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Telecom Professional Qualification study materials
Telecom Professional Qualification study resources

Telecom Professional Qualification: Optical and Access Networks (FTTx, xDSL, PON)

Telecom Professional Qualification · Work through your exam, one topic at a time.

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Optical and Access Networks (FTTx, xDSL, PON)

This domain covers the physical last mile: fibre characteristics and WDM, PON architecture and power budgeting, and copper access evolution from ADSL to G.fast. Power-budget arithmetic and wavelength assignment are the most common calculation items.

Fibre and WDM

  • Standard single-mode fibre (ITU-T G.652D) has zero-dispersion near 1310 nm and minimum loss near 1550 nm (about 0.2 dB/km, versus roughly 0.35 dB/km at 1310 nm). G.657 fibre is bend-insensitive for in-building drops.
  • EDFA amplifies the C-band (about 1530–1565 nm). DWDM channels follow the ITU-T G.694.1 grid anchored at 193.1 THz with 100 or 50 GHz spacing. Reach is limited by OSNR, chromatic and polarisation-mode dispersion, and non-linear effects.

PON

  • Architecture: OLT in the exchange - ODN with splitters and fibre - ONT at the premises. GPON runs 2.488 Gbit/s downstream and 1.244 Gbit/s upstream, with 1490 nm downstream, 1310 nm upstream and 1550 nm reserved for RF video overlay; typical reach 20 km and split 1:32 or 1:64.
  • XG-PON is 10/2.5 Gbit/s, XGS-PON is symmetric 10/10, NG-PON2 uses TWDM wavelengths.
  • Downstream is a shared broadcast encrypted per ONT; upstream uses TDMA granted by the OLT.
  • Power budget classes: class B+ about 28 dB, class C+ about 32 dB.

xDSL and FTTx

  • ADSL2+ reaches up to 24 Mbit/s downstream on short loops, using DMT across many subcarriers; VDSL2 profile 17a reaches roughly 100 Mbit/s; vectoring cancels far-end crosstalk; G.fast pushes higher rates over very short copper.
  • FTTC (cabinet, VDSL/G.fast), FTTB (building, copper or LAN in-building), FTTH (fibre to the home).

Worked example

A 1:32 splitter has an ideal loss of 10 log₁₀(32) = 15.05 dB, but real parts add excess loss, often 16.5–17 dB. Take 17 dB, plus 10 km × 0.35 dB/km = 3.5 dB, plus two connectors at 0.5 dB = 1 dB and splices at 0.5 dB: total ≈ 22 dB. Against a class B+ budget of 28 dB this leaves about 6 dB for ageing, repairs and temperature - acceptable but not generous, so a longer reach or higher split would need class C+.

Traps that cost marks

  • Confusing downstream (1490 nm) with upstream (1310 nm) wavelengths.
  • Forgetting that splitter loss rises about 3 dB for every doubling of the split ratio.
  • Quoting GPON downstream as 2.5 Gbit/s instead of 2.488 Gbit/s.
  • Treating xDSL rate as fixed rather than loop-length and crosstalk dependent.