Explain OSI Model.
SOLUTION
OSI Model (Open Systems Interconnection Model)
The OSI Model is a conceptual framework used to understand how different computer systems communicate over a network. It divides the communication process into seven layers, where each layer has specific functions and interacts with the layers directly above and below it. This layered approach helps standardize networking, making it easier for different hardware and software systems to work together.
Seven Layers of OSI Model
Physical Layer – Deals with the physical connection of devices (cables, signals, hardware).
Data Link Layer – Ensures error-free transfer of data between directly connected devices (uses MAC addresses).
Network Layer – Handles routing of data across networks (uses IP addresses).
Transport Layer – Provides reliable data transfer, error detection, and flow control (e.g., TCP, UDP).
Session Layer – Manages sessions (connections) between applications.
Presentation Layer – Translates, encrypts, or compresses data for applications.
Application Layer – Closest to the user; provides services like web browsing, email, and file transfer.
Seven Layers of the OSI Model
1. Physical Layer
This layer is responsible for the actual physical connection between devices. It deals with cables, switches, signals, voltage levels, and transmission of raw bits (0s and 1s).
2. Data Link Layer
It ensures error-free data transfer between two directly connected devices. It uses MAC (Media Access Control) addresses to identify devices within the same network and handles framing, error detection, and flow control.
3. Network Layer (Detailed Explanation)
The Network Layer is one of the most important parts of the OSI model because it is responsible for moving data from one computer to another across different networks.
Key Functions:
Logical Addressing (IP Addressing): Each device is assigned an IP address, which uniquely identifies it on a network. Unlike MAC addresses (which are fixed to hardware), IP addresses are logical and can change.
Routing: The network layer determines the best path for data to travel from the source to the destination. Routers operate at this layer.
Packet Forwarding: Data is broken into packets at this layer. Each packet is sent independently, and the network layer ensures they are delivered.
Fragmentation and Reassembly: If a packet is too large for a network, the network layer breaks it into smaller packets (fragmentation) and reassembles them at the destination.
Error Handling & Congestion Control: Helps detect delivery failures and may reroute traffic in case of congestion.
Protocols at Network Layer:
IPv4 / IPv6 – Logical addressing.
ICMP – Error reporting (e.g., ping command).
IPSec – Security for IP communication.
Example:
Imagine sending an email from your laptop in India to a server in the USA. The Network Layer ensures the email packets travel through different routers, across the internet, and finally reach the correct server using the destination IP address.
4. Transport Layer
Ensures complete and reliable data delivery with error detection and flow control. Uses TCP for reliable communication and UDP for faster, connectionless communication.
5. Session Layer
Manages sessions or dialogues between applications. It establishes, maintains, and terminates connections between applications (e.g., a video call session).
6. Presentation Layer
Responsible for data translation, encryption, and compression. It makes sure that data sent from one system is readable by another, regardless of format differences.
7. Application Layer
The topmost layer that directly interacts with users. Provides services like web browsing (HTTP), email (SMTP), file transfer (FTP), and DNS.