Physical Layer Practice Questions

4B/5B encoding: converts every 4 data bits into a 5-bit code selected to avoid problematic patterns. The 5-bit codes are chosen so that no code has more than one leading zero and no more than two trailing zeros - preventing long runs of zeros that cause synchronization loss with NRZ encoding. Out of 2⁵ = 32 possible 5-bit codes, only 16 are used (for the 16 4-bit data values); the remaining 16 are control codes or invalid. Overhead: 5/4 = 25% bandwidth overhead (efficiency = 80%). Used with NRZI in 100BASE-TX Fast Ethernet and FDDI. 8B/10B encoding (used in Gigabit Ethernet) similarly maps 8 bits to 10 bits with 20% overhead but also balances DC component.

Transmission delay = packet size / link rate = 1000 bits / (1 × 10⁶ bps) = 0.001 s = 1 ms. Propagation delay = 270 ms (given - one-way delay to geostationary orbit at ~36,000 km altitude, propagation speed ≈ speed of light ≈ 3×10⁸ m/s). Total end-to-end delay = transmission delay + propagation delay = 1 + 270 = 271 ms. Note: for geostationary satellite links, propagation delay dominates completely. The RTT (round-trip time) ≈ 540 ms, which is the primary reason satellite internet has high latency despite potentially high bandwidth. LEO (Low Earth Orbit) satellites like Starlink have much lower latency (~20-40 ms) due to closer orbit (~500 km altitude).

Bandwidth efficiency (spectral efficiency) = Data rate (bps) / Bandwidth (Hz), measured in bits/s/Hz. Higher efficiency means more data transmitted per unit of bandwidth.

Multipath propagation: the transmitted signal reaches the receiver via multiple paths - direct line of sight, reflections off buildings/walls/ground, diffraction around obstacles. Each path has a different length → different delay → the receiver sees multiple delayed copies of the signal. These copies add constructively or destructively depending on phase difference, causing frequency-selective fading (some frequencies attenuated, others boosted). OFDM mitigation: OFDM divides the wideband channel into many narrow subcarriers (e.g., 312.5 kHz per subcarrier in 802.11). Each narrow subcarrier experiences flat fading (approximately uniform attenuation) rather than frequency-selective fading. Equalization per subcarrier is simple (just multiply by 1/H for each subcarrier's channel response H). Cyclic prefix (guard interval) absorbs inter-symbol interference from multipath delay spread.

Types of noise in communication channels: A: TRUE - Thermal/Johnson noise: N = kTB where k = Boltzmann constant, T = absolute temperature, B = bandwidth. Present in ALL electronic components, irreducible at temperatures above absolute zero. Sets the fundamental noise floor. B: TRUE - Impulse noise: brief, high-amplitude spikes from external sources (lightning, switching transients, motor starts). Primary cause of errors in digital communication. Duration: microseconds; amplitude: very large. Hard to filter without specialized circuits. C: TRUE - Crosstalk: near-end crosstalk (NEXT) at the transmitting end; far-end crosstalk (FEXT) at the receiving end. Reduced by twisting (UTP) or shielding (STP). D: FALSE - white noise (thermal noise) has uniform power spectral density across ALL frequencies - it is NOT limited to high frequencies.

Shannon's theorem: $C = B \log_2(1 + S/N)$ bps, where $B$ is bandwidth (Hz) and $S/N$ is the signal-to-noise ratio. This gives the theoretical maximum for a noisy channel, regardless of encoding.

Nyquist's formula: Max data rate $= 2B \log_2 M$ bps, where $B$ is bandwidth in Hz and $M$ is the number of signal levels. This assumes a noiseless channel.

Baseband transmission: digital signals are sent directly on the medium using the entire available bandwidth for a single channel. Signals extend from 0 Hz up to some maximum frequency. Used in: Ethernet (LAN), USB, PCIe. No modulation - bits are encoded as voltage/current levels. Broadband transmission: the available bandwidth is divided into multiple channels using FDM, each carrying a modulated signal (analog or digital). Different channels carry different services simultaneously (cable TV + internet + telephone). The signal occupies a specific frequency range, not starting from 0 Hz. Used in: cable TV, DSL, WiFi (OFDM). Note: 'broadband' in common usage (high-speed internet) is different from its technical definition in transmission.