Guided and Unguided Media
🎬 This chapter has a 5:55 video: Transmission Media →Every transmission medium is one of two kinds, and the exam uses the words guided and unguided for them. They are the same two kinds you already know as wired and wireless — but a question that says “suggest a guided medium” is only easy if you know which word means which.
1The two words
The word does the explaining. A cable guides: put a signal in one end and it comes out the other, and nowhere else. Air does not guide: the waves spread out, and whoever is in range receives them.
2The six media, in their two families
Three cables. The first two carry electric signals through copper; the third carries light through glass.
- Twisted pair cable — electric signals
- Co-axial cable — electric signals
- Fibre-optic cable — light
Three kinds of electromagnetic wave, sent and received by antennas. They differ in frequency, and everything else follows from that.
- Radio waves — spread in all directions and get through walls
- Microwaves — a narrow beam in one direction; needs line of sight
- Infrared waves — very short range, blocked by walls
3What follows from being guided
The difference is not only about cables. Because a guided signal can only travel where its path goes, it is harder to reach, harder to disturb and harder to join — all three at once.
| Guided | Unguided | |
|---|---|---|
| Reaching a new device | Lay a cable to it | Be in range — nothing to lay |
| Who else receives the signal | Nobody: it stays in the cable | Anybody in range, which is why it needs encrypting |
| What disturbs it | Little; optical fibre is not disturbed by electrical noise at all | Walls, weather, distance and other signals on the same frequency |
| Devices that move | Cannot: the cable holds them in place | Can: a phone works while you walk |
4Which one the case study wants
The last question of the paper describes a campus and asks you to suggest a medium. Two words in it decide the answer.
It wants a cable. Say optical fibre when the question asks for high speed or a long distance, and twisted pair (Ethernet) when it is cheap and inside one building.
It wants an unguided medium: radio waves for a short hop inside the campus, microwaves between two distant buildings in line of sight.
▶Show the answer
Optical fibre cable. The question says guided, so the answer has to be one of the three cables, and it says high speed over a long distance, which among the three is optical fibre.
Answering “satellite” or “microwave” here loses the mark, however sensible it sounds: both are unguided.
5Guided or unguided?
Ask one question: is the signal led along something?
6Check yourself
Which of these is an unguided transmission medium?
Why is a guided medium usually the more secure of the two?
A hospital wants to connect a building across a busy main road, with no permission to dig. Which family should it use?