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Open Systems Interconnection Model Cheat Sheet (DRAFT) by [deleted]

Open Systems Interconnection Model

This is a draft cheat sheet. It is a work in progress and is not finished yet.


The Open Systems Interc­onn­ection reference model was developed by the Intern­ational Organi­zation for Standa­rdi­zation (ISO) to serve as a conceptual framework for unders­tanding how data is transf­erred between two endpoints in a network. Specif­ically, it catego­rizes the commun­ica­tions process into seven distinct “layers” or “funct­ions.” For example, whenever a computer forwards inform­ation to another computer, under the Open Systems Interc­onn­ection model, the data navigates through seven layers across the network to the receiving machine.

Each layer performs a specific function as data passes from one layer to the next. Most portrayals of the Open Systems Interc­onn­ection model take a top-down approach, descending from layer seven to layer one. Layers seven through five are regarded as the upper layers, while layers four through one are regarded as the lower layers. Let’s now explore each layer of the Open Systems Interc­onn­ection model in more detail.

Layer 1: the physical layer

The bulk of docume­ntation takes place at the physical layer, which serves as an electrical and physical repres­ent­ation of the system. It consists of the hardware required for forwarding and switching data, including cables, cords and other physical necess­ities. Whenever a networking error occurs, the physical layer is frequently checked to see if the cables are connected properly or whether a power cord may have been unplugged.

Layer 2: the data-link layer

The data-link layer provides node-t­o-node transfer and manages errors in the physical layer. This layer is actually split into two sub-la­yers, including the Media Access Control (MAC) layer and the Logical Link Control (LLC) layer. The former sub-layer determines how a network can access and transfer data, while the latter sub-layer manages traffic and checks for errors across the physical medium.

Layer 3: the network layer

The network layer consists of switching and routing techno­logies, and helps determine the optimal path to move data between nodes. Additi­onally, the network layer performs functions like sequencing packets, managing conges­tion, addres­sing, interw­orking and addressing errors.

OSI Model

Layer 4: the transport layer

The transport layer ensures an entire message is delivered without any inaccu­racies. In partic­ular, it helps determine how much data to send and when. It also ensures messages are error-free by looking for mistakes in the data upon arrival.

Layer 5: the session layer

The session layer is respon­sible for creating a “session’ so that two machines can commun­icate with each other. This typically involves setting up, coordi­nating, managing and termin­ating exchanges between applic­ations on both ends during a session.

Layer 6: the presen­tation layer

The presen­tation layer is typically a feature of an operating system. It translates the applic­ation format to network format, or the other way around, such as encrypting and decrypting data. The purpose of the presen­tation layer is to convert data into a form the applic­ation layer can accept.

Layer 7: the applic­ation layer

As the name suggests, everything about the applic­ation layer centers around applic­ation services, specif­ically for file transfers. At this layer, a commun­ica­tions partner is identi­fied, while network capacity is reviewed. Among all the layers, the applic­ation layer is the closest to the end-user. Users directly interact with applic­ations running at layer seven. Examples include Google, Skype and Microsoft Office.