The short answer
MPO cabling was standardised around the twelve-fibre ferrule, so almost all installed trunk plant is organised in groups of twelve. Modern parallel optics mostly use eight fibres — four transmit and four receive for a DR4 or SR4 — or sixteen for an SR8 or DR8.
Eight does not divide into twelve. Connect an eight-fibre transceiver to a twelve-fibre trunk and four fibres in every twelve do nothing at all. You installed a third more fibre than you can use, and on a large plant that is a very expensive rounding error.
What Is Base-12?
Trunks, cassettes and harnesses organised in groups of twelve, matching the standard MPO-12 ferrule. This is what almost all existing plant is, and what most catalogue parts still default to.
It works perfectly for twelve-fibre applications and for duplex breakout — a 12F trunk feeding six LC duplex ports uses every fibre. It only becomes wasteful when the far end is an eight-lane parallel optic.
What Is Base-8?
Trunks organised in groups of eight, matching eight-fibre parallel optics exactly. Nothing is stranded: one 8F group serves one SR4, PSM4 or DR4 port completely.
For a new build committed to parallel optics, this is simply the correct arrangement. The parts cost much the same; the difference is that you buy and install a third less fibre for the same number of live ports.
What Is Base-16?
Trunks organised in sixteens to match the MPO-16 ferrule used by 400GBASE-SR8, 800G SR8 and DR8.
Sixteen divides by eight, so a base-16 plant also serves eight-fibre optics without waste. It is the forward-looking choice where 800G is on the roadmap, and it is why MPO-16 assemblies have moved from exotic to mainstream in a short time.
Base-8 vs Base-12 vs Base-16
The utilisation row is the one to read. Everything else is a consequence of it.
| Base-12 | Base-8 | Base-16 | |
|---|---|---|---|
| Group size | 8 fibres | 12 fibres | 16 fibres |
| Matching ferrule | MPO-12, 8 positions used | MPO-12, all 12 used | MPO-16 |
| Serves SR4 / DR4 / PSM4 | 100% utilisation | 67% — 4 of 12 stranded | 100% |
| Serves SR8 / DR8 | Two groups per port | Poor fit | 100% utilisation |
| Serves duplex breakout | 4 duplex per group | 6 duplex per group | 8 duplex per group |
| Installed base | Growing | Very large | Growing fast with 800G |
| Fibre installed per live 8F port | 8 | 12 | 8 |
| Availability | Standard | Standard | Standard |
For plant that already exists, there is a third option that is often better than either replacing it or living with the waste: a conversion module. It re-groups two twelve-fibre trunks into three eight-fibre connections, recovering full utilisation for the cost of one extra mated pair — about 0.35 dB. Against stranding a third of an installed trunk, that trade is almost always worth making.
Quantify it before you argue about it. Put your port count and trunk type into the MPO cabling planner and it will tell you exactly how many fibres go unused and how many trunks you actually need.
Which One Should You Choose?
New build with parallel optics: base-8 or base-16
If the ports you are lighting are SR4, DR4 or PSM4, build in eights. If they are SR8 or DR8, build in sixteens — and note that sixteen also serves eight-fibre optics cleanly, so base-16 is the more future-proof of the two. Either way you install a third less fibre than a base-12 plant would need for the same live port count.
New build with duplex breakout: base-12 is fine
If the trunk lands on cassettes that fan out to duplex LC, twelve divides by two perfectly and nothing is wasted. The base-8 argument only applies when the far end is an eight-lane parallel optic. Do not re-architect a duplex plant for a problem it does not have.
Existing base-12 plant meeting 8-fibre optics: conversion modules
Do not pull new cable. A conversion module re-groups two 12F trunks into three 8F connections and recovers the whole stranded third for one extra mated pair of loss. On any plant of meaningful size this is dramatically cheaper than re-cabling, and it can be done rack by rack during normal change windows.
Mixed requirements: standardise the trunk, vary the ends
The trunk is the expensive, long-lived part; cassettes and harnesses are cheap and swappable. Pick one trunk arrangement for the site and adapt at the ends. A site with three trunk conventions is a site where nobody can safely plan a change.
Frequently Asked Questions
How many 100G-SR4 ports fit on a 24-fibre trunk?
Three, using all 24 fibres, because 24 divides evenly by 8. On a 12-fibre trunk you get one port and strand four fibres. This single fact is the strongest argument for base-8 or base-24 plant in a parallel-optics build.
Is base-8 more expensive than base-12?
Per metre, marginally — you are buying a slightly less standard part. Per live port, it is cheaper, because you install two-thirds of the fibre for the same number of working links. Compare on cost per live port, not on cost per trunk.
Can I mix base-8 and base-12 in the same data centre?
You can, and many sites end up doing so, but label ruthlessly. A base-8 harness on a base-12 trunk produces a link that comes up partially or not at all, and diagnosing it from the switch is unpleasant. If you must mix, keep the two plants in physically separate panels.
What about base-16 for 400G and 800G?
400GBASE-SR8 and 800G SR8/DR8 use sixteen fibres, so base-16 matches them exactly. Because sixteen also divides by eight, a base-16 plant serves DR4 and SR4 without stranding anything. For a build with 800G anywhere on the roadmap, base-16 is the safest single choice.
Does polarity change with the base?
The polarity scheme is independent of the group size, but you still have to pick one and apply it consistently through trunks, adapters and cassettes. See MPO polarity.
The Parts We Build
We build trunks, harnesses, cassettes and conversion modules in base-8, base-12 and base-16, to your polarity and polish.
