Copper and fibre transmission media

Why this matters

Objective 1.5 pairs media with transceivers because in practice you choose them together: the cable determines the distance and speed you can reach, and the transceiver has to match the cable. Get either wrong and the link either does not come up, or comes up and misbehaves — which is the worse outcome, because an intermittent physical fault looks like almost anything else.

Domain 5 examines cabling faults heavily. Nearly everything in this lesson is a fault you will be asked to recognise from symptoms.

The lesson

Wired and wireless are both "transmission media"

CompTIA groups them under one objective, so a media question can legitimately offer a wireless option. The division is simply whether the signal is guided by a physical path.

Wired media guide the signal: copper carries electrical current, fibre carries light. They are predictable, private by default, and unaffected by weather or neighbours.

Wireless media radiate. The headline distinctions you need here:

• 802.11 — wireless LAN. Short range, unlicensed spectrum, shared medium.
• Cellular — licensed spectrum, wide area, carrier-operated. LTE and 5G, increasingly used as a primary or backup WAN link at branch sites.
• Satellite — very wide coverage, including places nothing else reaches, at the cost of latency. Geostationary service adds roughly 500 ms round trip, which breaks interactive protocols; low-earth-orbit services cut that substantially but still sit above terrestrial links.

Copper: the 802.3 standards you must know

802.3 is the IEEE standard family for Ethernet. The naming is readable once you know it: speed, then signalling, then medium. 1000BASE-T is 1000 Mbps, baseband, twisted pair.

The 100-metre limit is the most examined number in this objective. It is a total channel length — patch lead to the wall, horizontal run, patch panel to switch — not just the long bit in the ceiling. A 95 m run plus two 3 m patch leads is over limit, and the symptom is not a dead link: it is intermittent errors, retransmissions and poor throughput. A question describing "works sometimes, slow, CRC errors" over a long run is testing exactly this.

Category ratings describe bandwidth and resistance to crosstalk. Cat 5e for gigabit, Cat 6 for 10 Gb over short runs, Cat 6a for 10 Gb to the full 100 m, Cat 7 and Cat 8 for higher speeds over shorter distances.

Coaxial, twinaxial and direct attach

Coaxial has a single central conductor inside a dielectric, surrounded by a braided shield and jacket. In networking it survives mainly as the cable-modem drop (RG-6) and in some CCTV installations. The shield gives it good noise immunity.

Twinaxial has two central conductors rather than one, and is used for very short, very fast links inside a rack.

Direct attach copper (DAC) is a twinaxial assembly with the transceivers moulded onto both ends, bought as one unit. It is cheap, draws less power than optics, and is the normal way to connect a server to a top-of-rack switch — but it is short (typically 5–7 m passive) and a fixed length. Because the ends are integral you cannot re-terminate it; you replace the whole cable.

Fibre: single-mode versus multimode

Fibre carries light in a glass core, so it is immune to electromagnetic interference, does not conduct (which matters between buildings, where copper can carry a lightning surge or a ground potential difference), and is far harder to tap.

Multimode (MMF) has a wide core — 50 or 62.5 microns — so light enters at multiple angles and travels several paths, or modes. Those paths differ in length, so the pulse spreads as it travels. That is modal dispersion, and it is what limits multimode distance: a few hundred metres at 10 Gb. It uses inexpensive LED or VCSEL sources; connectors are conventionally beige or aqua.

Single-mode (SMF) has a core around 9 microns — narrow enough that light takes essentially one path. No modal dispersion, so reach extends to tens of kilometres. It uses laser sources and more precise optics, so the electronics cost more; connectors are conventionally yellow.

The rule to carry into the exam: multimode inside a building, single-mode between buildings or across a city. If a question describes a campus link of several kilometres and offers multimode, that is wrong however attractive the rest of the option looks.

One practical trap: you cannot mix them. Single-mode fibre with multimode optics, or the reverse, will either not link or link badly. Matching the optic to the glass is part of the same decision.

Plenum versus non-plenum

A plenum is the air-handling space in a building, typically above a suspended ceiling, where air circulates for the HVAC system. Cable run through it must be plenum-rated: jacketed in a material such as FEP that resists flame and, critically, produces little smoke and low toxicity when it does burn. Ordinary PVC-jacketed cable in a plenum feeds a fire with smoke that the air handling then distributes through the building.

This is a building code requirement, not a performance one. Plenum cable costs more and is often stiffer. The exam scenario is always the same shape: cable is being run above a ceiling used for air return, so the answer is plenum, and the reason is life safety and code compliance rather than signal quality.

Riser-rated cable sits between the two, for vertical runs between floors.

Cable speeds, and choosing between media

The decision usually goes distance first, then speed, then cost:

• Under 100 m, gigabit or 10 Gb, inside a building: twisted pair. Cheapest to install and terminate, and PoE can power the device at the far end.
• Inside a rack, 10–100 Gb, a few metres: DAC. Cheaper and lower-power than optics at that distance.
• Between floors or buildings, or beyond 100 m: fibre. Multimode within a building, single-mode beyond it.
• Where interference is severe — a factory floor, runs alongside power, near motors: fibre, because it is immune; or shielded copper as a compromise.
• Where a lightning risk or ground potential difference exists between buildings: fibre, because it does not conduct. That is a safety answer, not a performance one.

Protocol over media

The final sub-topic under 1.5 is a reminder that media and protocol are separate choices. Ethernet is not the only thing carried on these cables: Fibre Channel runs over the same optics for storage networks, and cable and DSL services run their own protocols over coax and twisted pair. The transceiver and connector may be identical while the protocol differs — which is why an SFP that physically fits is not necessarily an SFP that works.