Transceivers, connectors and cable standards

Why this matters

This is the most directly visual part of the exam. You will be shown, or described, a connector and asked what it is and what it carries. It is also the part most people skip, because it looks like trivia — and then they lose marks that required no reasoning at all, only recognition.

There is a practical side too. A link that will not come up is very often a transceiver mismatch, and the error message rarely says so. Knowing which combinations are legal is a real troubleshooting skill.

The lesson

What a transceiver is

A transceiver transmits and receives — it converts between the electrical signals inside a device and whatever the medium carries, whether that is light on a fibre or current on copper. On a switch with fixed copper ports the transceiver is built in. On anything modular it is a small pluggable unit you insert into a cage on the device, which is what makes a switch port able to be copper today and long-range fibre tomorrow.

Two properties have to match the link: the form factor (will it physically fit the cage) and the optical or electrical spec (wavelength, mode, reach, speed). Physical fit is not compatibility. An SFP will slide into any SFP cage; a single-mode 10 km optic in a cage at the far end of a multimode patch lead will still not pass traffic.

Form factors

SFP (small form-factor pluggable) is the baseline: a compact module, typically 1 Gb. SFP+ is the same physical size at 10 Gb, and SFP28 at 25 Gb. Because the size is identical, exam questions about them turn on speed rather than shape.

QSFP (quad small form-factor pluggable) is the "quad" version: four lanes in one module, physically larger. QSFP+ carries 40 Gb (4 × 10 Gb) and QSFP28 carries 100 Gb (4 × 25 Gb). A useful property is that a QSFP+ port can often be broken out into four separate 10 Gb links with a breakout cable, which is how one high-speed switch port serves four servers.

Older names you may still meet: GBIC, the larger predecessor to SFP, and XFP, an early 10 Gb module.

The pattern worth remembering is simply that "Q" means four lanes, and that the number after the name is the per-lane speed.

Ethernet and Fibre Channel

Transceivers are not Ethernet-specific. Fibre Channel (FC) is a separate protocol family used for storage area networks, and it runs over the same optics and the same LC connectors. Speeds are quoted differently (8GFC, 16GFC, 32GFC) and the switches are different equipment.

The exam point is that identical-looking hardware can be carrying a completely different protocol, and that FCoE (Fibre Channel over Ethernet) exists to carry FC traffic across an Ethernet fabric so a data centre needs only one set of cabling.

Fibre connectors

Four to know, and they are commonly shown as images.

LC (local connector). Small, latching, almost always in duplex pairs — two fibres clipped together, one transmit and one receive. The dominant modern connector, and what you will find on almost every SFP. If a question describes a small duplex fibre connector on an SFP, it is LC.

SC (subscriber connector). Larger, square-bodied, push-pull. Remember it as "square connector" — the mnemonic is not the real expansion but it works. Common on older equipment and in some provider handoffs.

ST (straight tip). Round, with a bayonet twist-and-lock coupling. Remember it as "stab and twist". Older, frequently seen in legacy multimode installations.

MPO (multi-fiber push on). A rectangular connector carrying many fibres — typically 12 or 24 — in one body. This is how QSFP modules get their four lanes and how high-density trunk cabling is run between racks and patch frames. If a question mentions 40 Gb or 100 Gb over parallel fibres, MPO is the connector.

Copper connectors

RJ45. Eight positions, eight contacts. The standard Ethernet connector on twisted pair. Wired to either T568A or T568B — both are valid, but both ends of a cable must use the same standard, or you have inadvertently made a crossover. Modern equipment with auto-MDI-X will usually cope, which means a miswired cable can work in one place and fail in another, and that is a genuinely nasty intermittent fault.

RJ11. Six positions, usually two or four contacts. The telephone connector, and what a DSL modem plugs into. It is visibly narrower than RJ45, and that size difference is the intended exam cue.

F-type. A screw-on connector for coaxial cable, used for cable television and cable-modem service. The centre conductor of the coax itself forms the pin.

BNC (Bayonet Neill–Concelman). A twist-lock coaxial connector, named after its two designers. Historically used for 10BASE2 Ethernet, and still found on CCTV, some test equipment and serial links.

Matching, and the faults that come from not matching

A link comes up when everything agrees: form factor fits the cage, both ends run the same speed and standard, the optic's mode matches the glass, and the wavelength matches at both ends.

Common failures, all examinable:

• Mode mismatch. Multimode optic on single-mode fibre, or the reverse. May not link at all, or may link over a short patch and fail over distance.
• Wavelength mismatch. Two single-mode optics specified for different wavelengths will not talk to each other.
• Distance mismatch. A long-reach optic driven into a very short fibre can overload the receiver. The answer is an attenuator, not a different cable.
• Vendor lock. Many switches check a vendor code in the module and refuse third-party optics. The link is dead with no optical fault at all, and the log message names the module, not the fibre.
• Dirty connector. The most common fibre fault in the field by a wide margin. A fingerprint on an end face attenuates the signal enough to cause errors while the link still shows up. Clean before replacing anything.

The troubleshooting order that follows is worth carrying: check that the module is seated, that it is the right type for the glass, that both ends match each other, and that the end faces are clean — before you conclude the cable is bad.