MPO polarity is the fibre mapping that sends each transmitter to a receiver at the far end of a link. TIA-568 defines three MPO cable types: Type A (straight through, key-up to key-down), Type B (reversed, key-up to key-up) and Type C (pairs flipped, key-up to key-down). Methods A, B and C each combine one trunk type with specific adapters and patch cords.
Why MPO polarity matters
A fibre link has two directions. The transmit fibre at one end must arrive at the receive port at the other end. With duplex LC cabling this is handled by a crossover (A-to-B) patch cord. With MPO cabling there are 8 to 24 fibres in one connector, connectors can be mated key-up or key-down, and trunks can be built straight or reversed, so a polarity error is easy to make and is often found only when the link fails to come up.
TIA-568 (the fibre part is TIA-568.3; the 2016 edition is -D, which TIA-568.3-E replaced in 2022) defines a small set of cable types and connectivity methods so that a designer, an installer and a cable maker all work from the same map. The practical rule is simple: choose one method for the site, write it on the drawings, and order every trunk, cassette and patch cord to that method.
Key-up, key-down and the two adapter types
Fibre positions on an MPO connector are numbered 1 to 12 (or 1 to 16, or 1 to 24) across the ferrule. Viewed from the end face with the key up, position 1 is on the left. Through an adapter, the key orientation of the two plugs decides which positions meet.
- Type A adapter, key-up to key-down. One plug is turned over relative to the other, so position 1 meets position 1 and position 12 meets position 12.
- Type B adapter, key-up to key-up. Both plugs have their keys on the same side. Facing each other, their left and right swap, so position 1 meets position 12.
Angle-polished (APC) connectors can only be joined key-up to key-down. Their angled end faces lie flat against each other only when one connector is rotated 180 degrees relative to the other. Key-up to key-up adapters are therefore used with flat-polished multimode connectors, and single-mode APC channels are built on key-up to key-down adapters.
The three MPO cable types
TIA-568 names three MPO cable types by the way fibres are routed from one end to the other. The same names apply to trunks and to MPO patch cords.
| Type | Key orientation, end to end | Fibre map (12-fibre example) | In short |
|---|---|---|---|
| Type A | Key-up to key-down | 1 to 1, 2 to 2, through 12 to 12 | Straight through |
| Type B | Key-up to key-up | 1 to 12, 2 to 11, through 12 to 1 | Fully reversed |
| Type C | Key-up to key-down | 1 to 2, 2 to 1, 3 to 4, 4 to 3, through 12 to 11 | Adjacent pairs flipped |
A Type B MPO cable is the one most often asked for by name, because it is the cord that connects two parallel optic transceivers directly. Type C is used only in duplex designs, because flipping adjacent pairs does not line up the transmit and receive groups of a parallel transceiver.
Connectivity methods for duplex links
For duplex applications (10G, 25G, or any LC-based port), the MPO trunk usually ends in cassettes or modules that break out to LC duplex adapters on the front. Each method then specifies the trunk type and the duplex patch cords at each end.
| Method | Trunk | MPO adapters | Duplex cord at end 1 | Duplex cord at end 2 |
|---|---|---|---|---|
| Method A | Type A | Key-up to key-down | A-to-B (crossover) | A-to-A (straight) |
| Method B | Type B | Key-up to key-up | A-to-B | A-to-B |
| Method C | Type C | Key-up to key-down | A-to-B | A-to-B |
Method A is simple to build but needs two different patch cords, and the A-to-A cord at one end is easy to swap by mistake. Method B and Method C use standard A-to-B cords at both ends, which suits sites where patching is done by many people. Method B relies on key-up to key-up mating, so it is not used for single-mode APC links.
Polarity for parallel optics
Parallel transceivers such as 40GBASE-SR4, 100GBASE-SR4 and 400GBASE-DR4 transmit on positions 1 to 4 and receive on positions 9 to 12 of an MPO-12 port. End to end, the link must map position 1 to position 12, position 2 to position 11, and so on. That is a single reversal.
- Direct link: one Type B MPO patch cord between the two transceivers, with unpinned connectors at both ends.
- Method A channel: a Type A trunk, key-up to key-down adapters, a Type A MPO patch cord at one end and a Type B MPO patch cord at the other.
- Method B channel: a Type B trunk, key-up to key-up adapters and Type B MPO patch cords at both ends.
Eight-lane optics on MPO-16 follow the same principle of a single end-to-end reversal. If you are planning for 400G and 800G, our guide to parallel optics cabling covers MPO-16 and base-8 designs.
How to order the right polarity
- Pick one method for the site and record it on every drawing.
- State the polarity type on every MPO trunk and patch cord line in the order, along with gender at each end.
- For duplex cords, state A-to-B or A-to-A.
- Ask for a polarity check in the factory test report, and for the polarity type on the label at each end.
- Keep a few spare cords of each type on site, labelled clearly.
Radix builds MTP/MPO trunk cables and MTP/MPO patch cords in Type A, B and C to the customer's drawing. If you are unsure which method fits your equipment, list the transceivers or panels at each end in your quote request and ask for the fibre map to be confirmed on the drawing before production.
Questions buyers ask
What is a Type B MPO cable?
A Type B MPO cable has key-up connectors at both ends and reverses the fibre order, so position 1 at one end goes to position 12 at the other. It is the standard cord for connecting two parallel optic transceivers, such as 100GBASE-SR4 modules, directly.
Which MPO polarity do I need for 40G or 100G SR4?
The end-to-end channel must reverse the fibre order once, so transmit positions 1 to 4 reach receive positions 9 to 12. A single Type B cord does this directly. Through a trunk, follow Method A or Method B exactly as TIA-568 lists the components.
Can Type A, B and C cables be told apart by eye?
Only partly. A Type B cable has both keys on the same side when the ends are laid side by side, while Type A and Type C have opposite keys. Type A and Type C look identical outside, so the label is the only reliable way to tell them apart.
What is an A-to-B patch cord?
An A-to-B duplex patch cord is a crossover cord: the fibre in the A position at one end goes to the B position at the other. It is the standard duplex patch cord, and Methods B and C use it at both ends of a link.
Can I fix a polarity mistake on site?
Often yes, by changing a patch cord: a Type A cord for a Type B, or an A-to-A duplex cord for an A-to-B. That adds a non-standard part to the link, so record the change on the drawings. A wrong trunk type is better corrected by replacing the trunk.
Why can't APC MPO connectors use key-up to key-up adapters?
The end face of an APC connector is polished at 8 degrees relative to the key. Two such faces sit flat against each other only when one connector is turned 180 degrees, which is key-up to key-down. Key-up to key-up would leave the angled faces apart.