Copper, fibre, and terminating a cable that actually works

Why this matters

Cabling is the layer everything else sits on, and it is the layer that gets blamed last and causes problems first. A marginal cable does not fail cleanly. It produces errors, retransmissions and a link that negotiates at a tenth of its rated speed, and every one of those presents to a user as "the network is slow".

The exam asks for categories, distances, connector types and the two wiring standards by name. It also asks scenario questions where the answer is a cabling limit — a run that is too long, a category that cannot carry the required speed, or a fibre type chosen for the wrong distance.

Terminating your own cable is on the objective list, and it is worth doing once even if you never do it again, because making one wrongly on purpose teaches more about lesson 17's testers than any amount of reading.

The lesson

Twisted pair categories, what each supports, and why length is part of the specification

Twisted pair cable is rated by category, and each category is a statement about bandwidth — from which a speed and a distance follow together.

• Cat 5e — 1 Gbps to 100 m. Still everywhere, still adequate for most desks.
• Cat 6 — 1 Gbps to 100 m, 10 Gbps to about 55 m. The distance reduction at 10 Gbps is the examinable detail.
• Cat 6a — 10 Gbps to 100 m. Thicker, less flexible, better shielded.
• Cat 7 and Cat 8 — higher frequencies over shorter distances; Cat 8 is a data-centre cable rated to around 30 m.

100 metres is the number to remember, and it is 90 metres of solid-core horizontal cable plus up to 10 metres of stranded patch leads at the two ends. Beyond that, the electrical signal has degraded enough that the standard no longer guarantees anything — the link may work, may work slowly, or may fail intermittently as temperature changes. "It works, mostly" is the characteristic symptom of an over-length run, and a scenario describing a 150-metre run has already told you the answer.

Solid core and stranded are not interchangeable: solid for permanent runs inside walls, stranded for patch leads that move. A solid-core patch lead fractures internally after enough flexing and produces exactly the intermittent fault that is hardest to find.

Shielded cable exists for electrically noisy environments. It only helps if it is grounded at one end, and grounding it at both can make things worse by creating a current path through the shield.

Terminating to the standard: pin order, pairs, and the two wiring schemes

An eight-position modular plug holds four pairs. Two standard pin orders exist, T568A and T568B, and they differ by swapping the green and orange pairs.

T568B, which is the more common in practice:

1. white/orange
2. orange
3. white/green
4. blue
5. white/blue
6. green
7. white/brown
8. brown

T568A swaps pairs 2 and 3 so it runs white/green, green, white/orange, blue, white/blue, orange, and then the same brown pair.

The rules that matter more than which you choose:

• Be consistent. Both ends the same, and the whole site the same. Mixing schemes within an installation is how a crossover cable appears by accident.
• Keep the twist. Untwisting more than about 13 mm at the connector degrades the cable's noise rejection. This is the most common cause of a hand-made cable that passes a continuity test and performs badly.
• The jacket goes inside the plug so the strain relief clamps on the jacket, not the conductors.
• All eight conductors reach the end of the plug before crimping. Conductors that stop short produce intermittent contact.

Wall outlets and patch panels are punched down, not crimped, using a punchdown tool that seats the conductor into an insulation-displacement contact and trims the excess in one motion. The same colour scheme applies and is usually printed on the jack.

Straight-through and crossover, and why modern equipment hides the difference

A straight-through cable has the same pin order at both ends. A crossover swaps the transmit and receive pairs, historically needed to connect two devices of the same type: switch to switch, or computer to computer.

Modern equipment almost universally supports automatic detection, which senses the crossover requirement and reconfigures the port internally. That is why a crossover cable is rarely needed now — but the exam still asks, because older equipment exists and because understanding why it was needed is the point.

The reasoning: a network interface transmits on one pair and receives on another. Two devices that both transmit on the same pair shout at each other and hear nothing. A switch port is wired the opposite way round, which is what makes a straight-through cable work between a computer and a switch.

The other cable type worth naming is the rollover or console cable, which reverses all eight conductors and connects a computer's serial port to a network device's management console. It carries no network traffic at all.

Fibre: single-mode against multi-mode, connector types, and handling rules

Fibre carries light rather than electricity, which gives it immunity to electrical interference and vastly greater distance.

• Single-mode has a very narrow core and uses a laser. One path for the light, so no dispersion, and distances measured in tens of kilometres. More expensive optics, and the choice for anything long.
• Multi-mode has a wider core and typically uses an LED or VCSEL. Light takes multiple paths, which limits distance to a few hundred metres, but the optics are cheaper. The choice within a building or campus.

Connectors in scope: LC, small and now the most common; SC, square, push-pull; ST, round with a bayonet twist. All three appear in exam questions, and the practical identification is size and locking mechanism.

Handling rules matter more with fibre than with copper:

• Do not exceed the bend radius. A sharp bend causes light to leave the core, which shows as attenuation rather than a break.
• Keep the end faces clean and capped. A fingerprint on a fibre end face is enough to degrade a link, and dust caps exist for that reason.
• Never look into a fibre or a transceiver. The light is invisible and can damage your eye.
• Transceivers must match at both ends — same type, same wavelength, same fibre mode. A single-mode transceiver on multi-mode fibre will sometimes appear to work over short distances and is not a correct installation.

Coaxial and the places it still appears, from cable internet to older video

Coaxial cable has a centre conductor, a dielectric, a shield and a jacket, and it is still present in several places this exam cares about.

• Cable internet. RG-6 from the street to the modem, with an F-type screw-on connector. This is the most common coax a technician meets today.
• Older Ethernet. Thin and thick coaxial Ethernet used BNC connectors and bus topology, and are historical.
• Video distribution in buildings, and older analogue camera systems.

Worth knowing: RG-6 is used for television and internet distribution and RG-59 is the thinner, older cable found in legacy video installations. The impedance difference means they are not interchangeable in a specification, even though a connector will fit both.

The failure modes are mechanical and specific: a loose F-connector causes intermittent signal loss, a sharply bent cable changes its impedance, and water ingress at an outdoor connection corrodes the centre conductor. All three present as a service that works in dry weather and fails in wet, which is one of the more satisfying diagnoses in this job.