optional · CSS study notes
Transmission Media (2016)
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OSI Layer: Physical Layer (L1)
Definition
Transmission media are the physical or wireless pathways carrying signals between network devices. Classified as guided (wired) and unguided (wireless).
Guided Media
- Twisted Pair
- UTP (unshielded) — cheap, Cat5e/Cat6, 100 Mbps–10 Gbps, LANs
- STP (shielded) — metal foil reduces EMI, costlier
- Limitation: ~100 m segment length
- Coaxial Cable
- Inner conductor + insulator + braided shield
- Higher bandwidth and noise immunity than twisted pair
- Used in cable TV, broadband, legacy Ethernet
- Fibre Optic
- Transmits light through glass/plastic core using total internal reflection
- Single-mode — laser source, one path, long haul (tens of km)
- Multi-mode — LED source, multiple paths, shorter distance (modal dispersion)
- Immune to EMI, highest bandwidth, very low attenuation, high cost
Unguided Media
- Radio waves — omnidirectional, penetrates walls, Wi-Fi/FM (3 kHz–1 GHz)
- Microwave — line-of-sight, point-to-point towers and satellite links (1–300 GHz)
- Infrared — very short range, blocked by walls, remote controls
Comparison Table
| Medium | Bandwidth | Attenuation | EMI Immunity | Cost | Typical Use |
|---|---|---|---|---|---|
| UTP | Low–Medium | High | Poor | Very Low | LAN |
| Coaxial | Medium | Medium | Moderate | Low | Cable TV |
| Fibre | Very High | Very Low | Immune | High | Backbone |
| Radio | Low–Medium | High | Poor | Low | Wi-Fi, mobile |
| Microwave | High | Medium | Moderate | Medium | Satellite, WAN |
Numericals
Q1 — Nyquist (noiseless channel)
C = 2 × B × log₂L
Bandwidth = 3,000 Hz, 4 signal levels.
- C = 2 × 3,000 × log₂4 = 2 × 3,000 × 2 = 12,000 bps
Q2 — Shannon (noisy channel)
C = B × log₂(1 + S/N)
Bandwidth = 3,000 Hz, SNR = 3,161.
- C = 3,000 × log₂(3,162) ≈ 3,000 × 11.62 ≈ 34,860 bps (basis of ~33.6 kbps dial-up limit)
Q3 — SNR in decibels
SNR(dB) = 10 log₁₀(S/N). If S/N = 1,000 → 10 × 3 = 30 dB
Q4 — Attenuation
Signal enters at 10 mW, leaves at 5 mW.
- Loss = 10 log₁₀(5/10) = 10 × (−0.3) = −3 dB (half power lost = 3 dB rule)
CSS Exam Angle
- CSS 2016 Paper-II Q2(b), 7 marks, asks about transmission impairments. Answer attenuation, distortion and noise; the media comparison is related practice.
- Nyquist vs Shannon distinction (noiseless vs noisy) is a favourite short question
- Cite fibre in Pakistan: PTCL long-haul backbone, CPEC optical fibre via Khunjerab, submarine cables (SMW-4/5, AAE-1) and the impact of cable cuts on national internet
Likely Questions
- Practice: discuss transmission media with comparative merits and demerits. This is not the verified wording of the 2016 question.
- Differentiate single-mode and multi-mode fibre.
- State Nyquist and Shannon capacity formulas and solve a numerical.
📜 Past-Paper Questions & Structured Answers
Questions are paraphrased; answer focus is suggested guidance, not an official marking scheme.
2016 · Paper-II · Q2(b) · 7 marks
Question (paraphrased): Explain the impairments that alter signals during transmission.
Answer focus: Explain attenuation, distortion and noise; add causes, effects and remedies.
Introduction: a real channel changes signal strength and shape and introduces unwanted energy. These effects can cause incorrect bit decisions at the receiver.
| Impairment | Cause and effect | Example / remedy |
|---|---|---|
| Attenuation | Signal power falls with distance; the received signal may be too weak | A long copper cable weakens a signal. Use suitable media, shorter spans, amplifiers or digital regenerators. |
| Distortion | Frequency components experience unequal attenuation or delay, changing waveform shape | A pulse spreads and overlaps neighbouring pulses. Equalisation and appropriate bandwidth reduce intersymbol interference. |
| Noise | Unwanted energy is added to the useful signal | Thermal noise arises from random charge motion; crosstalk comes from adjacent wires; impulse noise can come from lightning or switching. Shielding, twisting, filtering and error control help. |
Numerical illustration: if received power is one tenth of transmitted power, gain = 10 log₁₀(Pout/Pin) = −10 dB, i.e. 10 dB attenuation. Signal-to-noise ratio compares signal and noise powers; a larger ratio generally makes detection easier.
Conclusion: amplification alone does not solve every impairment because it also amplifies noise. Reliable transmission combines suitable media, signal processing and error control.