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The 7 Layers of the OSI Model

January 21, 2019

When describing telecommunication networks, often we refer to layers (such as Layer 1, Layer 2 and so on). This article describes, in detail, what exactly these layers are, how they are defined and where precisely the terminology comes from.

The Open System Interconnection (OSI) is a model created by ISO (International Standards Organisation) and is most widely used to define these layers. It defines a networking framework, that is used to implement protocols in seven separate layers.

The first four layers are considered the lower layers, and are mostly used to move data around a network. Layers five to seven, are known as the upper layers and contain application-level data.

Each layer takes care of a specific task and then passes the data on to the next layer. This is the basic principle of all networks.

With the OSI model, control is passed on from one layer to the next and starts at the application layer (Layer 7) in one station and proceeds to the bottom layer (Layer 1), over the channel to the next station and back up the hierarchy.

In some network cases, two or three OSI layers may be incorporated into one depending on the types of equipment being used, but for the purpose of understanding we will define each layer separately.

Layer 7 – Application

Layer 7 is the layer that is closest to the end-user. The functions of Layer 7 are usually to identify communication partners, determine the availability of resources and synchronize communication. It is also at this point that the user authentication and privacy are considered.

Layer 6 – Presentation

Layer 6 is the layer which transforms data into the format that the application layer can accept. It formats and encrypts data which is to be sent across a network. It provides the freedom from compatibility issues and is often known as the Syntax later.

Layer 5 – Session

Layer 5 is where the connections between applications are established, managed and terminated. The session layer is able to set up, coordinate and terminate conversations, exchanges and dialogues between applications at each end of the network.

Layer 4 – Transport

Layer 4 is the transport layer. It is fairly self-explanatory in that is takes care of the transfer of data between end systems (or hosts), but it is also responsible for end-to-end recovery as well as flow control. It ensures complete data transfer is achieved without error, in sequence and with no losses or duplications.

Layer 3 – Network

Layer 3 provides switching and routing technologies that control the operations of the subnet, deciding which physical path the data should take. It creates logical paths (known as virtual circuits) which transmit the data from node to node. Routing and forwarding are functions of layer 3 as well as addressing, internetworking, error handling, congestion control and packet sequencing.

Layer 2 – Data Link

Layer 2 is where data packets are encoded and decoded into bits. It utilises the transmission protocol knowledge to manage and handle errors in the physical layer, flow control systems and frame synchronisation. This layer is divided into two sub layers, the Media Access Control known as the MAC later and the Logical Link Control (LLC) layer.

The MAC layer controls how computers on a network fain access to the data as well as permissions to transmit it. The LLC layer takes care of frame synchronisation, flow control and error checking.

Layer 1 – Physical

Layer 1 is where the transmission and reception of the unstructured raw bit-stream (electrical impulse, light or radio signals) are conveyed through the network at an electrical and mechanical level. It provides the hardware with a way of sending and receiving data on a carrier, including defining cables, cards and physical aspects.


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