The physical layer carries bits using electrical, optical, or electromagnetic energy. Choice of medium trades distance, bandwidth, immunity to interference, and cost. Understanding categories of copper, fiber modes, and RF bands explains real-world limits you see in link-up speeds.

Copper, light, air
When engineers pick each medium
MediumTypical useLimiting factor
Cat6a copperOffice horizontal cabling100 m channel, crosstalk
OM4 MM fiberData-center rack-to-rackModal dispersion distance
SM fiberCampus/MAN/WANOptical budget & splicing quality
Wi‑FiUser mobilityInterference, airtime, half-duplex shared medium
Signal journey — simplified
Bits
From NIC
Line coding
Manchester/PAM
Medium
Copper/fiber/RF
Receiver
Decode + clock
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Key terms

Attenuation weakens signal with distance; dispersion spreads pulses in time on fiber; NEXT/FEXT measure crosstalk between pairs. Installers certify cable plants with testers that plot these — if margins fail, link may flap at 1 Gb/s even though LEDs show link.
Cat6a vs Cat5e headroom
SM vs MM fiber distance
Wi‑Fi is shared half-duplex air
Certify cabling after install
Never exceed bend radius on fiber — micro-bends raise loss. Copper runs must respect cable trays and avoid tight kinks that alter impedance.