What is OSI model in computer network?

What is the OSI model in a computer network?

OSI Model Introduction:

OSI stands for Open System Interconnection. It is used for designing network architecture. It was designed by the International Standard Organization (ISO) in 1983. It provides a logical framework for any type of data communication through computer networks. This model provides the reference in a system but not the reality. Therefore, the OSI model is also known as the OSI Reference Model.

Purpose of OSI Model:

The purpose of the OSI model is to open communication between different systems, without requesting changes to the logic of the underlying hardware and software. The OSI model is not a protocol in IP; It is a model of the network system for understanding and designing network architecture. It deals with the open system. An open system is one, which is open to others for the purpose of information exchange.

Layers of the OSI Model:

OSI model consists of 7 layers. Each layer is responsible for performing a particular function in data communication. For example, one layer may be responsible for establishing connections between devices, while another layer may be responsible for error checking during data transfer and so on.

The Seven Layers of OSI models are as follows:

1. Physical Layer
2. Data Link Layer
3. Network Layer
4. Transport Layer
5. Session Layer
6. Presentation Layer
7. Application Layer

Physical Layer:

The physical layer of the OSI model is the bottom-most layer. This layer is concerned with the transmission media (i.e. physical part of the connects network components together) for data transmission. This layer controls and co-ordinates the transmission of data in the form of a bitstream over a physical medium such a coaxial cable, optic fiber cable, etc. It also defines the rules by which the bits are passed from one node in the network to the next.

Physical Layer provided services:

• Representations of bits into signals
• Activating and maintaining the physical links between systems
• Defining data rate
• Bit- Synchronization
• Multiplexing
• Defining transmission mode

Datalink layer:

The data link layer of the OSI model deals with the flow of data from one device to another. This layer provides the error-free transfer of any type of data from one computer to another computer. It checks whether any type of data has arrived properly and safely at the destination of a system. This layer is also responsible for the reliability of the physical link established at the physical layer.

The data link layer is divided into sub-layers in the OSI model:

• Logical Link-Control
• Media Access Control

The main services provided by Data Link Layer are:

Packetizing:

The data stream of bits is encapsulated packets of data. In LANs, packets are referred to as frames. In some WANs, a packet is referred to as a cell.

Addressing:

The Data link layer adds a header to the frame to define the physical address of the sender and receiver in a computer network system.

Error Control:

The network must be able to transfer data from one computer or device to another computer or device with complete accuracy. The data link layer is responsible for data error detection and correction.

Flow Control:

The Data link layer is also responsible for controlling the flow of data.

Access Control:

When two or more devices are connected to the same link, then Data Link Layer determines, which device has control over the link at any given time.

Network Layer

The network layer of the OSI model makes routing decisions to transfer data between source and destination. In a larger network, there may be several intermediate networks between source and destination computers.

The main duties of the network layer are:

• Packetizing:

The network layer receives packets from the upper layer. It makes new packets out of them through the Network layer protocol, called IP (Internet Protocol).

• Internet Working:

The internetwork is the network of the network or the internet. The network layer performs its main functions in internetworking for the delivery of data from source to destination.

• Addressing:

The addressing of devices or nodes at the data link layer is applicable only in the local area network. But for data communication on the internet or WAN, another addressing system is used to uniquely identifies each device.

For example: in a telephone system, each telephone subscriber has a unique telephone number. (such as country code, area code, and actual telephone number). The network layer adds a header to the frame coming from the upper layer.

• Routing:

Whenever there are multiple routes to a destination, we must make a decision and choice one suitable route on the basics of some criteria. On the internet, there are multiple routes to a destination in a computer system. The connecting devices such as routers or gateways route the packets to their final destination through any one of the available routes.

Transport Layer

The transport layer of the OSI model provides a mechanism for the exchange of data between source and destination. The transport layer also sends an acknowledgment of receiving data to the sender. This layer is the core of the internet model. 

The main duties of the transport layers are:

• Packetizing
• Connection Control
• Flow Control
• Error Control

Session Layer

The session layer of the OSI model establishes, maintains, and manages connections between applications. The connection between applications running on separate computers is called a session.

The main services provided by the session layer are the following.

1. Dialogue Control
2. Synchronization

Presentation Layer

This layer of the OSI model performs data transformations to provide a common interface for user applications. It means that data of any type is translated between the formats the network requires and the format the computers system of the end-users expect.

1. Translation
2. Encryption & Decryption
3. Compression

Application Layer:

The application layer is the topmost layer of the OSI model in a computer network system. It provides services directly to the application programs, to access the network, It provides a user interface to send and receive information by the user

The main services provided by the Application layer are:

1. Network Virtual Terminal
2. Accessing Files
3. Mail services