Data Transmission Media
1. What Transmission Media Is
Transmission media is the physical path between the transmitter and the receiver over which data travels as electrical signals, light pulses or electromagnetic waves. It is classified by whether the signal is confined to a physical path:
| Category | Meaning | Examples |
|---|---|---|
| Guided (bounded / wired) | Signal travels along a solid physical path | Twisted pair, coaxial cable, optical fibre |
| Unguided (unbounded / wireless) | Signal radiates through space | Radio waves, microwaves, infrared |
2. Guided Media
(a) Twisted Pair Cable
Two insulated copper conductors twisted around each other; the twist cancels electromagnetic interference and crosstalk between the pairs.
- UTP (Unshielded Twisted Pair) — cheap, flexible, most common in LANs; four pairs; connector RJ-45; category ratings define bandwidth: Cat3 (16 MHz, telephony), Cat5e (100 MHz, 1 Gbps up to 100 m), Cat6 (250 MHz, 1 Gbps at 100 m / 10 Gbps at 55 m), Cat6a (500 MHz, 10 Gbps at 100 m), Cat7/8 (shielded, 600 MHz-2 GHz, data centres).
- STP (Shielded Twisted Pair) — metallic foil/braid shield reduces noise; bulkier and costlier.
- Limits: attenuation rises with distance — the practical LAN limit is 100 m; susceptible to EMI and tapping.
(b) Coaxial Cable
A central copper core, an insulating layer, a braided metallic shield and an outer jacket. The shield gives excellent immunity to noise and supports higher bandwidth than twisted pair.
- Thinnet (10Base2, 185 m) and Thicknet (10Base5, 500 m) were used in early bus LANs; RG-6 coaxial is still used for cable TV and broadband internet, and RG-59 for CCTV.
- Advantages: higher bandwidth and better noise immunity than UTP, longer runs. Disadvantages: thick, stiff, expensive compared with UTP, harder to install and troubleshoot, and largely replaced by UTP/fibre in LANs.
(c) Optical Fibre
A hair-thin glass or plastic core surrounded by cladding of lower refractive index; data travels as light and stays inside the fibre by total internal reflection. Because it carries light, not electricity, it is immune to electromagnetic interference and cannot be tapped easily.
- Single-mode fibre (SMF) — core ~8-10 µm, one light path, laser source, very low attenuation, used for long-haul and metro backbones, 50-100 km between repeaters.
- Multimode fibre (MMF) — core 50-62.5 µm, several light paths, LED/VCSEL source, cheaper and easier to splice, used inside buildings and data centres (up to a few hundred metres at 10 Gbps).
- Advantages: enormous bandwidth (Tbps in the lab, structured to 10/40/100 Gbps), very low attenuation and loss, immune to EMI and crosstalk, no fire/electrical hazard, highly secure, light in weight and small in size. Disadvantages: fragile, expensive to install and splice, needs specialist technicians and equipment, and unidirectional — two fibres (or two wavelengths) are needed for bidirectional communication.
3. Comparison of Guided Media
| Basis | Twisted pair (UTP) | Coaxial cable | Optical fibre |
|---|---|---|---|
| Signal | Electrical | Electrical | Light |
| Bandwidth | Low-medium (up to 1-10 Gbps short) | Medium (hundreds of Mbps-Gbps) | Very high (Gbps-Tbps) |
| Max distance without repeater | 100 m (LAN) | 185-500 m | 2 km (MMF) to 50-100 km (SMF) |
| Noise / EMI immunity | Poor (better if shielded) | Good | Excellent |
| Security (tapping) | Easy to tap | Moderate | Very hard to tap |
| Cost | Lowest | Moderate | Highest |
| Installation | Easiest, most flexible | Moderate, bulky | Difficult, skilled labour |
| Fire/electrical hazard | Yes | Yes | No |
| Typical use | LAN, telephony, CCTV | Cable TV, broadband, legacy LAN | Backbone, ISP/mobile towers, data centres |
4. Unguided Media
(a) Radio Waves — frequency roughly 3 kHz to 1 GHz. Omni-directional (a single antenna radiates in all directions), so sender and receiver need not be aligned; waves penetrate walls and buildings. Used for AM/FM radio, television, cordless phones, Wi-Fi (2.4 GHz and 5 GHz), Bluetooth and cellular networks. Susceptible to interference, and licences are required for many bands.
