Transmission media

The essentials

A transmission medium carries signals between devices. Guided (wired) media carry signals along a physical path. Unguided (wireless) media carry signals through the air or space as electromagnetic waves.

Key points

• Twisted pair cable: pairs of insulated copper wires twisted together to reduce interference. Cheap and easy to install; used in telephone lines and LANs. Types: UTP (unshielded) and STP (shielded, better protection from noise).
• Coaxial cable: a central copper conductor surrounded by insulation, a braided metal shield and an outer cover. Better shielding and higher bandwidth than twisted pair; used for cable TV.
• Ethernet: the most common wired LAN technology. Computers connect through Ethernet cards, usually using twisted pair cables with RJ45 connectors, to a switch or hub.
• Optical fibre: carries data as pulses of light through thin glass or plastic fibres, using total internal reflection. Very high bandwidth, low loss over long distances, and immune to electromagnetic interference. It is costly and harder to install and join.
• Radio waves: travel long distances, can pass through walls, and are used for FM radio, mobile phones and Wi-Fi.
• Microwaves: high-frequency waves that travel in straight lines (line of sight), so towers must face each other; used in long-distance and satellite links.
• Infrared: very short range, line of sight, cannot pass through walls; used in TV remote controls.
• Bluetooth: short-range wireless technology (about 10 metres for common devices) for connecting phones, earphones and keyboards in a PAN.
• WLAN (Wi-Fi): a wireless LAN in which devices connect to an access point using radio waves.

Exam tips

• Case questions: "two buildings far apart, high speed, electrical noise" points to optical fibre; "TV remote" points to infrared.
• Remember that microwaves and infrared need line of sight.

Common mistakes

• Saying infrared can pass through walls. It cannot.
• Calling optical fibre an electrical cable; it carries light, not electric current.