Breakout & MPO cabling: polarity, Base-8/12/16, pinning

Parallel optics moved the hard part of a link from the module to the cable. A 400G DR4 port needs eight fibres arriving in the right order, with the right polish and the right gender at the connector; a 4 × 100G breakout needs the far ends to be four separate ports. Most "the link is dead but the module is fine" cases on parallel optics are cabling — this page lays out the rules.

Breakout: which ports can split

Parent port	Breakout options	Requires
40G QSFP+ (SR4, PSM4, CR4)	4 × 10G	parallel optics or breakout DAC/AOC; host support
100G QSFP28 (SR4, PSM4, CR4)	4 × 25G, 2 × 50G	as above; FEC per lane consistent
100G QSFP28 (LR4, CWDM4)	none	WDM optics cannot split
200G QSFP56 (SR4, DR4, CR4)	4 × 50G, 2 × 100G	—
400G QSFP-DD / OSFP (DR4, SR8, CR8)	4 × 100G (DR4 → 4 × 100G DR), 8 × 50G, 2 × 200G	CMIS applications listing the breakout; host breakout config
400G FR4 / LR4	none	WDM
800G OSFP (DR8, 2×DR4, 2×FR4)	2 × 400G, 8 × 100G, 4 × 200G	twin-port modules present as two logical ports

Rules: the parent must be parallel (one fibre pair per lane); the host must support breakout on that port (often only on some ports or with neighbouring ports disabled); speed and FEC per child must match the far end; for CMIS modules the breakout must be an advertised application (Speed & rate, CMIS). A DR4 broken out to four DR modules works; broken out to four FR1 modules does not (different wavelength/reach class).

MPO connector basics

Attribute	Options	Rule
Fibre count	MPO-12 (8 or 12 used), MPO-16, MPO-24	SR4/DR4/PSM4 use 8 of 12; SR8/DR8 use MPO-16 (or 2 × MPO-12)
Polish	UPC (MMF and some SMF), APC (all SM parallel: PSM4, DR4, DR8)	never mate APC to UPC
Gender	pinned (male) / unpinned (female)	transceivers are pinned → the patch cord into a module is unpinned; trunks and cassette rear ports are pinned; every mating has exactly one pinned side
Key	key-up / key-down	defines fibre position numbering; polarity methods depend on it
Base	Base-8, Base-12, Base-16	fibre grouping of trunks and cassettes; must match the optics' lane count

Connector details and polish colours: Connectors & fibre.

Polarity methods (TIA-568)

The goal is simple — Tx of lane n must arrive at Rx of lane n — but a trunk, two cassettes or adapters and two patch cords can each flip or not flip the order.

Method	Trunk	Patch cords	Cassettes / adapters	Notes
A	straight (key-up to key-down), fibre 1 → 1	one straight (A-to-B), one flipped (A-to-A) at one end	key-up/key-down adapters	duplex flip done in one patch; easy to get wrong when both patches are the same type
B	key-up to key-up, fibre 1 → 12 (pairwise reversed)	both straight	key-up/key-up	most common for parallel optics (SR4/DR4 direct MPO-to-MPO); every component identical
C	pairwise flipped (1↔2, 3↔4 …)	both straight	—	duplex breakout only; unusable for parallel 8-fibre lanes

For parallel optics (MPO to MPO, module to module) use Method B end to end: trunk Type B, patch cords Type B. Mixing a Type A patch into a Type B channel lands every lane on the wrong fibre — all four lanes dark, both directions (Physical mismatches).

Base-8 vs Base-12 vs Base-16

Base-12 — the legacy MPO-12 trunk with all 12 fibres used, meant for duplex breakout (6 pairs). SR4/DR4 use only 8 → 4 fibres wasted per trunk.
Base-8 — MPO-12 connectors, 8 fibres populated, trunks in multiples of 8; maps 1:1 onto 40G/100G/400G 4-lane optics. Conversion modules or harnesses connect Base-12 trunks to Base-8 optics.
Base-16 — MPO-16 for SR8/DR8/800G; APC for single-mode. Not compatible with MPO-12 without a harness.

Structured cabling patterns

Pattern	Components	Typical use
Direct MPO-to-MPO	one Type B trunk or patch	in-row parallel links, spine cabling
Trunk + cassettes	MPO trunk, MPO-to-LC cassettes both ends	duplex optics over a parallel backbone
Trunk + breakout harness	MPO trunk, MPO-to-4×LC-duplex harness	4 × 25G from a 100G SR4 port to four SFP28 ports
Conversion cassette	Base-12 trunk → Base-8 MPO ports	reusing 12-fibre plant for 8-fibre optics
Breakout DAC/AOC	one QSFP end, four SFP ends	in-rack breakout without fibre plant

Configuration and verification

Configure breakout on the parent port (NOS-specific; often requires a reload or disables adjacent ports) and set child speed/FEC to match the far ends.
Check the module advertises the mode (QSFP28: parallel PMD codes; CMIS: application list) — Check transceiver.
Inspect and clean every MPO end-face (APC scopes for APC).
Verify per-lane Rx power on both ends — a lane at −40 dBm with others fine is a polarity or dead-lane problem (Per-lane diagnostics).
Trace polarity with a visual fault locator through the channel if lanes are swapped.

Common failures

Symptom	Cause
All lanes dark both ways	Type A patch in a Type B channel; pinned-to-pinned mismatch
Lanes 1↔4 swapped	Method C component or mislabelled trunk
One lane dark	broken fibre, contaminated fibre in the array, one dead laser
Works on 8 fibres, fails on 16	MPO-12 cable on MPO-16 optics
High loss on all lanes	APC/UPC mismatch, dirty MPO, Base-12 ferrule with unused fibres against Base-8 optics
Only some child ports link	breakout not enabled for all lanes, FEC mismatch on one child

In CodingBox

CodingBox reads the module side of the equation: connector code (0Ch MPO-12, 28h MPO-16), lane count, breakout-capable applications and per-lane DDM on the bench. If all four lanes light on the bench and the link is dark in the rack, the cable is the suspect.