Fibre Guide
Type A, B and C trunks. Methods A, B and C. Key-up, key-down, A-to-B cords. Here is MPO/MTP polarity in plain English — and how to stop it breaking your 100G and 400G links.
High-density MPO/MTP connectivity in a data centre rack.
Why polarity matters: every optical link must connect a transmitter at one end to a receiver at the other. With duplex LC links that is one fibre pair. With MPO connectors carrying 8, 12 or 24 fibres, the positions must line up across trunks, cassettes and patch cords — and the polarity type of each component decides whether they do.
An MPO connector has a key on one side of its body. Fibre positions are numbered 1 to 12 (for a 12-fibre connector) across the end-face. The three trunk types defined in TIA-568 differ in how positions map from one end to the other:
| Type | Connector keys | Position mapping |
|---|---|---|
| Type A — straight | Key up to key down | 1→1, 2→2 … 12→12 |
| Type B — reversed | Key up to key up | 1→12, 2→11 … 12→1 |
| Type C — pair-flipped | Key up to key down | 1→2, 2→1, 3→4, 4→3 … |
Type A keeps positions straight through; Type B reverses them end to end.
A complete channel is more than a trunk. TIA-568 defines connectivity methods that combine a trunk type with matching adapters, cassettes and patch cords so that, end to end, transmit always meets receive.
Uses a Type A straight trunk with key-up to key-down adapters. Because positions are not flipped in the trunk, the flip has to happen in the patch cords: a standard A-to-B duplex cord at one end and an A-to-A cord at the other. It is simple to install, but the two ends need different patch cords — a common source of mistakes.
Uses a Type B reversed trunk with key-up to key-up adapters. The flip happens in the trunk, so standard A-to-B patch cords can be used at both ends. Method B is widely used for parallel optics such as 40G/100G SR4 and 400G DR4, because the position reversal lines up transmit and receive lanes directly.
Uses a Type C pair-flipped trunk. Each fibre pair is crossed inside the trunk, so standard A-to-B duplex cords work at both ends for duplex applications. It is less suited to parallel optics, where lanes span the whole connector rather than adjacent pairs.
For duplex applications (10G, 25G, 100G over LC pairs), MPO trunks feed cassettes that break out to LC ports — any method works if every component matches. For parallel optics, the transceiver itself has an MPO port and uses several fibres in each direction. Here the trunk and MPO patch cords must deliver the transmit lanes to the receive lanes exactly, which is why choosing the method — and sticking to it across the site — is so important.
MPO connectors are either pinned (male, with two guide pins) or unpinned (female). A mated pair must be one of each. Trunks are typically pinned where they plug into cassettes or adapters, and transceivers usually have pinned ports, so patch cords to optics are normally unpinned. Mating two pinned connectors damages the guide pins; mating two unpinned ones gives a misaligned, high-loss joint.
FAQ
Type A trunks keep fibre positions straight through (1 to 1) with key-up to key-down ends. Type B trunks reverse positions (1 to 12) with key-up to key-up ends.
Parallel-optic 400G links are commonly built with Method B because the reversal in the trunk lines up transmit and receive lanes directly. The right choice depends on the components already installed on site.
It is possible but risky. Mixing methods without clear labelling is a common cause of links that will not come up. Standardise on one method where you can.
With an MPO tester that maps every fibre position from end to end and measures loss per lane, after inspecting and cleaning every connector.
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