Connector Identification Notes: LC, SC, ST and MPO

Every optical installation has a moment when someone stands in front of a panel, holding a patch cord, and asks what connector this is. The answer matters, because a patch cord with the wrong connector at either end will not mate, and a crew that guesses will waste a visit. This note teaches connector identification by sight, by size and by context, covering the four types a buyer is most likely to meet: LC, SC, ST and MPO.

LC is the connector you will meet most often in a modern installation. It is small — roughly half the size of SC — and it latches like a small telephone plug, with a plastic lever that clicks into the adapter. It usually appears in pairs, two LC connectors in a duplex housing, one for transmit and one for receive, and the housing is often colour-coded: beige or aqua for multimode, blue for single-mode with a standard polish, green for single-mode with an angled polish. If the connector is small, latching and paired, it is almost certainly LC, and the equipment it plugs into is almost certainly modern: SFP, SFP+, XFP or QSFP28 duplex variants, and modern patch panels.

SC is the connector you will meet in older plant and in telecom equipment. It is larger than LC, roughly twice the size, and it latches by pushing the connector body straight into the adapter, with no lever to squeeze. It also appears in pairs for duplex links, and its housing follows the same colour pattern: beige or aqua for multimode, blue for single-mode. If the connector is chunky, push-pull and paired, it is SC, and the plant it belongs to is likely older, or telecom, or both. A site with SC panels and new LC equipment needs mixed cords, with LC at the equipment end and SC at the panel end, and that mixed cord is a standard product.

ST is the connector you will meet in the oldest installations. It is a bayonet connector: it twists and locks, like a bayonet light bulb, with a metal sleeve and a notch that engages the adapter. It appears as single connectors rather than duplex housings, one for transmit and one for receive, each twisted separately. If the connector twists to lock and stands alone rather than in a pair, it is ST, and the plant it belongs to was installed when optical networking was new. A site with ST panels is rare today, but it exists, and a buyer connecting to it needs a mixed cord with ST at the panel end.

MPO is the connector that looks like none of the others, which makes it the easiest to identify once you know what to look for. It is a flat, rectangular connector, wider than LC or SC, with a single ferrule that carries many fibers in a row — eight or twelve, most commonly — and it latches with a push-pull housing like SC but much wider. It appears where parallel optics is used: QSFP and QSFP28 links, where four lanes run in parallel and need eight fibers in one plug. If the connector is flat, wide and rectangular, with a row of fibers visible in the ferrule, it is MPO, and the link it serves is a parallel-optics link.

Context is the second identification method, and it often settles what sight leaves uncertain. The equipment tells you the connector: a modern switch with SFP+ slots takes LC; a telecom shelf takes SC; a legacy panel takes ST; a QSFP port takes MPO. The panel tells you the connector: the adapters in the panel match the cords that plug into them, and a panel of small duplex adapters is LC, while a panel of large push-pull adapters is SC. The jacket colour tells you the fiber type, which narrows the connector: yellow usually means single-mode, aqua or orange usually means multimode, though colour codes vary and should not be trusted alone.

The practical advice is short. Read the connector off the equipment and the panel, not off a plan, because panels are sometimes changed without the records being updated. Photograph the connector and the panel if you are reporting back to someone else, because a photograph settles what a description leaves uncertain. State the connector at each end during the inquiry, together with the fiber type and the length, and the cord can be confirmed as a pair. Before ordering, confirm quantity, price and delivery time during the inquiry. Certificates are available upon request.

Angled polish is the detail that confuses connector identification most, and it deserves a paragraph because it is easy to miss and costly to get wrong. A standard connector has a flat end face, polished straight across, and its housing is usually blue for single-mode. An angled connector has an end face polished at an angle, usually eight degrees, and its housing is usually green for single-mode. The angle reduces back-reflection, which matters in analogue video, in passive optical networks and in any link sensitive to reflected light. The catch is that an angled connector and a flat connector must never be mated together: the angle prevents a proper physical contact, and the resulting link will be lossy or dead. A buyer who sees green housings should treat them as angled connectors and order angled mates, not flat ones, and should state the polish during the inquiry.

Cleaning and inspection tools are the practical side of connector identification, because a connector cannot be identified reliably if its end face is dirty. A handheld inspection scope, which magnifies the end face so dirt, scratches and damage can be seen, is the single most useful tool in optical installation, and it costs less than one wasted site visit. A cleaning kit — lint-free wipes or a cassette cleaner, and the correct solvent — is the second. The practice is simple: inspect before mating, clean if dirty, inspect again after cleaning, and mate only a clean connector. A site that follows that practice will have fewer failed links than a site with better modules and worse practice.

Labelling and record-keeping close the loop on connector identification. A panel whose adapters are labelled by connector type, fiber type and destination saves every future visit, because the next crew does not have to identify anything from scratch. A record that lists, for each link, the connector at each end, the fiber type, the length and the installation date turns fault-finding from detective work into routine work. The time spent labelling at installation is repaid at the first fault, and the record is what allows a remote engineer to diagnose a link without travelling to it. Before ordering, confirm quantity, price and delivery time during the inquiry. Certificates are available upon request.