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MPO: Multiple Push On/Off
MTP is a proprietary refinement of MPO, and is fully compatible with MPO. All MTP connectors are MPO, but not all MPO connectors are MTP.
The “n” in “MPO-n” means that is the number of cores.
QSFP and MPO
QSFP transceivers require MPO-8 (8 cores, 4 pairs). They can be used with MPO-12, but the inner 4 strands will be unlit.
The MPO-8 interface uses eight of the 12 fiber positions of the industry standard MPO-12 (a QSFP transceiver does not use the center four fiber positions). Because leaving fibers unused is undesirable, cables and connectors can be constructed with MPO-12 connectors that omit or do not terminate the center four fibers (see figure 4). — Choosing the Right Multimode MPO System for Your Data Center | Data Center Frontier
12-fiber MTP®/MPO cables are the earliest developed and most commonly used solution in 10G-40G, 40G-100G connections. If they are used in 40G QSFP+ or 100G QSFP28 transceivers, 4 fibers will be idle, resulting in low fiber utilization. — What Are MTP®/MPO Cables and How to Choose Them? | FS Community
It appears there is no current or upcoming version of ethernet (up to 800gbe) that requires more than 8 strands.
Polarity
Type A (Straight Through) - Side A = Key up, Side B = Key down
Type B (Cross) - Side A = Key up, Side B = Key up
Type C (Cross Pair / Interleave) - Side A = Key up, Side B = Key down
Universal Polarity
There is a new standard (as of 2023) called Universal Polarity, and uses polarities named “U1” (Same as Type A, Key up and Key down) and “U2” (Same as Type B, Key up and Key up). Universal Polarity Explained: Basis, Benefits & FAQs | FS Community
Gender, Male vs Female
Both connectors have flat, polished fibre surfaces. The gender refers to the locating pins that are present on Male but absent on Female.
This means that direct coupling with passive couplers requires one of the cables to be Male and one to be Female, unlike with LC-to-LC couplers. (MPO MTP Male and Female Connector Differences - ServeTheHome)
You can change the gender with a housing removal tool (not cheap!) and swap put the pins or a channels (vary on the cable manufacturer).
Connector to Cable compatibility
A connector expecting a 2/4/8 core connection will work with an MPO-12 cable. This is because, in Single Row MPO (2-12 strands), the pairs start from the outside in.
However, Dual Row MPO (MPO-24) is NOT compatible with Single Row MPO, since the rows are offset equally from the centre.
