Guided and other transmission media

Transmission media is the physical path that carries signals from sender to receiver. It is divided into guided (wired) and unguided (wireless) media.

1. Twisted pair cable (guided)

Two insulated copper wires twisted together to reduce interference. Types: UTP (unshielded, Cat5e/Cat6, used in LANs, up to 100 m) and STP (shielded, better noise protection, costlier). Cheap and easy to install, but short range and lower bandwidth, and sensitive to noise.

2. Coaxial cable (guided)

A central copper conductor, surrounded by insulation, a braided metallic shield and an outer jacket. Better shielding and higher bandwidth than twisted pair. Types: 50 ohm baseband (Thinnet/Thicknet Ethernet) and 75 ohm broadband (cable TV). Used in cable TV and older Ethernet. Bulkier than twisted pair; connectors are BNC.

3. Optical fibre (guided)

A glass or plastic core surrounded by cladding; data travels as light pulses by total internal reflection.

• Single-mode: thin core, laser, very long distance (tens of km).
• Multimode: thicker core, LED, shorter distance. Advantages: very high bandwidth, low attenuation, immune to electromagnetic interference, secure. Disadvantages: costly, fragile, difficult to splice.

4. Radio waves / microwave / infrared (unguided)

• Radio waves (3 kHz to 1 GHz): omnidirectional, penetrate walls; used in FM, mobile and Wi-Fi.
• Microwaves (1 to 300 GHz): unidirectional, line-of-sight, needs towers or satellites; used in cellular and satellite links.
• Infrared: short range, line-of-sight, cannot pass walls; used in remote controls.