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The OSI Model

Published 7 May 2020

A conceptual framework that describes the functions of a networking or telecommunication system. Characterizes and standardizes the communications between different computer systems. It allows a variety of communication systems to communicate using a standard protocol. The model splits up the communication into 7 layers, each layers communication having moved through the previous layer in the chain. The OSI model helps us understand the flow of data across a network.

7 layer OSI model diagram

Layer 7: Application

The application layer is the layer closest to the user. It’s the layer that directly interacts with data from the end user applications. At this layer we’ll be using protocols like HTTP when requesting a webpage in a browser or SMTP when downloading email into our desktop email client.

  • HTTP
  • SMPT

Layer 6: Presentation

The presentation layer is responsible for making the data presentable to be consumed by the application layer. Any encryption, compression or translation of the data will take place at this layer. Given 2 applications using different encodings methods trying to communicate, the data is translated into what each device can understand. If communication is over an encrypted network, it’s the presentation layer which handles the data encryption on both the sender and receivers ends. To help increase speed and efficiency of the data flow from layer 7 to layer 5, layer 6 reduces the data size by applying compression.

  • MOV, JPEG, ZIP

Layer 5: Session

The session layer creates, manages, and terminates the connections between the applications on a network. In order for the communication between 2 devices to occur a session needs to be created. The session layer ensures that a session is opened in order to start communication. It makes sure that the session stays open for the right amount of time to allow all data to be transferred and makes sure a session is closed to prevent wasting resources.

  • Session management

Layer 4: Transport

The transport layer is responsible for taking data from the session layer and breaking it up into chucks that can be sent to the network layer. A web page for example is built up of many packets received over the network from the server. This layer manages the flow and error control of the data. Flow referring to the speed at which the data is sent and processed. Error control referring to the order of data received at the destination. It ensures that the data is repackaged and complete and if not will request a retransmission of the data. The protocols referred to at this layer are TCP and UDP.

  • TCP
  • UDP

Layer 3: Network

The network layer facilitates data transfer between nodes on a different network. It deals with IP addresses and routing. This layer is only required if the data flow is between 2 different networks. The network layer breaks up data from the transport layer and creates smaller packets on the senders devices. It is also responsible for reassembling fragmented packets on the receivers device. Routing is a fundamental part at this layer, it refers to finding the most efficient physical path for the data to reach the intended receiving device.

  • IP address
  • Routing

The data link layer facilitates the data flow between 2 directly connected nodes on the same network. It is at this layer that physical error correction also takes place. On an ethernet network the data link layer refers to communication between MAC addresses on the network. At this layer packets from the network layer are taken and broken down into smaller blocks called frames.

  • Switches
  • MAC addresses

Layer 1: Physical

The physical layer is the equipment, electrical and physical, that’s involved in allowing the data to be transported. It’s where the signal gets converted to bitstreams. The physical layer comprises of cables and switches. When trouble shooting one should always start at layer 1, the physical layer, and work your way up to the application layer. Eg: someone’s says their internet is not working, you would start by checking if the wifi router is plugged in or their ethernet cable is still connected.

  • cables
  • fibre optics

When we receive data it always enters through the physical layer 1 and moves to the application layer 7. When we are transmitting data it moves from the application layer all the way down to the physical layer.