How to read optics names
The suffix usually tells you the reach and fibre type. A summary for standard Ethernet:
- SR (Short Reach): multimode, 70–400 m depending on speed and fibre grade.
- DR / FR: single-mode, 500 m and 2 km — common in new 100G/400G datacenters.
- LR (Long Reach): single-mode, 10 km.
- ER / ZR: single-mode, 40 and 80 km (more for coherent 400G ZR modules).
- BiDi: transmit and receive on one strand (or, for multimode variants, reuse of existing duplex LC cabling).
- DAC: direct-attach copper for in-rack links (1–5 m), cheapest and lowest power.
- AOC: active optical cable with fixed ends, up to about 100 m; lighter and more flexible than DAC.
Selection logic
candidates = modules where speed = selected
AND medium matches the fibre type
AND reach(fibre grade) ≥ distance
sort by reach ascending (least over-spec first), then DAC < RJ45 < AOC < SR < BiDi < DR < FR < LR < ER < ZR
The shortest-reach option that covers the distance is usually the cheapest and lowest-power one. For links close to the reach limit, compute the optical power budget including connectors and splices.
MPO or LC?
SR4, SR8 and DR4 modules use parallel fibres on MPO connectors and need MPO trunk cabling; LR4, FR4 and BiDi modules run on duplex LC and fit existing cabling more easily.