Open System Interconnection OSI  Model

The OSI, or Open System Interconnection model, was developed by the International Standards Organization (ISO). It helps different communication systems talk to each other using standard protocols. The OSI model provides a layered framework that shows how communication should happen between different systems. Simply put, it sets a standard for computers to communicate with one another.

The model divides data flow in a communication system into seven layers. These layers work together to manage communication step by step. The seven layers of the OSI model are:

Each layer has a specific role in ensuring smooth communication, as shown in the diagram below.

The Seven Layers of the OSI Model

1. Physical Layer

The Physical Layer is the lowest layer of the OSI Model. It is responsible for the actual transmission of raw data bits (0s and 1s) over a physical medium. This layer defines hardware elements such as cables, switches, connectors, voltage levels, signal types, data rates, and transmission modes (simplex, half-duplex, full-duplex). It does not understand data or addresses; it only focuses on sending and receiving electrical, optical, or radio signals. Examples: Ethernet cables, fiber-optic cables, hubs, repeaters.

2. Data Link Layer

The Data Link Layer ensures reliable data transfer between two directly connected devices. It organizes raw bits from the Physical Layer into frames and detects and corrects errors that may occur during transmission. This layer also controls how devices access the shared transmission medium. It uses MAC (Media Access Control) addresses to identify devices on the same network.

This layer is divided into two sublayers:

• LLC (Logical Link Control) – manages error control and flow control.
• MAC (Media Access Control) – handles physical addressing.

Examples: Switches, Ethernet, Wi-Fi (IEEE 802 standards).

3. Network Layer

The Network Layer is responsible for routing data from the source device to the destination device across different networks.It determines the best path for data transmission and handles logical addressing using IP addresses. If data packets are too large, this layer may break them into smaller packets and reassemble them at the destination.  Examples: Routers, IP (Internet Protocol), ICMP.

4. Transport Layer

The Transport Layer ensures complete, reliable, and orderly delivery of data from sender to receiver. It manages flow control, error control, and segmentation of data. Data received from upper layers is divided into smaller units called segments, and each segment is assigned a sequence number.

There are two main transport protocols:

• TCP (Transmission Control Protocol) – reliable, connection-oriented.
• UDP (User Datagram Protocol) – fast, connectionless, but less reliable.

Examples: TCP, UDP.

5. Session Layer

The Session Layer manages communication sessions between applications on different devices. It establishes, maintains, and terminates connections (sessions). This layer also handles synchronization, allowing data transfer to resume from a specific point if the connection is interrupted.

Examples: Session checkpoints, login sessions, video calls.

6. Presentation Layer

The Presentation Layer acts as a translator between the Application Layer and the network.
It converts data into a format that the receiving system can understand. This layer also handles encryption, decryption, and data compression, ensuring data security and efficiency.

Examples: SSL/TLS encryption, JPEG, MP3, video formats.

7. Application Layer

The Application Layer is the topmost layer and is closest to the user. It provides network services directly to user applications. This layer does not represent the application itself but provides networking support for applications like browsers, email clients, and file transfer programs.

Examples: HTTP (web), FTP (file transfer), SMTP (email), DNS.

Transmission Control Protocol/Internet Protocol (TCP/IP) model

The Transmission Control Protocol/Internet Protocol (TCP/IP) model is a simplified version of the OSI Model. It explains how data is sent and received over a network. The TCP/IP protocol suite was developed before the OSI Model. Its layers were designed for practical use on real networks. The layers of the TCP/IP model do not exactly match the layers of the OSI Model. Some OSI layers are combined into a single layer in TCP/IP. Because of this, TCP/IP is simpler and more widely used in the real world.

TCP/IP Protocol

• The original TCP/IP protocol model uses four layers: Host-to-Network, Internet, Transport, and Application.
• The Host-to-Network layer in the TCP/IP model is equal to the combination of the Physical layer and Data Link layer of the OSI model. It deals with hardware, network interfaces, and data transmission.
• The Internet layer of TCP/IP is similar to the Network layer of the OSI model. It is responsible for logical addressing and routing of data packets between networks.
• In the TCP/IP model, the Application layer performs the functions of the Session, Presentation, and Application layers of the OSI model. It provides services directly to user applications.
• In TCP/IP, the lower layers handle physical standards, network access, internetworking, and data transport. These layers work together to ensure smooth communication between devices.
• The top three layers of the OSI model (Application, Presentation, and Session) are combined into a single Application layer in the TCP/IP model.

Layers of the TCP/IP Model

1. Application Layer

The Application layer provides network services to application software. It allows programs on a computer to communicate with the network. This layer uses protocols such as HTTP, POP3, and SMTP. It acts as an interface between the user’s software and the network.

2. Transport Layer

The Transport layer is responsible for end-to-end data delivery. It includes Transmission Control Protocol (TCP) and User Datagram Protocol (UDP). TCP provides reliable data transfer to applications. UDP provides faster data transfer without reliability checks.

3. Internet Layer (Network Layer)

The Internet layer is similar to the Network layer (Layer 3) of the OSI model. It stores IP addresses and routing information. This layer is used when data is sent between different networks. Protocols used in this layer include IPv4, IPv6, ICMP, and routing protocols.

4. Host-to-Network (Link) Layer

The Host-to-Network layer is also called the Link layer or Network Interface layer. It handles the physical transmission of data over the network. It sends IP packets to the next router or host using hardware details. This layer is the lowest layer of the TCP/IP model. It is a combination of the Physical layer and Data Link layer of the OSI model.

Comparison Between the OSI Model and the TCP/IP Model

Introduction to Protocols

In computer networks, a protocol is a set of rules that allows two or more devices to communicate with each other. Protocols define how data is sent, received, and interpreted across a network. Without protocols, computers and other devices would not understand each other, and communication would fail.

Compiled by Er. Basant Kumar Yadav