OSI Model (History & Explanation)

In early nineteens 19’s, only the devices of same company could connect to each other. The devices of different companies could not be connected to each other (for example Microsoft devices could connect to other Microsoft devices but not with devices of other companies e.g. IBM couldn’t connect to Microsoft devices). This was a problem until 1947 when an organization was established. The organization was named as International Standard Organization (ISO). The organization worked and created a set of protocols. By utilizing these set of rules any device can connect to the other devices. It was known as Open System Interconnection (OSI) model. OSI model was presented in late 1970’s. The major purpose of OSI model was to establish a network between different devices without changing device’s software or hardware underlying architecture.

The OSI model is a layered based model which allows computers of different types to communicate with one another. The OSI model consists of seven layers/steps. Each step is connected to each other and at the same time they are completely different from the other. These all layers work together to establish a network between the devices which are connected to each other to transfer information per data. OSI model has seven layers which are.

7. Application

6. Presentation

5. Session

4. Transport

3. Network

2. Data link

1. Physical

Layered architecture

OSI model has seven orders layers connected to each other, these layers are basically are the protocols. Such layers are already mentioned above. OSI model presents a map how message is sent from one device to another and it travels from layer to layer to reach the receiver.

When a sending device “A” sends data to a receiving device “B”, data passes through many intermediate nodes. The intermediate nodes normally involve first three layers of OSI model.

When the model was being developed, the developers designed the model in such a way so that the information or data could be sent to its most fundamental elements. Each layer performs a different set of functions which is distinct from those of other layers. By defining these set rules, the developers, established a rigid yet a flexible model. These rules allow complete interoperability between otherwise incompatible systems.

Peer-to-Peer Process

Within a device each layer is connected to one another, each layer performs its services on the data and send it to the next layer for example, if it is a sending device then layer one (1) will perform its services and then send the data to layer two (2), this layer will perform its functions and put an envelope on the given data each layer will put on an envelope on the data and so on to the layer seven (7).

When data receives on the receiving side each layer will perform its functions and send the data to the next layer, for example when the data is received, layer seven (7) will remove the envelope which was put on layer seven (7) on the sender side and layer seven (7) of receiving side will remove that. This communication of layers at sender and receiver side is called peer-to-peer process. The data link layer of sender side will communicate with the data link layer of the receiving side this is called peer-to-peer processing.

Communication of the layers

The communication of layers within a same device is called as interface between the layers. The passing of data or information from upper layers to lower layers at sending side and passing of data from lower layers to the upper layers at receiving side got possible by interface between the layers. Each layer defines the information and services provided by the upper layers. Each layer also provides the modularity to the network. As long as the layer is providing its assigned information, the specific implementation of that layer can be modified or replaced without changing the surrounding layers.

These seven layers are further sub divided into three parts, which are given below:-

Group 1: layer 1 , 2 , 3

Group 2: layer 5 , 6 , 7

Group 3: layer 4

Group 1

In group 1, there are three layers layer 1, 2, 3. Which are physical layer, data link layer and network layer? The layers of this group are also called Network support layers. The purpose of these layers is to move data from sender to receiver and the all the aspects which involves in moving the data. These aspects involve physical addressing, reliability, transport timing and electrical specifications.

Group 2

In this group we have session, presentation, and application layers. This is layer 5, 6, 7. These layers are supposed be used as user support layers. These layers are there to provide support the user to connect to the network.

Group 3

Last group has only one layer in it which is the 4th layer which is also known as transport layer. The purpose of this layer is transfer or to keep the two groups connected to each other. This layer makes sure that the data which is transmitted in such a form the upper layers could use it. It is like a bridge between two groups.

Note: upper layers of the OSI model are always implanted in software and the lower layers are implanted in both software and hardware except for the physical layer which is mostly hardware.


As we have discussed in peer-to-peer processing that layer x of sending side will communicate with the layer x of receiving side. Just like that when the data is being send from the sending device the layer 7 of that device will encapsulate the data and after that the data is converted to a packet which is than given to layer 6 then layer 6 will encapsulate the packet and give it to the lower layer and this process continues till the layer 1. Data is converted into packet that frame and then it is converted to a bit-stream which than travels through the physical layer and reach its destination. This process is called encapsulation.

Header and Trailer

Normally header is added by each layer of the OSI model to the information so that when the data receive at the receiving device it can take it off and transfer the information to the upper layers. Trailer is usually added by the layer two (2) which contains some routing information about the data. Both header and trailer contain some valuable information which is used by our computer and network to make sure that our data reach at the desired destination.

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