Chapter 1. Network Concepts section 1.1 OSI Reference model

Open Systems Interconnection

OSI Model

• 7-layer conceptual networking model

• Explains how data moves from sender to receiver

• Used for troubleshooting and network design

• Layers (Top → Bottom):

  • Application

  • Presentation

  • Session

  • Transport

  • Network

  • Data Link

  • Physical

OSI Layer 1 – Physical

• Transmits raw bits (0s and 1s)

• Defines cables, connectors, voltages, signaling

• Examples: UTP, fiber, coax, RJ-45

• Devices: Hub, Repeater

OSI Layer 2 – Data Link

• Physical addressing using MAC addresses

• Framing and error detection (CRC)

• Media access control

• Protocols: Ethernet (802.3), Wi-Fi (802.11), ARP

• Devices: Switch, Bridge, NIC

• Data type: frames

OSI Layer 3 – Network

• Logical addressing and routing

• Determines best path between networks

• Protocols: IP, ICMP, IPSec

• Devices: Router, Layer 3 switch

• Uses IP addresses

• Data type: packets

OSI Layer 4 – Transport

• End-to-end communication

• Segmentation and reassembly of data

• Flow control and error recovery

• Protocols:

  • TCP (reliable, connection-oriented)

  • UDP (fast, connectionless)

• Uses port numbers

• Data type: segments

OSI Layer 5 – Session

• Establishes, manages, and terminates sessions

• Controls dialog (full-duplex / half-duplex)

• Provides session synchronization

• Examples: NetBIOS session, RPC

OSI Layer 6 – Presentation

• Formats data for the receiving system

• Handles encryption and decryption

• Handles compression

• Examples: SSL/TLS, ASCII, JPEG, MP3

OSI Layer 7 – Application

• Closest layer to the end user

• Provides network services to applications (not the app itself)

• Protocols: HTTP, HTTPS, FTP, SMTP, POP3, IMAP, DNS, SNMP

• Devices: Proxy, Application firewall

OSI Model – Exam Questions (Sample)

• Q: Which OSI layer handles encryption?

  • A: Layer 6 (Presentation)

• Q: TCP and UDP operate at which OSI layer?

  • A: Layer 4 (Transport)

• Q: Which OSI layer is responsible for IP addressing and routing?

  • A: Layer 3 (Network)

• Q: A cable unplugged or damaged relates to which OSI layer?

  • A: Layer 1 (Physical)

• Q: Switch and Router operate at which OSI layers?

  • A: Switch → Layer 2, Router → Layer 3