--- title: "Transmission impairments" subject: "Data Communication" chapter: 1 key: "semester_5/data_communication:transmission-impairments" source: https://www.bctengineer.com/q/semester_5/data_communication/transmission-impairments site: BCT Engineering --- # Transmission impairments Data Communication · Chapter 1: Introduction - Asked in: 2076 Asoj, 5 marks; 2075 Asoj, 6 marks; 2074 Chaitra, 5 marks; 2074 Asoj, 4 marks; 2072 Chaitra, 6 marks; 2070 Chaitra, 3 marks (asked 6 times) - Video: [Transmission Impairments | Data Communication | Lec-04 | Bhanu Priya](https://www.youtube.com/watch?v=GYAKUoeuFRQ) — Education 4u - Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14 - Page: https://www.bctengineer.com/q/semester_5/data_communication/transmission-impairments - Whole chapter: https://www.bctengineer.com/md/semester_5/data_communication/chapter-1 ## Question Describe the transmission impairments of a data communication system with suitable examples. ## Short answer Transmission impairments are attenuation (signal strength loss with distance, measured in dB), distortion (signal shape change due to varying delays for frequency components, causing inter-symbol interference), and noise (thermal, intermodulation, crosstalk, impulse). These degrade signal quality and increase bit errors. ## Answer Transmission impairments are the imperfections in a channel that make the received signal differ from the transmitted one. They cause errors in digital systems and loss of quality in analog systems. The three main types are attenuation, distortion and noise. #### 1. Attenuation Loss of signal strength with distance because energy is absorbed or dissipated in the medium. It increases with distance and frequency. It is measured in decibels: dB = 10 log₁₀(P₂/P₁), a negative value for loss. - Example: a signal falls from 10 mW to 5 mW, so loss = 10 log₁₀(0.5) = -3 dB. - Remedy: amplifiers (analog) or repeaters (digital). #### 2. Distortion Change in the shape of a composite signal because its frequency components travel at different speeds in the medium (delay distortion) or are attenuated unequally (attenuation distortion). - Example: the harmonics of a square pulse arrive at different times, so the pulse spreads and overlaps with the next one (inter-symbol interference). - Remedy: equalisers. #### 3. Noise Unwanted signals added between transmitter and receiver. - **Thermal noise**: random motion of electrons, N = kTB. - **Intermodulation noise**: sum or difference of frequencies when several signals share a non-linear medium. - **Crosstalk**: coupling from a neighbouring wire, for example overhearing another call on a telephone line. - **Impulse noise**: short, high-amplitude spikes from lightning or switching. It destroys a burst of bits. #### Effect The quality of a link is measured by SNR = signal power / noise power. A low SNR increases the bit error rate. By Shannon, the channel capacity C = B log₂(1 + SNR) drops when noise rises. ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. A signal falls from 10 mW to 5 mW. What is the loss in dB? A. -3 dB B. -5 dB C. +3 dB D. -0.5 dB Answer: A. dB = 10 log10(5/10) = 10 log10(0.5), about -3 dB. (Key point: Attenuation calculation) 2. What causes delay distortion? A. Signal energy is absorbed with distance B. Frequency components of a signal travel at different speeds in the medium C. Coupling from a neighbouring wire D. Lightning spikes Answer: B. Different speeds spread the pulse and cause inter-symbol interference; equalisers correct it. (Key point: Distortion cause) 3. Which noise causes a burst of bit errors, such as from lightning or switching? A. Thermal noise B. Crosstalk C. Impulse noise D. Intermodulation noise Answer: C. Impulse noise consists of short, high-amplitude spikes that destroy a group of bits. (Key point: Impulse noise) 4. What happens to Shannon capacity when noise rises (SNR falls)? A. It rises B. It is unchanged C. It becomes infinite D. It drops Answer: D. C = B log2(1 + SNR), so lower SNR means lower capacity and higher BER. 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