Lecture on OSI and TCP/IP Models

Overview

• OSI Model: Open Systems Interconnection model
• TCP/IP Model: More commonly used in class
• Both are conceptual models for internet design
• OSI has evolved into the TCP/IP model

Layering Concept

• Both models use a layering approach
• OSI Model: 7 layers
  • Physical
  • Data Link
  • Network
  • Transport
  • Session
  • Presentation
  • Application
• TCP/IP Model: 5 layers
  • Physical
  • Data Link
  • Network
  • Transport
  • Application
• TCP/IP combines session, presentation, and application layers

Purpose of Layering

• Provides abstraction
• Each layer performs a specific job
• Higher layer protocols operate under assumptions lower layers work correctly

Data Link Layer

• Handles message delivery across a single device or hop
• Example: Computer to router

Network Layer

• Provides end-to-end delivery across multiple hops
• Relies on data link layer for single hop delivery

Transport Layer

• Ensures data reaches the correct application on the correct machine
• Works under the assumption the network layer delivers to the correct machine

Communication Protocols

• Set of strict rules for data communication
• Key properties: syntax, semantics, synchronization, error recovery
• Early examples: Homing pigeons, hydraulic semaphore

Importance of Protocols

• No ambiguity allowed in computer protocols
• RFCs (Request for Comments) define detailed protocol rules

Historical Protocols

• Homing Pigeons: Unidirectional communication
• Hydraulic Semaphore: Distinction between transmission medium and protocol
• Optical Telegraph: Early example of end-to-end encryption

Physical Layer

• Deals with bit-by-bit data delivery
• Concerns encoding on transmission mediums (copper, optical, radio waves)

Transmission Mediums

• Copper Cables: Voltage modulation, uses network interface card

  • Issue: Crosstalk, minimized by twisted pair design

• Fiber Optic Cables: Light modulation, glass tubes

  • Issues: Attenuation loss (smearing), don't carry power
  • Used for internet backbone due to high bandwidth

Channel Types

• Simplex: Unidirectional (e.g., TVs)
• Duplex: Bidirectional
  • Half Duplex: One party sends at a time (e.g., walkie-talkies)
  • Full Duplex: Both parties can send simultaneously

Historical Networking Devices

• Hub
  • Physical layer device, duplicates signals
  • Creates collision domains (degraded connection)

Collision Domain Solutions

• Carrier Sense Multiple Access (CSMA)

  • Only send if no signal is received

• CSMA with Collision Detection (CSMA/CD)

  • Stop transmitting if another transmission is detected

• CSMA with Collision Avoidance (CSMA/CA)

  • Adds randomized time delay to avoid livelock

Key Takeaways

• Networking built on structured layers and protocols
• Historical models and devices inform modern networking practices
• Understanding of protocols and layers vital for networking applications