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 Cat 5 e for 1000 BASE-T, Cat 6/6 a for 10 GBASE-T (Cat 6 limited to about 55 m at 10 G). A common specified balanced-copper Ethernet channel model has a 100 m maximum including patch cords, with a 90 m permanent-link model; check the actual application and standard.
- 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 10 BASE-T; 4 B/5 B by 100 BASE-FX; 8 B/10 B by 1000 BASE-X; MLT-3 by 100 BASE-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).
- Simple parity detects any odd number of bit errors and can miss an even number. A suitable two-dimensional parity scheme can locate a single-bit error under its stated model.
- CRC uses modulo-2 division by a generator polynomial; the remainder is the FCS. For a suitable degree-r CRC generator, all burst errors of length at most r are detected under the standard error model; longer-burst detection depends on the polynomial and error pattern.
- 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; VCSEL lasers are commonly used with multimode fiber; select the specified transceiver, fiber class and supported reach together.
- Forgetting that 1000 BASE-T uses all four pairs and Cat 5 e, 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.