(b) Microwaves — frequency roughly 1 GHz to 300 GHz. Line-of-sight propagation, so parabolic dish antennas must be exactly aligned; the curvature of the earth and obstacles limit each hop to about 40-50 km, requiring repeaters. Used for long-distance telephone and TV links, satellite communication (uplink/downlink), Wi-Fi at higher bands, radar and microwave ovens (2.45 GHz). Cheap compared with laying cable across difficult terrain, but affected by rain, fog and atmospheric absorption, and insecure unless encrypted.
(c) Infrared — frequency roughly 300 GHz to 400 THz, just below visible light. Very short range, cannot pass through walls, and needs a clear path (or at least line of sight). Ideal for remote controls, wireless keyboards/mice and short indoor links. It is highly secure (it stays inside the room) and free of licensing, but limited by distance and by sunlight interference.
5. Wireless Technologies at a Glance
| Technology | Medium | Range | Data rate | Use |
|---|---|---|---|---|
| Bluetooth | 2.4 GHz radio | ~10 m | 1-3 Mbps | Headphones, peripherals, PAN |
| Wi-Fi 802.11 a/b/g/n/ac/ax | 2.4/5/6 GHz radio | 30-100 m | 11 Mbps - 9.6 Gbps | LAN, home/office internet |
| ZigBee | 2.4 GHz radio | 10-100 m | 250 kbps | IoT sensors, smart home |
| Cellular 4G/5G | Licensed radio bands | km | 10 Mbps - 1 Gbps+ | Mobile internet, voice |
| Microwave link | 4-11 GHz | 40-50 km per hop | 100 Mbps - Gbps | Backhaul, ISP links |
| Satellite | 1-40 GHz | Global | Mbps | Remote areas, TV, GPS, Starlink |
| Infrared | 300 GHz-400 THz | Few metres | kbps-Mbps | Remote controls, short links |
6. Choosing a Medium — Selection Criteria
Data rate required · distance between nodes · cost of cable plus installation and maintenance · environment (EMI from motors, welding, machinery; water, heat, chemicals) · required immunity to interference and tapping · flexibility for future upgrades · number of connections needed · whether mobility is required (then wireless). Rule of thumb: UTP for in-building LAN drops up to 100 m; fibre for backbone, long distance and electrically noisy plants; Wi-Fi where users move; microwave/satellite where cabling is impossible.
7. Capacity Formulae Worth Knowing
- Nyquist (noiseless channel): C = 2B × log₂ M bits per second, where B is bandwidth in Hz and M is the number of signal levels.
- Shannon (noisy channel): C = B × log₂(1 + S/N), where S/N is the signal-to-noise ratio (SNR in dB = 10 × log₁₀(S/N)).
Example: a 3 kHz telephone channel with 30 dB SNR: S/N = 1000, so C = 3000 × log₂(1001) ≈ 3000 × 9.97 ≈ 29.9 kbps. - Attenuation (signal loss) is measured in decibels (dB) and grows with distance and frequency; this is why repeaters/amplifiers and distance limits exist.
8. Exam Points
- Classify transmission media: guided vs unguided, with examples — 5 marks, almost certain.
- Explain twisted pair, coaxial and optical fibre with construction, advantages and disadvantages — 5-7 marks, very frequent.
- Compare single-mode and multimode fibre — 3-5 marks.
- Compare UTP, coaxial and fibre (bandwidth, distance, immunity, cost) — 5 marks, frequent.
- Explain radio waves, microwaves and infrared with their frequency bands and applications — 5 marks.
- Numerical on Nyquist/Shannon capacity — 3-5 marks.
- Why is optical fibre preferred for backbone networks? — 3 marks.