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

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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)

Radio

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

Frequencies

• 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
    

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Link

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

• 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)

Different modulation types link

Pulse Code Modulation (PCM)

Pulse Width Modulation

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Serial Clock (SCLK)