Wired (guided) media
In guided media the signal travels along a physical cable.
Twisted pair cable
Two insulated copper wires twisted together; many pairs sit in one cable. The twisting cuts noise from nearby wires. Types: UTP (unshielded) and STP (shielded, less noise). Cheap and easy to fit; used in LANs and telephone lines. Short range (about 100 m) and lower speed.
Coaxial cable
A copper core, an insulating layer, a woven metal shield and an outer cover. The shield blocks outside noise. Used in cable TV. Better than twisted pair for distance and speed, but thicker and costlier.
Optical fibre
Thin strands of glass or plastic that carry data as light pulses by total internal reflection. Very high speed, very long distance, no electrical noise, hard to tap. But costly, fragile and hard to join.
Wireless (unguided) media
In unguided media signals travel through air or space.
- Radio waves (about 3 kHz – 1 GHz): spread in all directions, pass through walls, travel far. Used in FM radio, Wi-Fi, mobile phones. Easy to interfere with.
- Microwaves (about 1 – 300 GHz): travel in a straight line (line of sight), so towers must see each other. Used for satellite links and long-distance towers. Cannot go through buildings; affected by rain.
- Infrared (300 GHz – 400 THz): very short range, line of sight, cannot pass walls. Used in TV remotes. Safe from eavesdropping outside the room.
Bluetooth is a short-range radio technology for devices close by.
Devices that connect a computer: modem, Ethernet card, RJ45, Wi-Fi card
- Modem (MOdulator–DEModulator): computers use digital signals; telephone and cable lines carry analog signals. The modem modulates digital to analog when sending and demodulates analog to digital when receiving.
- Ethernet card / NIC (Network Interface Card): a card in the computer that connects it to a wired network. Each NIC has a unique 48-bit MAC address.
- RJ45 (Registered Jack 45): an 8-pin plug at the end of an Ethernet twisted-pair cable that fits into the NIC port.
- Wi-Fi card: a wireless NIC with a small antenna that connects the computer to a Wi-Fi network by radio waves.
Devices that carry and direct data: repeater, hub, switch, router, gateway
- Repeater: signals become weak (attenuate) over long cables. A repeater receives a weak signal, regenerates it and sends it on at full strength.
- Hub: joins many computers in a LAN. Data arriving on one port is copied to every port. Simple, but causes extra traffic and collisions. Active hubs boost the signal; passive hubs do not.
- Switch: also joins computers, but reads the destination MAC address and sends data only to the right port. Less traffic, more secure, faster than a hub.
- Router: joins different networks (like your home LAN and the Internet). It reads the IP address of each packet and chooses the best path.
- Gateway: the entry/exit point of a network that connects networks using different protocols, translating between them. A home router often also works as the gateway to the Internet.
Try it: spot the media and devices at home or school
Walk around your home or computer lab. Look for: a blue/grey Ethernet cable with an RJ45 plug, a round TV cable (coaxial), a fibre box from the internet company, the Wi-Fi router's antennas, and the TV remote (infrared). Make a table: item, wired or wireless, what it does. Then in the 3D free-play step, send a message through a hub, a switch and a router and see who receives it.
Key formulas and definitions
- Guided: twisted pair < coaxial < optical fibre (speed and distance)
- Unguided: radio (all directions) · microwave (line of sight) · infrared (short range)
- Hub → all ports · Switch → one port (MAC) · Router → between networks (IP) · Gateway → between protocols
- Modem: digital ↔ analog
Worked examples
1. Which medium would you choose to join two buildings 5 km apart with very high speed? Why?
Optical fibre. It carries light over long distances at very high speed and is not affected by electrical noise.
2. Why does a TV remote not work from another room?
It uses infrared, which works only in a straight line over a short distance and cannot pass through walls.
3. Computer A sends data to computer C on a 4-port hub. Which computers receive it? What if it is a switch?
Hub: B, C and D all receive it (hub copies to every port); only C keeps it. Switch: only C receives it (switch uses C's MAC address).
4. A school's LAN cable runs 180 m between two labs and data is being lost. What device fixes this?
A repeater placed in between, because twisted pair signals weaken after about 100 m.
5. Which device connects a school LAN to the Internet and chooses paths for packets?
A router (it reads IP addresses and routes packets between networks).
6. Why is a modem needed on a telephone-line internet connection?
The computer sends digital signals but the phone line carries analog signals. The modem converts digital to analog and back.
Common mistakes
- Saying a hub sends data only to the target; that is a switch.
- Mixing router and switch: a switch works inside one network (MAC); a router joins different networks (IP).
- Thinking microwaves go through buildings; they need line of sight.
- Calling RJ45 a cable; it is the plug (connector) on the cable.