Connectors & fibre types

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

Type	Core / cladding	Jacket colour	Bandwidth (850 nm)	Typical reach	Used with
OM1	62.5 / 125 µm	orange	200 MHz·km	1G 275 m; 10G 33 m	legacy campus
OM2	50 / 125 µm	orange	500 MHz·km	1G 550 m; 10G 82 m	legacy
OM3	50 / 125 µm	aqua	2 000 MHz·km	10G 300 m; 40/100G SR4 100 m	data centre
OM4	50 / 125 µm	aqua / violet (Erika)	4 700 MHz·km	10G 400 m; 100G SR4 100–150 m	data centre standard
OM5	50 / 125 µm	lime green	wideband 850–953 nm	as OM4 + SWDM4	multi-wavelength MMF
OS1 / OS2	9 / 125 µm	yellow	—	10 km … 80 km+	all single-mode optics

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

Connector	Ferrule	Where
LC	1.25 mm, duplex or simplex	the standard on SFP/SFP+/SFP28, QSFP LR4/FR4, 400G FR4
SC	2.5 mm, push-pull	GBIC, PON (SC/APC), patch panels, older plant
FC	2.5 mm, screw-on	test equipment, legacy telecom
ST	2.5 mm, bayonet	legacy multi-mode
E2000	2.5 mm with shutter	European telecom, APC
MPO / MTP	12/16/24-fibre array	SR4/SR8/DR4/DR8/PSM4 parallel optics; MTP is a premium MPO variant
SN / CS	dual 1.25 mm, very small	400G/800G breakout (2×LC in one QSFP-DD faceplate)
MDC	dual 1.25 mm, very small	high-density 400G/800G
MU	1.25 mm	legacy telecom

Polish: UPC vs APC

Polish	End-face	Boot colour	Return loss	Use
PC / UPC	flat / ultra-polished, 0°	blue (SMF), beige/black (MMF)	≥ 50 dB	Ethernet, FC, data centre
APC	angled 8°	green	≥ 60 dB	PON, DWDM, analogue video, 400G DR4/800G DR8 MPO

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.