Layer 2: Data Link Layer (OSI Model)

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📘 Overview

The Data Link Layer is Layer 2 of the OSI Model.
It is responsible for reliable delivery of data across a physical network link. It ensures that frames are properly sent and received between directly connected devices.

Think of it as:
“How do devices on the same local network communicate accurately?”

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🔧 Key Responsibilities

• Framing: Breaks packets from Layer 3 into frames
• MAC Addressing: Adds hardware addresses for sender and receiver
• Error Detection: Adds CRC/FCS for integrity checks
• Flow Control: Prevents sender from overwhelming receiver
• Media Access Control: Decides when devices can transmit (e.g., CSMA/CD)

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📦 Data Unit: Frame

A frame includes:

• Destination MAC Address
• Source MAC Address
• Data
• Error Check (FCS)

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📑 Two Sublayers

Sublayer	Function
LLC (Logical Link Control)	Identifies protocols and manages connections
MAC (Media Access Control)	Controls access to the physical transmission medium

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🔄 Devices Operating at Layer 2

Device	Purpose
Switch	Forwards frames based on MAC Address
Bridge	Connects two network segments
Wireless Access Point	Layer 2 in wireless networks

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🔁 Protocols at Layer 2

• Ethernet (802.3)
• Wi-Fi (802.11)
• PPP (Point-to-Point Protocol)
• HDLC (High-Level Data Link Control)

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🧪 Troubleshooting at Layer 2

• Use arp -a to view MAC tables
• Inspect switch configuration
• Verify MAC address resolution
• Monitor frame drops or collisions

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🧠 Real-Life Scenario

Your laptop sends a file to a printer on the same network.
Layer 2 ensures the frame is delivered to the correct MAC address using a Switch.

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📊 Visual Aids

• Ethernet Frame Diagram
• MAC Address Table Diagram

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🔁 Related Pages

• MAC Address
• Switch
• Layer 1: Physical Layer
• Layer 3: Network Layer
• Ethernet

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Return to: OSI Model Overview