As we know computers are connected through each other and it forms a network of computers. The communication and data transfer in the network is by using packets. A packet has a header and a data part where the header contains the sender and receiver information and the data part contains data. There are many protocols and layers included in sending and receiving these packets.
OSI or the Open System Interconnection model, which can be called a reference model that describes these layers and protocols, associated in sending and receiving the packets of data. It starts from the question of How an application of one computer sends and receives a packet through the layers to the physical medium to the application of another computer.
OSI model was developed in 1984 by the ISO organization, which consists of seven layers, and each layer is independent and has its own function in sending and receiving a packet of data.
It has many different points describing the need for a reference model in the network which are
OSI model is an international model in the network so it must be made as per international guidelines.
Each layer must be independent in functionality so that changes in one layer may not make changes in another layer.
Make different functions in different layers and there should not be different functions in the same layer. Also, do not make too many layers to make the architecture complex.
Total layers in an OSI model are divided into two which are application layers, which are the upper layers, and other network layers, which are the lower layers.
Application layers are close to a user or an application, which is doing all the application-related issues. Mostly application layers deal with or communicate with the applications that are running in the systems in the network.
The lower layers of the OSI model are designed with the network and physical medium of data transfer. The physical layer is the lower end of the OSI model, which deals with all physical medium issues.
It was started in 1970 when the ISO conducted a seminar for making some international standard rules for networking
The need for higher-level protocols was identified in 1973 in an experiment in a packet switch system.
In 1983 the first model of OSI was initially developed but in 1984 the ISO accepted the OSI model architecture.
As we said, different layers are independent and assigned different functions in the data transfer. Let us examine the OSI layers and their functions in detail which is essential to know about network and cyber security.
This is the lowest layer in the OSI model that is related to the physical medium of data transfer. The physical layer is not dependent on any of the protocols like the higher layer in the OSI model. This layer is responsible for establishing and maintaining the connection between the physical medium and the system for data transfer. The physical layer is responsible for defining the electrical and mechanical specifications needed for transfer.
This is the next layer above the physical layer so the data from a physical layer enters the data link layer where the error-free transfer of data frames happens.
The data link layer makes a format for data and establishes a protocol for the data transfer and communication of devices in the network.
From this layer, we have the IP address of a device to identify each unique device (logical addressing) in the network.
For easy understanding of the functions of the data link layer, it is divided into two sub-layers that are
The network layer is the third layer of the OSI model that is above the data link layer. The network layer is responsible for proper routing and forwarding of the data.
This layer can able to find and locate the devices in a network and able to send the data using the best routes to reach the receiver after analyzing the network conditions.
In this layer, we are using protocols such as IP, IPV4, and IPV6 for proper routing of packets which are called network protocols.
The transport layer is the 4th layer of the OSI model and is also called the heart of the OSI model. The transport layer is responsible for sending the data completely without any loss.
This layer makes sure that the data is transmitted in the perfect order as that sent without any duplicate content.
The transport layer is the layer, which makes an end-to-end connection with the sender, and receiver to ensure the data is sent reliably.
The transport layer divides the data packets into smaller parts which can be called data segments.
As we said this transport layer is responsible for end-to-end connection and data transfer without any loss or duplication. There are two different protocols used in this layer.
TCP is a protocol that helps the data to be sent over the networks and it allows the devices in a network to communicate. It makes a connection and maintains the connection between the sender and receiver.
In TCP protocols, the data is divided into different small parts that we call segments; these segments are sent to the receiver using the best route. Different segments take different routes and reach the receiver in different order. The TCP reorders them to get the correct data.
UDP is another protocol used in transport layers but it is not a reliable protocol so it is not as much used as TCP.
The problem with the UDP protocol is that the receiver will not have the acknowledgment mechanism to inform the sender that the data is reached correctly. This lack of acknowledgment makes this UDP less reliable.
The session layer is above the transport layer and this layer is more closely related to applications and processes running on the system.
The session layer is important to make and maintain the connections and communications between the sender and receiver applications.
The session layer is responsible for handling all the login and its credentials.
Session make maintain and close the sessions.
The presentation layer is above the session layer, which is also called the syntax layer because it mainly concerns the syntax and semantics of the data that is transferred.
The presentation layer is a part of the Operating system that will convert the data from one format to another format before sending it to the receiver which is called encryption.
This is the top layer in the OSI model which will act as an interface to the user and the process to use the network. This layer’s responsibility includes transparency and resource allocation. This layer offers services to end users for using the network for example file transfer, email, remote login, etc.
