices communicate over a network. However, the OSI model is helpful for troubleshooting network problems, such as a website going down, as it can help break down a network problem into one layer of the OSI model, avoiding a lot of unnecessary work.

Layers

Data travels through these layers when devices communicate with each other. It starts with the physical layer of the transferring host and ends at the application layer. Data will be processed by the receiving host when it reaches the application layer.

Application layer (Layer 7) - The interaction layer where devices can access the network services. It is used by end-user applications, such as web browsers. The HTTP connection to a website is an example of the use of the application layer.
Presentation layer (Layer 6) - Ensures the usability of sent data and is responsible for presenting the data to the application layer. Data encryption occurs at the presentation layer.
Session layer (Layer 5) - Maintains connections and is responsible for controlling, synchronising, establishing, and terminating the ports and sessions.
Transport layer (Layer 4) - Transfers data using TCP and UDP. It is responsible for delivering data to the appropriate location.
Network layer (Layer 3) - Decides the physical path the sent data will take. It is responsible for routing data packets from the source host to the destination host.
Data-link layer (Layer 2) - Specifies the format of the data sent over the network. It is responsible for converting data stream to signals.
Physical layer (Layer 1) - Transfers bits over the physical medium, such as Wi-Fi or Ethernet. It is responsible for sending bits from one host to another.

DDoS attacks target specific layers of the OSI model. Application layer DDoS attacks target the application layer and protocol layer attacks target the network and transport layer.

History

The Basic Reference Model for Open Systems Interconnection, often known as the Open Systems Interconnection Reference Model, OSI Reference Model, or simply OSI model, was created in 1983 when the CCITT and ISO texts were merged together. Both the ISO and the renamed CCITT (now called the Telecommunications Standardisation Sector of the International Telecommunication Union or ITU-T) released it in 1984 as standard ISO 7498 and standard X.200.

OSI consisted of two main components: an abstract networking model known as the Basic Reference Model or seven-layer model, and a set of specialised protocols. The OSI reference model was a huge step forward in standardising network concepts. It promoted the idea of a consistent protocol layer model for the purpose of defining network device and software connectivity.

Charles Bachman's work at Honeywell Information Systems provided the seven-layer model concept. Experiences with the NPL network, ARPANET, CYCLADES, EIN, and the International Networking Working Group had an influence on a number of elements of OSI architecture (IFIP WG6.1). A networking system was separated into layers in this model. One or more entities implement the functionality of each layer. Each entity only communicated with the layer directly below it and provided capabilities for the layer above it to use.