What is the OSI Model? A Simplified Overview
Understanding the Open Systems Interconnection Model
Table of contents
OSI(Open Systems Interconnection Model) is a set of rules that explains how the data is actually being transmitted over the internet between two parties(client and server). OSI model was developed by the International Organization for Standardization(ISO).
The OSI model has 7 layers and each layer has its own responsibility. OSI model provides a clear structure for data transmission and managing networking issues.
Layers of OSI model -
There are seven layers of OSI model as mentioned in the above picture. Let us see their responsibilities.
Physical layer - The physical layer is the bottom most layer of the OSI model. It consists of various networking devices such as plugs, connectors, receivers, cables etc. Its main function is to send data bits from one device to another. The physical Layer defines the types of encoding (that is how the 0’s and 1’s are encoded in a signal).
Functions :
Maintains the data rate(how many bits of data is transferred).
Synchronization of bits.
Provides physical medium and interface decision.
Helps in taking decision about the physical topology(star, mesh, bus, ring).
It provides an interface between devices connected in the network.
Data Link layer - The data link layer is the second layer from the bottom of the OSI model. Its main function is to provide error free transmission of data. It is responsible for the node-to-node delivery of data. It also helps is encoding, decoding and organizing the outgoing and incoming data.
Functions :
DLL receives packets from the Network layer and divides them into small frames, then sends each frame bit-by-bit to the physical layer.
The data link layer encapsulates the sender’s and receiver’s MAC address to ensure node-to-node delivery.
detect the error in the transmitted data and correct it using error detection and correction techniques respectively.
Network layer - Network layer is the third layer from the bottom of the OSI model. It is the most important layer which plays an important role in data transmission. It handles routing, it chooses the best path for the data packet to travel from the sender to the receiver. It also maintains the quality of the data.
Functions :
Network layer is solely responsible for assigning logical addresses to devices which are either sending or receiving data packets.
Host-to-Host delivery also known as Forwarding is the process in which the network layer transmits or forwards the data packets via routers, after determining the best path/route.
Network Layer also allows a bigger network to be divided into smaller chunks of network known as Logical Subnetting.
Network Layer also check for errors and handles them.
Transport layer - Transport layer is the fourth layer from the bottom of the OSI model. It controls the network traffic between the two parties(client and server) to guarantee full data flow. Data volume, destination, and rate are all controlled by transport-layer protocols including TCP, UDP, DCCP, and SCTP.
Functions :
Transport Layer splits the total amount of data it gets from the applications running at the top layers into smaller units known as segments.
Transport Layer creates a connection between the source and the destination.
In order to prevent data overload, it regulates the data transfer rate.
Checksum is a technique used by transport layer for error detection.
Session layer - Session layer is the fifth layer from the bottom of the OSI model. This layer is responsible for setting up, coordinating, and terminating conversations, exchanges, and dialogues between the applications at each end.
Functions :
Session Layer works as a dialog controller through which it allows systems to communicate in either half-duplex mode or full duplex mode of communication.
This layer is also responsible for token management.
Presentation layer - Presentation layer is the sixth layer from the bottom of the OSI model. The main responsibility of this layer is to provide or define the data format and encryption. The presentation layer is also responsible for maintaining the proper syntax of the data for transmission.
Functions :
It formats and encrypts the data for transmitting.
It also carries out data compression to reduce the bandwidth.
This layer deals with the presentation part of the data.
This layer deals with the syntax and semantics of the messages.
This layer also performs serialization.
Application layer - Application layer is the last layer from the bottom of the OSI model. The application layer provides the functionality to send and receive data from users. It acts as an interface between the user and the application.
Functions :
Allows users to access, retrieve and manage files in a remote computer.
It handles issues such as network transparency, resource allocation and so on.
This layer provides services which include: e-mail, transferring files, distributing results to the user, directory services, network resources and so on.
Conclusion 🚀
In conclusion, the OSI (Open Systems Interconnection) model helps us understand how data travels. Each layer has specific responsibilities and interacts with the layers directly above and below it. Since it is a conceptual model, but the OSI framework is still widely used to troubleshoot and understand networking issues in networking.