MPO polarity explained: Methods A, B and C
How Type A, B and C MPO trunks and A-to-B or A-to-A patch cords keep transmit connected to receive, which method suits parallel optics, and how to state polarity in an order.
Why polarity matters
A duplex link has two fibers: one carries transmit (Tx) one way, the other carries it back. Somewhere along the channel the fibers must cross so that Tx at one end lands on receive (Rx) at the other. With 12- or 24-fiber MPO trunks there are many fibers to keep in order, so TIA-568 standardises three ways to do it.
The three methods
| Method | Trunk type | Patch cords | Where the flip happens |
|---|---|---|---|
| A | Type A: key up to key down, straight through | A-to-B at one end, A-to-A at the other | In one patch cord |
| B | Type B: key up to key up, fiber 1 to fiber 12 | A-to-B at both ends | In the trunk |
| C | Type C: each pair flipped | A-to-B at both ends | In the trunk, pair by pair |
Which method for parallel optics
Parallel optics such as 40GBASE-SR4, 100GBASE-SR4 and 400GBASE-SR8 send on several fibers and receive on others within one MPO connector. Method B maps these directly and is the usual choice. Method A also works with the right cords. Method C suits duplex links but is awkward for parallel optics.
Pinned or unpinned
MPO connectors align with two guide pins. One side of each mated pair must be pinned (male) and the other unpinned (female). Trunks are usually unpinned at both ends because the cassettes and transceivers they plug into are pinned.
Ordering
Write the method, trunk type, Base-8 or Base-12, fiber type, pinned or unpinned ends and lengths in your BOM. We build, test and label every assembly to it. See MPO cabling.