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Parallel optics eat fibre. Enter how many ports of what type you are lighting and how you want to land them, and this works out the fibre count, the number of trunks, and the cassettes or harnesses at each end — including how much of a base-12 trunk you would strand on base-8 optics.
Why fibre count is the whole design. A duplex 100G-LR4 port needs two fibres. A parallel 100G-SR4 port needs eight, and an 800G-SR8 port needs sixteen. Move a row of 32 ports from duplex to parallel and your fibre requirement goes from 64 to 512. That is the number that decides whether the trunk you already own is enough.
Base-8 versus base-12. Most legacy MPO plant is base-12, because that is what 40G-SR4 and 100G-SR4 were built around when the connector was 12-fibre. Modern parallel optics use eight fibres, which does not divide into twelve — so a base-12 trunk strands a third of itself. Base-8 trunks fix it for new builds; a conversion module fixes it for plant you already have.
Cassette, harness or direct. Direct MPO panel-to-panel is the lowest loss and the cheapest, but the front of the panel is MPO — fine if the equipment is parallel too. A harness fans out to duplex with one fewer mated pair than a cassette. A cassette costs about 0.35 dB per end but keeps the trunk connector out of daily handling and lets you re-patch freely.
Three, using 24 of the 24 fibres — 24 divides evenly by 8, so nothing is stranded. On a 12F trunk you get one port and strand four fibres. This is the single strongest argument for base-8 or base-24 trunk plant in a parallel-optics build.
Twenty per cent is a common starting point, but think in whole trunks rather than percentages — the difference between four and five trunks is one pull today versus a change window and a contractor later. Fibre is cheap; access to the duct is not.
Single-mode parallel optics — DR4, DR8, PSM4 — are APC. Multimode SR4 at 40G and 100G is conventionally UPC, but do not treat that as a rule at 400G and above: several widely deployed twin-port 400G and 800G multimode SR modules specify MPO-12 APC. Read the transceiver datasheet before you order trunk — APC and UPC must never be mated, so the polish is a decision for the whole channel.
Pick a standard and get the wavelength plan, fibre type, connector, reach and planning loss budget
Open tool →Total channel loss, power budget and pass/fail margin for single-mode and multimode links
Open tool →ITU channel number ⇄ frequency ⇄ wavelength, for 100/50/25/12.5 GHz DWDM and the 20 nm CWDM grid
Open tool →Transceivers, MPO assemblies, WDM and patch cords — built and tested to the figures you just calculated.