Physical Layer

Physical Layer

  • Take pulses of light, electricity, or radio and translate to 1's and 0's.
  • Pass received data up to data link layer
  • Take binary data-link info and turn to pulses

Communication Media

  • The channel or system through which a sender transmits a message to the receiver
  • Example: Sound waves through air when we talk

Communication Media

  • Digital - Specific values. Often 1 or 0
  • Analog - Continuously variable

Medium types

  • Electricity over a wire
  • Radio waves (wi-fi, mobile, satellite/dish)
  • Light via fiber optics (or IR remote)

Digital Over a Wire

  • Electricity on - 1, off - 0
  • Electricity pos - 1, neg - 0

Signal Issues

  • Electricity through a wire creates a magnetic field
  • The causes loss, and smoothing of signal, called attenuation
  • Magnetic fields create signal in the wire, called noise or interference



Balanced Line

Differential Signaling

Twisted Pair (TP)

Cable Standards

  • 8 wires in a cable
  • Cat 5 - 100 Mbps
  • Cat 5e - 1 Gbps
  • Cat 6 - 10 Gbps
  • Connectors are called RJ45 connectors
  • (Old phones used RJ11 - 4 wires)

Wire types

  • Solid wire transmits a better signal, but stiff
  • Stranded wire is more flexible
  • Shielded cable has foil around it

Coaxial Cable (Coax)


1040 KHz (1.04 MHz) - AM Radio

90.1 MHz - FM radio

2.4 GHz and 5.0 GHz Wi-Fi

700 MHz 4G Phone

850 MHz, 24 GHz 5G Phone



  • Data amount
    • Higher frequency - more data
  • Radio wave propagation - easier to block
    • Lower frequency - bends with Earth, passes through things
    • High frequency, straight line, easy to block
  • Electromagnetic spectrum
    • United States Spectrum Allocation Chart



Starlink in Low Earth Orbit (LEO)

35 ms vs 500 ms

Transmission Types

  • Simplex - One way - broadcast
  • Half-duplex - Two ways, can't talk and listen at the same time - CB
  • Full duplex - Talk and listen at the same time. Requires two channels/frequencies.


  • Modulation - take one signal and put on a different medium.
  • Demodulation - take a signal off the medium and convert it back
  • Modem - Modulator / Demodulator - Very common term from 90's when we used modems to modulate/demodulate binary data onto audio phone lines. Less common term now as they are often combined with routers and wi-fi devices.

Radio modulation types

  • CW (Morse code)
  • Amplitude Modulation (AM)
  • Single Side Band
  • Frequency Modulation (FM)

Pulse Code Modulation (PCM)

Pulse Width Modulation


Serial Clock (SCLK)

Networking - Physical Layer

By Paul Craven

Networking - Physical Layer

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