What is the OSI Model?

The Open Systems Interconnection (OSI) model is the best means to get an understanding of the working of the Internet and the communication of computers over a network system. It is considered to be the first standard model for network communication. It was adopted by all major computer and telecommunication companies in the early 1980s. The OSI model describes seven layers by which the computer systems communicate over a network.

The OSI model was introduced in 1983 by representatives of the major computer and telecom companies, and was later adopted by ISO as an international standard in 1984. However, the modern internet is based on the simpler TCP/IP model and not on OSI model. The OSI 7- layer model is used widely, which helps to visualize and communicate  how the network operates, and helps to isolate and troubleshoot networking problems.

7 Layers of Open System Interconnection (OSI) Model

The OSI model contains seven different layers, each consisting of a packet of protocols and are named according to their functions. We will discuss the OSI layers “top down” from the application layer that serves the end user directly, towards the physical layer. The 7 layers of OSI model are explained below:

1. Application Layer- Network applications make use of the application layer. Network applications are computer applications that use the internet, which includes Google Chrome, Zoom, Skype, and Outlook. This provides application-layer protocols that helps the applications to operate correctly on the network. Few examples of application layer protocols are the Hypertext Transfer Protocol (HTTP), File Transfer Protocol ( FTP), Post Office Protocol ( POP), Simple Mail Transfer ( SMTP), and Domain Name System (DNS). This layer is mainly used by the end-user software such as web browsers and email clients.
2. Presentation Layer- The data from the application layer is transmitted to the presentation layer. Here, the data is converted into a machine-readable format, which is a combination of 0 and 1, for outgoing messages and incoming messages. The machine-readable binary format is then converted  into a human-readable format. This process is known as translation. It defines how two devices should encode, encrypt, and compress data, to receive it correctly on the other end. In this layer, the size of the data is reduced, resulting in data compression. Data compression reduces the amount of space used to store the original files, which reduces the time taken to reach the destination. 
3. Session Layer-  The session layer creates a channel of communication between the devices. It sets up and manages the connection between two devices for sending and receiving data, followed by opening and closing of the session. The session layer can also set checkpoints during a data transfer, thus if the session is disconnected or there is a crash of data transmission, the devices can resume data transfer from the last checkpoint. This layer contains its own command sets namely Application Program Interfaces (APIs). A popular example of APIs is the Network Basic Input Output System (NetBIOS), that allows applications on different computers to communicate.
4. Transport Layer- This layer transfers the packets of data from the network layer to the session layer. It splits up the data into segments for sending the data and reassembles the segments into data in the receiving device. The transport layer carries out flow control, sending data at a rate that matches the connection speed of the receiving device, and error control. The transport layer uses two protocols: a) Transport Layer Protocol (TCP) and b) User Datagram Protocol (UDP). It performs both the connectionless transmission and the connection-oriented transmission. Here, UDP performs the connectionless transmission and TCP performs connection-oriented transmission. UDP is faster as compared to TCP as it does not provide any feedback on whether the data was delivered. 
5. Network Layer- The network layer collects the data segments from the transport layer and helps in transmission of data from one device to another device in two different networks. It breaks the data into smaller units known as packets in the sender’s device and reassembles these packets on the receiver’s device. It has three main functions- the first one is logical addressing, another is to split the segments into network packets, and reassembling the packets on the receiver end, and the other is routing packets by discovering the best path across a physical network. Internet Protocol (IP) addressing is done in the network layer, which is known as logical addressing. Each device in a network that can be connected to the internet has an IP address by way of digital identification. Routing is the process to transfer data packets from source to destination that is based on the logical address. Selecting the best possible route for data delivery from a sender to receiver is performed in this layer. 
6. Data Linked Layer- The data link layer creates and terminates a connection between two physically connected nodes on a network. It takes packets of data from the network layer and converts the data packets into frames and sends them from source to destination. The data packets contain the IP addresses of both the sender and receiver. There are two types of addressing: logical addressing and physical addressing. Logical addressing occurs in the network layer, where the sender and receiver are assigned IP addresses to each data segment to form data packets. Physical addressing, on the other hand, occurs in the data link layer where MAC (Media Access Control) addresses of the sender and the receiver are included in each data packet to form a frame. This layer allows the OSI model’s upper layer to access media (optical fibers, wireless antenna, etc) using the framing technique.
7. Physical Layer- The physical layer is responsible for the physical cable or wireless connection between the network nodes. It includes all physical equipment associated with data transfer. The binary bitstreams are converted into different signals on the receiving side, and reverse the conversion on the sending side. 

In brief, we can conclude that the data from the application layer has been segmented by the transport layer, converted into packets in the network layer, framed by the data link layer, by way of sequence of bitstreams. At last, the physical layer converts this bitstream into signals and transmits it into the media. The signals produced by the physical layer depend on the type of media used to connect the two devices. The signal can be an electrical signal for copper wire media, a light signal for optical fiber, and radio signal for wireless media.

Advantages of OSI Model

The following points describes the advantages of the Open Systems Interconnection (OSI) model:

• The OSI model helps the users and operators to determine the required hardware and software to build their network.
• It helps to understand and communicate the process followed by components communicating across a network.
• It helps in performing troubleshooting, by identifying which network layer is generating a problem and focusing efforts on that layer.
• It creates devices and software that can communicate with products from any other vendor, allowing open interoperability.
• It communicates to users on which network layers their product operates- such as on application layer or across the stack.
• It defines the part of a network their products should work with.

 Conclusion

In the present article, we presented a brief overview of the different layers of the OSI model. It is an important part to know about the functioning of the OSI model to properly understand the working principle of the internet. With the help of the OSI model, we can understand the working process of the internet in a simplified way. For the operation of the internet, many interconnected networking systems are utilized. For example, a device which is connected to another device through an optical fiber cable network. On the other hand, mobile phones and computers are connected to the internet via Wi-Fi or cellular network. Hence, our devices are connected to this optical fiber cable network.  

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