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Ruankao Network Engineer (Intermediate) study materials
Ruankao Network Engineer (Intermediate) study resources

Ruankao Network Engineer (Intermediate): Physical Layer and Data Communication

Ruankao Network Engineer (Intermediate) · Work through your exam, one topic at a time.

All guide sections

Physical Layer and Data Communication

This domain covers what actually carries bits: media, line coding, modulation, multiplexing, error detection and the dumb devices that regenerate signals. Questions are typically one-line discriminators plus a Hamming or CRC calculation.

Transmission media

  • Twisted pair: UTP Cat5e for 1000BASE-T, Cat6/6a for 10GBASE-T (Cat6 limited to about 55 m at 10G). Channel length limit 100 m for horizontal copper.
  • Coaxial: legacy bus Ethernet and cable TV/HFC access.
  • Multimode fibre: LED or VCSEL sources, 850/1300 nm, short reach, large core - used inside buildings and data centres.
  • Single-mode fibre: laser sources, 1310/1550 nm, small core, kilometres of reach - used for campus and carrier backbones.

Encoding and modulation

  • Manchester encoding is self-clocking and used by 10BASE-T; 4B/5B by 100BASE-FX; 8B/10B by 1000BASE-X; MLT-3 by 100BASE-TX.
  • Basic modulation: ASK (amplitude), FSK (frequency), PSK (phase), QAM (combined amplitude and phase). QAM-16 carries 4 bits per symbol, QAM-64 carries 6.

Multiplexing and error detection

  • FDM (frequency), TDM/STDM (time / statistical time), WDM and DWDM (wavelength), CDMA (code).
  • Parity detects single-bit errors only; the two-dimensional variant can correct.
  • CRC uses modulo-2 division by a generator polynomial; the remainder is the FCS. It detects bursts shorter than the polynomial degree.
  • Hamming code: r check bits for m data bits require 2^r ≥ m + r + 1. Check bits sit at positions 1, 2, 4, 8…

Worked example

Four data bits need a Hamming code. Try r = 2: 2^2 = 4 < 4 + 2 + 1 = 7, fails. Try r = 3: 2^3 = 8 ≥ 4 + 3 + 1 = 8, satisfied. So a 7-bit codeword results, and the receiver can correct any single-bit error.

Physical-layer equipment

Repeaters and hubs regenerate and amplify but do not filter; a hub is one collision domain. Modems perform digital-to-analogue conversion; media converters bridge copper and fibre.

Traps that cost marks

  • Mixing multimode and single-mode light sources; lasers do not pair with multimode in exam answers.
  • Forgetting that 1000BASE-T uses all four pairs and Cat5e, with a 100 m limit.
  • Using the wrong Hamming inequality or placing check bits at 3, 5, 6 instead of powers of two.
  • Confusing baud rate with bit rate when a symbol carries more than one bit.