OSI Model

OSI stands for Open System Interconnection is a reference model that describes how information from a software application in one computer moves through a physical medium to the software application in another computer.
OSI consists of seven layers, and each layer performs a particular network function.
OSI model was developed by the International Organization for Standardization (ISO) in 1984, and it is now considered as an architectural model for the inter-computer communications.
OSI model divides the whole task into seven smaller and manageable tasks. Each layer is assigned a particular task.
Each layer is self-contained, so that task assigned to each layer can be performed independently.

Characteristics of OSI Model:

The OSI model is divided into two layers: upper layers and lower layers.
The upper layer of the OSI model mainly deals with the application related issues, and they are implemented only in the software. The application layer is closest to the end user. Both the end user and the application layer interact with the software applications. An upper layer refers to the layer just above another layer.
The lower layer of the OSI model deals with the data transport issues. The data link layer and the physical layer are implemented in hardware and software. The physical layer is the lowest layer of the OSI model and is closest to the physical medium. The physical layer is mainly responsible for placing the information on the physical medium.

There are the seven OSI layers. Each layer has different functions. A list of seven layers are given below:

The main functionality of the physical layer is to transmit the individual bits from one node to another node.
It is the lowest layer of the OSI model.
It establishes, maintains and deactivates the physical connection.
It specifies the mechanical, electrical and procedural network interface specifications.

This layer is responsible for the error-free transfer of data frames.
It defines the format of the data on the network.
It provides a reliable and efficient communication between two or more devices.
It is mainly responsible for the unique identification of each device that resides on a local network.
It contains two sub-layers:
- Logical Link Control Layer
  - It is responsible for transferring the packets to the Network layer of the receiver that is receiving.
  - It identifies the address of the network layer protocol from the header.
  - It also provides flow control.
- Media Access Control Layer
  - A Media access control layer is a link between the Logical Link Control layer and the network’s physical layer.
  - It is used for transferring the packets over the network.

It is a layer 3 that manages device addressing, tracks the location of devices on the network.
It determines the best path to move data from source to the destination based on the network conditions, the priority of service, and other factors.
The Data link layer is responsible for routing and forwarding the packets.
Routers are the layer 3 devices, they are specified in this layer and used to provide the routing services within an internetwork.
The protocols used to route the network traffic are known as Network layer protocols. Examples of protocols are IP and Ipv6.

The Transport layer is a Layer 4 ensures that messages are transmitted in the order in which they are sent and there is no duplication of data.
The main responsibility of the transport layer is to transfer the data completely.
It receives the data from the upper layer and converts them into smaller units known as segments.
This layer can be termed as an end-to-end layer as it provides a point-to-point connection between source and destination to deliver the data reliably.

The two protocols used in this layer are:

Transmission Control Protocol
- It is a standard protocol that allows the systems to communicate over the internet.
- It establishes and maintains a connection between hosts.
- When data is sent over the TCP connection, then the TCP protocol divides the data into smaller units known as segments. Each segment travels over the internet using multiple routes, and they arrive in different orders at the destination. The transmission control protocol reorders the packets in the correct order at the receiving end.
User Datagram Protocol
- User Datagram Protocol is a transport layer protocol.
- It is an unreliable transport protocol as in this case receiver does not send any acknowledgment when the packet is received, the sender does not wait for any acknowledgment. Therefore, this makes a protocol unreliable.

It is a layer 3 in the OSI model.
The Session layer is used to establish, maintain and synchronizes the interaction between communicating devices.

A Presentation layer is mainly concerned with the syntax and semantics of the information exchanged between the two systems.
It acts as a data translator for a network.
This layer is a part of the operating system that converts the data from one presentation format to another format.
The Presentation layer is also known as the syntax layer.

An application layer serves as a window for users and application processes to access network service.
It handles issues such as network transparency, resource allocation, etc.
An application layer is not an application, but it performs the application layer functions.
This layer provides the network services to the end-users.