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The module is only half of a link; the other half is the glass and the connector that mates to it. Mismatched fibre or polish costs decibels silently, and a dirty ferrule is the single most common cause of a "bad module". This page is the field reference.

Fibre types

Single-mode subtypes: G.652.D (standard, low water peak — the default), G.657 (bend-insensitive, for tight indoor routing), G.655 (non-zero dispersion-shifted, legacy DWDM plant), G.654 (low-loss, submarine/long-haul).

Rules: never mix 50 and 62.5 µm multi-mode on one link (up to 4 dB loss at the joint); never plug a multi-mode module into single-mode fibre or vice versa — the link may come up marginally and fail later. Multi-mode reach is set by modal bandwidth, not attenuation; single-mode reach by attenuation and dispersion (Wavelength bands).

Connectors

Polish: UPC vs APC

Never mate UPC to APC — the angled and flat faces cannot touch; expect several dB of loss and possible end-face damage. High-speed single-mode MPO (DR4/DR8) is APC, and InfiniBand NDR insists on it (InfiniBand optics).

MPO essentials

• Pinned vs unpinned — a pinned (male) connector mates only with an unpinned (female); transceivers are usually pinned, so patch cords are female–female.
• Fibre count — MPO-12 for SR4/DR4 (8 fibres used), MPO-16 for SR8/DR8, MPO-24 for legacy SR10.
• Polarity — Type A (straight), B (reversed), C (pair-flipped) cabling; a wrong polarity mix gives Tx-to-Tx and no link. Standardise on one method per site.
• Key up / key down and APC angle orientation matter — follow the labels.

Cleaning and inspection

One 2–5 µm particle adds 0.5–2 dB; a clean LC pair costs 0.1–0.2 dB. Inspect → clean → re-inspect, dry cleaning after wet; one-click cleaners for LC/SC, dedicated MPO cleaners for arrays. Details and the diagnostic ladder: Link flapping, CRC & dirty connectors.

Cables with fixed ends

DAC (copper) and AOC (active optical) cables carry module ends permanently attached — no connector to clean, but nothing to replace except the whole cable (DAC vs AOC, Copper & DAC).

What the module memory says

The connector type is a byte in the identity block (SFF-8472 A0h byte 2, SFF-8636 byte 130), coded per SFF-8024: 07h = LC, 01h = SC, 0Ch = MPO 1×12, 0Dh = MPO 2×16, 21h/22h = copper pigtail/RJ45, 23h = no separable connector (AOC/DAC). Length fields (bytes 14–19) declare the fibre types the module is rated for. CodingBox decodes both on the Check transceiver screen; see the Memory map quick reference.

What goes wrong when fibre, polish or polarity do not match — and how it shows up — is in Physical-layer mismatches.

How MPO connectors are used in structured cabling — polarity methods, trunks and cassettes, Base-8 vs Base-12: Breakout & MPO cabling.

Full reach matrix per PMD × OM1–OM5/OS2, plus FC, InfiniBand, PON and cable lengths: Reach tables; latch, jacket and boot colour codes in one place: Labels & colour codes.

The physics behind these grades — modes, numerical aperture, mode field diameter, bend loss: How light propagates in fibre; the rest of the passive plant these connectors live in — cables, panels, pigtails, splices, splitters, filters: Passive plant.

The standards behind the fibre column — ITU-T G.652–G.657, IEC multi-mode codes, specialty fibres and dissimilar-fibre joints: Fibre types; eye safety around live connectors and the handling rules for modules and cords: Safety and handling.

Endface grading against IEC 61300-3-35 — zones A–D, allowed scratches and defects per connector type, contamination types and cleaning methods including MPO and transceiver receptacles: Endface inspection & cleaning.