Network Design: Topologies, Budgets, and Architecture
Topology and architecture
Design starts with requirements: distance, bandwidth, growth, and reliability. Know point-to-point, star, ring, and mesh topologies; in fiber networks, common architectures include home-run (centralized), collapsed backbone, and passive optical LAN. Distinguish active Ethernet (point-to-point switches) from PON (point-to-multipoint with splitters). Redundancy options include dual homing, ring protection, and diverse paths.
Link budget and loss
Every design needs a power budget: transmitter output minus receiver sensitivity gives available loss; subtract connector, splice, splitter, and fiber losses plus margin. Splitter loss is logarithmic: a 1:2 splitter is about 3.5 dB, 1:4 about 7 dB, 1:8 about 10.5 dB, 1:16 about 14 dB, 1:32 about 17.5 dB (including excess loss). Remember that splitters are bidirectional and that PON budgets are class-based (e.g., Class B+).
Media and optics selection
Match fiber type to optics: multimode OM3/OM4 with short-reach transceivers; single-mode for long reach and WDM. Consider wavelength planning (CWDM/DWDM), connector cleanliness, and future capacity. Design for test access, labeling, and documentation.
Traps and worked example
Traps: forgetting splitter loss; using worst-case connector loss inconsistently; ignoring dispersion and reach limits; and assuming a budget passes without margin. Worked example: a PON with 20 dB budget, 10 km fiber at 0.35 dB/km (3.5 dB), two connectors at 0.5 dB each (1.0 dB), two splices at 0.1 dB (0.2 dB), and a 1:16 splitter at 14 dB totals 18.7 dB, leaving only 1.3 dB margin, which is tight; consider a 1:8 splitter or shorter reach.