🌐 Unit 2 – Computer Networks · Lesson 17
Data Communication & Media
What is data communication?
Data communication is the exchange of data between two devices through a transmission medium – sending a WhatsApp photo, streaming a video or printing over Wi-Fi.
Components
- Sender – device that sends the data.
- Receiver – device that receives it.
- Message – the data: text, image, audio, video.
- Medium – the path: cable or air.
- Protocol – the agreed set of rules for communication.
Bandwidth & data rate
Bandwidth is the range of frequencies a channel can carry, measured in Hertz (Hz); higher bandwidth carries more data. Data transfer rate is the number of bits sent per second (bps, Kbps, Mbps, Gbps).
Python
units = ['bps', 'Kbps', 'Mbps', 'Gbps', 'Tbps']
for i, u in enumerate(units):
print('1', u, '=', 1000 ** i, 'bps')1 bps = 1 bps
1 Kbps = 1000 bps
1 Mbps = 1000000 bps
1 Gbps = 1000000000 bps
1 Tbps = 1000000000000 bps
Python
# Time to download a file = size / speed (1 byte = 8 bits)
def download_time(size_mb, speed_mbps):
bits = size_mb * 8 # megabits
return bits / speed_mbps # seconds
for speed in [10, 50, 100, 1000]:
t = download_time(700, speed) # a 700 MB video
print(str(speed).rjust(4), 'Mbps ->', round(t, 1), 'seconds') 10 Mbps -> 560.0 seconds
50 Mbps -> 112.0 seconds
100 Mbps -> 56.0 seconds
1000 Mbps -> 5.6 seconds
Communication modes
➡️
SimplexOne direction only. Keyboard → computer, TV broadcast.↔️
Half duplexBoth directions, one at a time. Walkie-talkie.⇄
Full duplexBoth directions at the same time. Phone call.Switching techniques
Circuit switching
- A dedicated path is set up first
- Path stays reserved for the whole call
- Data arrives in order, no delay
- Wastes capacity when idle · old telephone
Packet switching
- Data broken into small packets
- Each packet may take a different route
- Packets reassembled at destination
- Efficient use of lines · the Internet
Wired (guided) media
| Medium | How it works | Speed / range | Use |
|---|---|---|---|
| Twisted pair (UTP/STP) | Pairs of copper wires twisted to reduce noise; RJ45 | Up to 10 Gbps, ~100 m | LAN, telephone |
| Coaxial cable | Copper core, insulation, metal shield | Better than twisted pair | Cable TV, CCTV |
| Optical fibre | Light pulses in a thin glass core | Very high, many km | Backbone, between buildings, undersea |
Wireless (unguided) media
📻
Radio wavesTravel long distances and through walls. FM, Wi-Fi, mobile.📡
MicrowavesLine-of-sight between towers; satellite links.🔦
InfraredVery short range, cannot pass walls. TV remote.🛰️
SatelliteCovers huge areas: DTH TV, GPS.🔷
BluetoothShort range radio for PAN devices.Mobile telecommunication (1G–5G)
- 11G (1980s) – analog voice calls only
- 22G (1990s) – digital voice, SMS (GSM)
- 33G (2000s) – mobile Internet, video calls
- 44G (2010s) – high-speed data, HD streaming (LTE)
- 55G (now) – very high speed, very low delay, IoT and smart cities
Choosing a medium
Between buildings far apart → optical fibre; inside a lab → twisted pair (Ethernet); across a hilly area or river → microwave / radio; very remote places → satellite.
Frequently Asked Questions
What is the difference between half duplex and full duplex?
In half duplex both sides can send but one at a time (walkie-talkie). In full duplex both can send at the same time (phone call).
Which switching technique does the Internet use?
Packet switching: data is broken into packets that may take different routes and are reassembled at the destination.
Which wired medium is fastest and immune to electrical interference?
Optical fibre, because it carries data as pulses of light through glass.
