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Layering Concepts (OSI & TCP/IP)

Short Notes

Layering is a design principle to divide network functionality into manageable pieces.

Models

  • OSI (7 Layers): Physical, Data Link, Network, Transport, Session, Presentation, Application.
  • TCP/IP (4/5 Layers): Network Access (Link), Internet (Network), Transport, Application.

Key Theories & Formulas

1. Data Units (PDU)

  • Application: Message
  • Transport: Segment (TCP) / Datagram (UDP)
  • Network: Packet
  • Data Link: Frame
  • Physical: Bits

2. Encapsulation

Headers are added as data moves down the stack; stripped as it moves up.

Example Problems

Problem: At which layer is the IP address added?

  • Result: Network Layer.

Hardest GATE Questions

Topic: Functions per Layer and Multi-layer interactions Tricky Question (GATE 2011/2015/2018): Which layer is responsible for Dialog Control and Token Management?

  • Analysis: Session Layer.
  • The "Trap": "Routing vs Forwarding".
  • Routing (finding the path) and Forwarding (moving packet from input to output) are both Network Layer functions.
  • Hard Aspect: Identifying the layer of a specific device.
  • Hub: Physical.
  • Bridge/Switch: Data Link.
  • Router: Network.
  • Gateway: All layers.
  • Complexity: Comparison of OSI and TCP/IP philosophies. (OSI is a reference model, TCP/IP is the implementation)

References