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
🔌
Twisted pair (UTP/STP)
📡
Coax
💎
Fiber SM
🔷
Fiber MM
📶
Radio (Wi‑Fi)
📱
Cellular / licensed
When engineers pick each medium
| Medium | Typical use | Limiting factor |
|---|---|---|
| Cat6a copper | Office horizontal cabling | 100 m channel, crosstalk |
| OM4 MM fiber | Data-center rack-to-rack | Modal dispersion distance |
| SM fiber | Campus/MAN/WAN | Optical budget & splicing quality |
| Wi‑Fi | User mobility | Interference, airtime, half-duplex shared medium |
Signal journey — simplified
🔢
Bits
From NIC
→
📟
Line coding
Manchester/PAM
→
🌈
Medium
Copper/fiber/RF
→
📥
Receiver
Decode + clock
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.