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Module 1: Physical Layer & Data Communication

202214m

Explain the duties of different layers in TCP/IP model.

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The TCP/IP protocol suite is organized into four layers, each with distinct duties:

+-------------------------------+
|   Application Layer           |  HTTP, FTP, SMTP, DNS, Telnet
+-------------------------------+
|   Transport Layer (Host-Host) |  TCP, UDP
+-------------------------------+
|   Internet Layer              |  IP, ICMP, ARP, IGMP
+-------------------------------+
|   Network Access Layer        |  Ethernet, Wi-Fi, PPP
+-------------------------------+

1. Network Access (Link) Layer
Combines the OSI Physical + Data Link layers. Responsible for:

  • Physical addressing (MAC addresses)
  • Framing data into frames for the transmission medium
  • Media access control and error detection at the link level
  • Interfacing directly with the physical hardware (NIC, cabling, wireless medium)

2. Internet Layer
Equivalent to the OSI Network layer. Responsible for:

  • Logical (IP) addressing of hosts
  • Routing packets across interconnected networks, choosing best paths
  • Fragmentation and reassembly of packets to fit different network MTUs
  • Handled mainly by IP, with ICMP for error reporting and ARP for address resolution

3. Transport (Host-to-Host) Layer
Equivalent to the OSI Transport layer. Responsible for:

  • End-to-end, process-to-process delivery between applications
  • TCP: connection-oriented, reliable, ordered delivery with flow and congestion control
  • UDP: connectionless, best-effort, low-overhead delivery
  • Segmentation of application data and reassembly at the receiver

4. Application Layer
Combines OSI's Application, Presentation, and Session layers. Responsible for:

  • Providing network services directly to user applications
  • Data formatting, encryption/compression (presentation concerns)
  • Session management between communicating processes
  • Implemented by protocols like HTTP (web), FTP (file transfer), SMTP (email), DNS (name resolution)

Each layer only interacts with the layers directly above and below it, passing data downward by encapsulation (adding headers) on the sending side, and upward by decapsulation (removing headers) on the receiving side.

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