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OSI Model – Networking Fundamentals

The OSI (Open Systems Interconnection) model is a conceptual framework used to describe how data is transmitted across a network.

It divides networking into seven logical layers, each responsible for a specific function.
The OSI model is primarily used for understanding, communication, and troubleshooting, rather than direct implementation.

Layer 7 – Application

Function:
Provides network services directly to user-facing applications.

Examples: - HTTP / HTTPS - FTP - SMTP - DNS - SSH

Typical issues: - Websites not loading - Email services failing - Application-level errors

Layer 6 – Presentation

Function:
Responsible for data formatting, encryption, and compression.

Examples: - TLS / SSL encryption - Character encoding (UTF-8) - Data compression

Typical issues: - Certificate errors - Encryption failures - Incompatible data formats

Layer 5 – Session

Function:
Manages sessions between communicating devices.

Responsibilities: - Session establishment - Session maintenance - Session termination

Typical issues: - Session timeouts - Dropped connections

Layer 4 – Transport

Function:
Controls end-to-end data transmission and reliability.

Protocols: - TCP – reliable, ordered delivery - UDP – fast, connectionless delivery

Key concepts: - Ports - Flow control - Error handling

Typical issues: - Blocked ports - Slow or unreliable connections

Layer 3 – Network

Function:
Handles logical addressing and routing between networks.

Examples: - IP addressing (IPv4 / IPv6) - ICMP - Routing tables

Devices: - Routers - Layer 3 switches

Typical issues: - No route to host - Incorrect IP configuration

Function:
Manages physical addressing and local network delivery.

Examples: - MAC addresses - ARP - VLANs

Devices: - Switches - Network Interface Cards (NICs)

Typical issues: - VLAN misconfiguration - ARP resolution failures

Layer 1 – Physical

Function:
Transmits raw data over physical media.

Examples: - Ethernet cabling - Fibre optics - Wireless signals

Typical issues: - Damaged cables - No link light - Hardware failure

OSI Model in Troubleshooting

The OSI model provides a structured method for diagnosing network issues:

  • Physical connection issues → Layer 1
  • VLAN or MAC address issues → Layer 2
  • IP addressing or routing issues → Layer 3
  • Port or protocol issues → Layer 4
  • Encryption or certificate issues → Layer 6
  • Application service issues → Layer 7

This layered approach supports efficient fault isolation and resolution.