The Physical Layer converts bits into signals. The transmission medium is the physical path between sender and receiver. Two categories: guided (data travels through a solid medium) and unguided (data travels through air/space).
Guided Media (Wired)
| Medium | Bandwidth | Distance | Interference | Cost |
|---|---|---|---|---|
| Twisted Pair | Up to 10 Gbps | 100m (unboosted) | Moderate | Low |
| Coaxial | Up to 1 Gbps | 500m | Low | Moderate |
| Fiber Optic | 100+ Gbps | 40+ km | None (immune to EMI) | High |
Twisted pair — copper wires twisted together to cancel electromagnetic interference. The twists reduce crosstalk between pairs. Most common in Ethernet (Cat5e, Cat6, Cat6a).
Coaxial — center conductor surrounded by insulating layer and braided shield. Used in cable TV, older Ethernet (10Base2/5). Better shielding than twisted pair.
Fiber optic — light pulses through glass/plastic core. Highest bandwidth, longest distance, immune to EMI. Two types: single-mode (laser, long distance, expensive) and multi-mode (LED, shorter distance, cheaper).
Unguided Media (Wireless)
| Type | Range | Directional | Use case |
|---|---|---|---|
| Radio waves | Long | Omnidirectional | WiFi, cellular, Bluetooth |
| Microwaves | Long | Unidirectional (line-of-sight) | Satellite, point-to-point links |
| Infrared | Very short | Line-of-sight | Remote controls, short-range links |
Radio waves penetrate walls and travel omnidirectionally — convenient but shared with other devices. Microwaves require line-of-sight but offer high bandwidth for point-to-point links (e.g., between buildings). Infrared cannot penetrate walls and is limited to short range.
Q: What’s the difference between guided and unguided media?
A: Guided media use a physical conductor (copper wire, fiber optic cable) to contain the signal. Unguided media transmit through air/space via electromagnetic waves (radio, microwave, infrared).
Q: Why is fiber optic cable better than copper?
A: Fiber uses light — much higher bandwidth (100+ Gbps), lower signal loss over long distances (40+ km vs 100m), and immune to electromagnetic interference. It’s the backbone of the modern internet.
Q: Why are wires in Ethernet cable twisted?
A: To cancel electromagnetic interference (crosstalk). Each pair is twisted at a different rate so interference on one wire is cancelled by the interference on the opposite wire in the pair.
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