Ref: https://learn.cantrill.io/courses/2022818/lectures/45636946

YouTube: https://www.youtube.com/watch?v=a5SMTyhn0U8

• Summary:
• Contents:

OSI L1 (Physical) - Key Concepts

• 🔧 Standards/specifications for physical transmission and reception of data over a communications channel (physical medium)
  • Raw data → Bit streams (1s and 0s)
  • Defined characteristics of communication medium: voltage levels, signaling methods (modulation, timing, rates…), distances, connector types…
• Common shared physical mediums:
  1. Copper (Electrical signal)
  2. Optical fiber (Light signal)
  3. WiFi (Radio Frequency - RF signal)
• Example: Connect two laptops over a physical cable to play a game

L1 Hubs and L1 Limitations

• 🔧 A L1 Hub can connect multiple devices at L1
  • Blindly repeats (broadcasts) signal it receives in one port to all other ports
  • Has no awareness of upper layers (L1 device)
• Problems
  • No uniquely identified devices → data sent to ALL devices
    • 💡 Like shouting into a room full of people, without using names
  • Collision: if more than one device transmits at the same time → Data corruption
    • And all errors and collisions are retransmitted through medium
  • 1 Broadcast and 1 Collision domain → scales very poorly
• ‼️ L1 Limitations
  • No media access control (i.e. can't control which devices can transmit at given moment)
  • No collision detection
  • No device addressing (Broadcast medium)