Cabling and Connectors: Cat6, Fiber, SFP

The one-line rule for choosing a cable

Pick the cable from the distance and speed the scenario gives you: twisted pair copper (UTP) for runs up to 100 m, multimode fiber for building and campus runs, single-mode fiber for long distances, and direct attach copper (DAC) for a few metres between devices in the same or neighbouring racks. Then pick the connector that matches the port: RJ45 for twisted pair, LC or SC for fiber, F-type or BNC for coaxial. Most exam questions are solved by those two steps.

Copper: categories, distances and pin-outs

Ethernet over twisted pair is defined by IEEE 802.3, and the cable categories come from the TIA cabling standards. The practical limit for the common speeds is a 100 m channel: 90 m of permanent cabling plus up to 10 m of patch cords.

The Cat 6 figures (100 m, and 55 m for 10GBASE-T) are documented on the Category 6 cable reference page. The exam-safe summary is: Cat 5e or better for 1 Gbps, Cat 6a for 10 Gbps over a full 100 m.

UTP vs STP. Unshielded twisted pair relies on the twists to cancel interference. Shielded twisted pair adds foil or braid for noisy places such as next to motors or heavy power cabling.

Plenum vs non-plenum. Plenum-rated cable has a jacket that burns with less smoke and is required (by building and fire codes) when the cable runs through air-handling spaces such as the area above a ceiling used for air return. Non-plenum (PVC) cable is cheaper and fine elsewhere. A “which cable above the drop ceiling” question points to plenum.

Pin-outs. An RJ45 plug (8P8C) has eight pins. The two wiring standards are T568A and T568B, which differ by swapping the orange and green pairs. A cable with the same standard on both ends is straight-through (switch to router, PC to switch). A crossover has T568A on one end and T568B on the other, and was needed to join like devices such as switch to switch. Modern ports detect this themselves with auto-MDI/MDIX, so a crossover is rarely required, but “pin-out” and “TX/RX transposed” faults still appear in the outlines. RJ11 is the smaller telephone connector with fewer pins and never fits an Ethernet port.

Fiber: single-mode, multimode and the connector family

Fiber carries light, so it is immune to electrical interference and reaches further than copper. The core size decides which type it is.

• Multimode (MMF) has a wide core, uses cheaper short-wave optics, and suits runs inside a building or campus. OM3 and OM4 grades carry 10 Gbps over a few hundred metres, and 1000BASE-SX runs roughly 220 to 550 m depending on the grade.
• Single-mode (SMF) has a very narrow core, uses laser optics, and reaches kilometres: 1000BASE-LX at about 5 km and LX10 at 10 km. It costs more in optics, not in the glass.

The reach values above come from the Gigabit Ethernet reference. The rule that matters for the exam: the cable and the transceiver at both ends must match. A single-mode transceiver on multimode fiber, or the reverse, gives high attenuation and a link that is down or full of errors.

Transceivers and DAC

Switches often have cages for pluggable modules. An SFP is the small form factor for 1 Gbps; SFP+ carries 10 Gbps in the same size. QSFP is the larger form factor that bundles four lanes for 40 Gbps and higher. The module decides the medium: an SFP can have an RJ45 copper port, an LC multimode port or an LC single-mode port. A DAC (twinaxial) cable has its transceiver ends permanently attached to copper, so it is cheap but short-reach.

Worked example: choosing and diagnosing

A company links two switches in different buildings, 1.2 km apart, at 10 Gbps. Copper is out (100 m limit), and multimode 10G optics will not cover 1.2 km, so the answer is single-mode fiber with 10G single-mode SFP+ modules and LC connectors at both ends.

After installation the link shows down. The counters are empty and the light level at the far end is very low. A mismatch (multimode module on single-mode glass) or dirty or damaged connectors are the first checks, with a visual fault locator or optical power meter to confirm. If instead the link is up but the interface shows rising CRC errors and runts, think of crosstalk, interference, a kinked cable or a duplex or speed mismatch, which is how CCNA infrastructure questions frame “interface and cable issues”.

Which exams test this

How exam questions are built around cabling

The trap is choosing by speed and forgetting distance, or the reverse. Look for the constraint words: “between buildings”, “1.2 km”, “across the campus” (fiber, usually single-mode), “above the ceiling” (plenum), “next to heavy machinery” (shielded or fiber), “same rack” (DAC), “needs power for a phone or access point” (PoE over twisted pair). Fault questions give symptoms: no link and no light (wrong or dirty fiber, transposed TX/RX), CRC errors and runts (interference, bad termination), link up but slow or error-prone (duplex or speed mismatch). If a question says “Layer 1”, the answer is usually a cable, connector or transceiver; the OSI vs TCP/IP guide explains the layers, and network troubleshooting commands and methodology gives the order to work in.

Go deeper

• IEEE 802.3-2022 Standard for Ethernet, the source of the Ethernet physical layer variants
• Category 6 cable reference and Gigabit Ethernet reference for the distances above
• Cisco: Ethernet auto-negotiation, duplex and interface error counters
• Related: Network+ exam hub, CCNA exam hub, common ports cheat sheet