OSI Layer: Physical Layer (L1)
Definition
Transmission media are the physical or wireless pathways carrying signals between network devices. Classified as guided (wired) and unguided (wireless).
Guided Media
- Twisted Pair
- UTP (unshielded) — cheap, Cat5e/Cat6, 100 Mbps–10 Gbps, LANs
- STP (shielded) — metal foil reduces EMI, costlier
- Limitation: ~100 m segment length
- Coaxial Cable
- Inner conductor + insulator + braided shield
- Higher bandwidth and noise immunity than twisted pair
- Used in cable TV, broadband, legacy Ethernet
- Fibre Optic
- Transmits light through glass/plastic core using total internal reflection
- Single-mode — laser source, one path, long haul (tens of km)
- Multi-mode — LED source, multiple paths, shorter distance (modal dispersion)
- Immune to EMI, highest bandwidth, very low attenuation, high cost
Unguided Media
- Radio waves — omnidirectional, penetrates walls, Wi-Fi/FM (3 kHz–1 GHz)
- Microwave — line-of-sight, point-to-point towers and satellite links (1–300 GHz)
- Infrared — very short range, blocked by walls, remote controls
Comparison Table
| Medium |
Bandwidth |
Attenuation |
EMI Immunity |
Cost |
Typical Use |
| UTP |
Low–Medium |
High |
Poor |
Very Low |
LAN |
| Coaxial |
Medium |
Medium |
Moderate |
Low |
Cable TV |
| Fibre |
Very High |
Very Low |
Immune |
High |
Backbone |
| Radio |
Low–Medium |
High |
Poor |
Low |
Wi-Fi, mobile |
| Microwave |
High |
Medium |
Moderate |
Medium |
Satellite, WAN |
Numericals
Q1 — Nyquist (noiseless channel)
C = 2 × B × log₂L
Bandwidth = 3,000 Hz, 4 signal levels.
- C = 2 × 3,000 × log₂4 = 2 × 3,000 × 2 = 12,000 bps
Q2 — Shannon (noisy channel)
C = B × log₂(1 + S/N)