OSI Model: Networking Layers, Grouping, ‘N’ Notation, PDUs & SDUs

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The OSI Reference Model is a valuable tool for explaining how the network functions, and also for describing the relationships between different networking technologies and protocols. In this blog, I will discuss what are the OSI Model layers, Notations used in the Model, and important data encapsulation concepts like Protocol Data Units (PDUs) and the Service Data Units (SDUs).

Read Introduction To The Open Systems Interconnection (OSI) Reference Model.

OSI Reference Model Network layers

The most important part of the OSI Reference Model is to understand each layer of the model. The OSI Reference Model is composed of seven conceptual layers each of which is assigned a number from 1 to 7.

Each layer represents a specific task in the networking field.

The first and lowest layer is the physical layer, which is where low-level signaling and networking hardware are implemented. The Seventh and highest layer is the application layer which sends and receives the data.

Lower Layers (Layer 1,2,3,and 4)

The lower layers of the OSI Reference Models are the physical, data link, network, and transport layers. These are primarily concerned with formatting, encoding, and transmission of data over the network.

Upper Layer (Layer 5,6, and 7)

The higher layers of the model – session, presentation, and the application layer are concerned with interaction with the user and implementing the applications that run over the network.

‘N’ Notation

There are many terminologies that are used to describe the OSI Model and its layers, one such terminology is the N Notation.

The letter N is often used to generically refer to ta number within the computer world.

In the OSI Reference Model, each layer is numbered from 1 to 7. Suppose you are talking about the transport layer i.e the 4th layer of the Model. In N Notation form this layer will be the Nth layer and the layer above and below it will be called (N+1) and (N-1) layer.

If you are looking at the network layer (layer 3), then the N+3 layer is the presentation layer (layer 6).

Data Encapsulation, Protocol Data Units (PDUs) and Service Data Units (SDUs)

Protocols are the rules that control horizontal communication, that is, a conversation between processes that run at corresponding layers within the OSI Reference Model.

A PDU i.e Protocol Data Unit is a complete message that has been implemented by the Nth layer protocol. This PDU is then passed to layer N-1, and it becomes the data for that layer and gives its service to the data. Thus the layer N’s Protocol Data Unit i.e PDU is called layer N-1’s Service Data Unit i.e SDU.

This process of passing PDU’s and the next layers takes it as SDU’s is called data encapsulation. Encapsulation is wrapping up of data and each higher layer of data of the model is wrapped by the next lower layer.

The process reverses when the data travels from the lower layer to the higher layer and the wrapping is removed by each layer.

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