<p><strong class="ql-font-monospace">The OSI (Open Systems Interconnection) </strong><span class="ql-font-monospace">Model defines how different computer systems communicate over a network. It was developed by the International Organization for Standardization (ISO). It consists of 7 layers and each layer has specific functions and responsibilities.</span></p><p><br></p><p><span class="ql-font-monospace">In this topic, we will discuss:</span></p><p><span class="ql-font-monospace">- What are the layers included in the OSI layers?</span></p><p><span class="ql-font-monospace">- What are the roles of each layer?</span></p><p><span class="ql-font-monospace">- How the Data is handled in each layer?</span></p><p><br></p><p><span class="ql-font-monospace">Below diagram shows how the data is handled per the the OSI layers</span></p>

<h3><strong>OSI layers:</strong></h3><ol><li>Application layer: This layer is application running on the computers, like web browsers.</li><li>Presentation layer: In this layer, the data structure is define for the application it self, in other word, it defines how is the data will be handled by the application it self, the type of data like (and how the data is loaded, save etc... so the application can handle the data or the received computer can load or read it.):</li></ol><ul><li class="ql-indent-1">String</li><li class="ql-indent-1">Integer.</li><li class="ql-indent-1">Boolean</li></ul><ol><li>Session layer: This layer defines the structure of the data in order to send it over the network from one computer or another.</li><li>Transport layer: This layer controls how the traffic is transmitted between the computers, like:</li></ol><ul><li class="ql-indent-1">Connection-oriented communication</li><li class="ql-indent-1">Same order delivery</li><li class="ql-indent-1">Reliability</li><li class="ql-indent-1">Flow control</li><li class="ql-indent-1">Congestion avoidance</li><li class="ql-indent-1">Port Multiplexing.</li></ul><ol><li>Network layer: This layers is responsible for:</li></ol><ul><li class="ql-indent-1">Handling packet routing</li><li class="ql-indent-1">The logical addressing of network computers</li><li class="ql-indent-1">Handles the quality of service:</li><li class="ql-indent-2">like the traffic priority</li><li class="ql-indent-1">Detect data errors</li><li class="ql-indent-1">Flow control of the data across the network devices</li><li class="ql-indent-1">Etc...</li></ul><ol><li>Data link layer: This layer is responsible for:</li></ol><ul><li class="ql-indent-1">Physical address of the computers in the network segment</li><li class="ql-indent-1">Provides the control of the network transmission between the computers in the same network segment</li><li class="ql-indent-1">Provides the mechanism how the computers learn the physical address of other computers in the same segment</li><li class="ql-indent-1">Detect and manage the transmission congestion.</li><li class="ql-indent-1">Etc...</li></ul><ol><li>Physical layer: this layer provide the mechanism of how the data will be sent between the computers over certain media, like:</li></ol><ul><li class="ql-indent-1">Wire network where the data is sent as electrical charges over copper cables,</li><li class="ql-indent-1">As optics over fiber cables</li><li class="ql-indent-1">As electromagnetic waves over the air</li><li class="ql-indent-1">Etc...</li></ul>