A+ Core 1 3.2: Network Cables

In one line: twisted pair cancels interference by twisting equal-and-opposite pairs at differing rates, the category and IEEE 802.3 decide what speed runs how far, the shielding code on the jacket tells you what’s inside, and where the cable goes — buried or above a drop ceiling — decides what jacket it needs.

Companion to: Professor Messer, CompTIA A+ 220-1201 Core 1, objective 3.2 (Network Cables). The full version is the Network Cables class notes; the section overview is the Section 3 sheet.

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Twisted pair: why twisted

Point	Detail
Pairs	Four: blue, green, orange, brown
Signals	Equal and opposite — Transmit+ and Transmit− on the two wires
Why twisted	Both wires pick up the same interference, so the receiver subtracts what’s common and keeps the difference
Twist rates	Different for every pair, so the receiver can compare across all four and separate noise from signal (also cuts crosstalk)
Practical	Keep the twists right up to the termination — untwisting undoes the protection

Categories and distances

A cable has no speed — the signalling and coding do. The category states what the cable was tested to carry, and IEEE 802.3 defines the minimum category per Ethernet standard.

Standard	Minimum category	Max distance
1000BASE-T	Cat 5	100 m
1000BASE-T (new cable)	Cat 5e	100 m
10GBASE-T	Cat 6 unshielded	55 m
10GBASE-T	Cat 6 shielded	100 m
10GBASE-T	Cat 6A	100 m

• Cat 5 is deprecated — new purchases are Cat 5e (“e” = Enhanced, extra qualification tests). Existing Cat 5 still runs gigabit to 100 m.
• Cat 6A — “A” = Augmented.
• The category is printed on the jacket.
• Beyond the exam: TSB-155 puts unshielded Cat 6 at 37–55 m depending on alien crosstalk (interference from neighbouring cables in the bundle). Specify Cat 6A for any new 10G install.

Coaxial

Point	Detail
Name	“Co-axial” — two or more forms sharing a common axis
Build	Inner conductor carries the signal; outer shield protects it
On networks	Cable modems and digital cable — high-speed internet over cable infrastructure

Shielding: UTP vs STP

Code	Meaning
U	Unshielded
S	Shield — braided
F	Foil

Format: overall shielding / per-pair shielding + TP

Marking	Overall	Each pair
U/UTP	None	None — plain UTP
F/UTP	Foil	None
S/FTP	Braid	Foil

A shield needs shielded jacks, panels and patch leads throughout, and grounding at the correct point — badly terminated STP can perform worse than plain UTP.

Direct burial STP

Feature	Why
Built for burial	Goes straight into the ground, often without conduit
Waterproof gel inside	Repels water around the pairs
Shielded	Interference protection, grounding, and extra strength
Drain wire	Runs the whole length of the cable as an electrical ground
Structure	Same four pairs as indoor STP, plus the gel and tougher jacket

Inter-building copper carries surge and ground-potential risk — fibre avoids it entirely.

Plenum-rated cable

Term	Meaning
Plenum space	Open void above a drop ceiling where return air circulates
Not a plenum	Return air is ducted; the void air isn’t circulating
The risk	Fire, smoke and toxic fumes travel freely through that open space — and into the air handling
Standard jacket	PVC (polyvinyl chloride) — not for plenum
Plenum jacket	FEP (fluorinated ethylene polymer) or low-smoke PVC
Trade-off	Electrically identical, but the jacket is less flexible and harder to work with

Above a drop ceiling used for return air → plenum-rated cable, no exceptions. It’s a building code requirement.

🔐 Security notes

• Copper radiates, fibre doesn’t. Twisted pair emits a weak field (the same physics an inductive probe uses); shielding reduces it, fibre eliminates it and can’t be tapped without breaking the light.
• Cable routes are attack surface: ceiling voids, risers and shared spaces can be reached, tapped or cut. Lock and document cable paths like comms rooms.
• Inter-building runs leave your perimeter — physical protection at both ends, and don’t implicitly trust the far segment.
• Wrong category is a slow-burn availability fault: Cat 6 stretched past 55 m produces errors that look like an application problem.
• Plenum ratings are life safety, not preference — PVC jacket is what produces the toxic smoke.
• Grounding and surges kill switch ports: shields grounded at both ends, or unprotected copper between buildings, carry surges inside.

Practice drills

1. Why are the wires twisted?

So both wires pick up the same interference; the receiver subtracts what’s common to both (they carry equal and opposite signals) and keeps the real data.

2. Why is each pair twisted at a different rate?

So the receiver can compare interference across all four pairs and separate noise from signal — and to reduce crosstalk between pairs.

3. Does a cable have a speed?

No. The signalling and coding at each end have a speed; the category says whether the cable can carry it cleanly. IEEE 802.3 defines the minimum.

4. Minimum category and distance for 1000BASE-T?

Cat 5, 100 m — though Cat 5 is deprecated, so new cable is Cat 5e (Enhanced).

5. The three 10GBASE-T combinations?

Cat 6 unshielded 55 m · Cat 6 shielded 100 m · Cat 6A 100 m (“A” = Augmented).

6. Decode S/FTP and F/UTP.

S/FTP = braided shield overall, foil on each pair. F/UTP = foil overall, pairs unshielded.

7. What's a drain wire?

A wire running the full length of the cable used as an electrical ground — found on direct burial STP.

8. Cable above a drop ceiling carrying return air — what do you specify?

Plenum-rated cable: FEP or low-smoke PVC jacket, not ordinary PVC.

Key takeaways

• Twisted pair cancels interference: equal and opposite signals, both wires exposed equally, different twist rate per pair.
• Cables have no speed — the category plus IEEE 802.3 tell you what will run how far.
• 1000BASE-T: Cat 5/5e, 100 m. 10GBASE-T: Cat 6 (55 m UTP / 100 m shielded) or Cat 6A (100 m).
• Shielding code = overall / per-pair + TP, with U unshielded, S braid, F foil.
• Direct burial = waterproof, gel-filled, shielded, with a drain wire. Plenum space = FEP or low-smoke PVC.