Transmission media

Data travels along a transmission medium, either guided (along wires) or unguided (through the air).

Guided (wired) media:

• Twisted pair cable: pairs of copper wires twisted together to reduce interference; unshielded (UTP) or shielded (STP). Used in telephone lines and LANs.
• Coaxial cable: a central copper wire inside insulation and a metal shield; it carries higher frequencies, as in cable TV.
• Optical fibre: thin strands of glass that carry data as pulses of light. An outer layer, the cladding, keeps the light bouncing inside. It is very fast, carries data over long distances, and is not affected by electrical interference.

Unguided (wireless) media use electromagnetic waves, from about 3 KHz to 900 THz, sent and received by antennas:

• Radio waves (3 KHz-1 GHz) spread in all directions and pass through walls: used for radio, mobile phones and Wi-Fi.
• Microwaves (1-300 GHz) travel in straight lines, so the sender and receiver must be in line of sight: used by towers and satellites.
• Infrared (300 GHz-400 THz) works only over short distances and cannot pass through walls: used in TV remote controls.

Wireless technologies:

• Bluetooth uses the 2.4 GHz band, over about 10 m, at 1-2 Mbps. Devices form a small network, a piconet, with one master and up to 7 active slaves.
• Wireless LAN (the IEEE 802.11 standard, or Wi-Fi) connects devices to a network through access points.
• WiMax covers much larger areas, such as a city (a MAN).