Overview#
A cable is a private medium in a way radio never is. Nobody else’s traffic shares your patch lead, the path is bounded and measurable, and a station can listen to the wire while transmitting on it — which is why wired Ethernet could detect collisions where wireless can only try to avoid them.
What limits a wired link is the cable itself: how far a signal travels before it degrades, how much interference it picks up, and how much bandwidth its construction supports. Those limits are why cable categories, fiber grades, and distance ratings exist, and why “it plugged in and linked up” is not the same as “it will work reliably.”
However convenient wireless is, nearly every network is wired somewhere — and wired media are generally the fastest, most reliable, and hardest to intercept of the three. This page covers the Ethernet standards, copper conductors, fiber optics, and the material properties that bound each one.
The IEEE defines Ethernet in the 802.3 family, which specifies how data crosses physical cabling in a wired LAN. A link can be full-duplex, sending in both directions simultaneously, or half-duplex, sending in one direction at a time.
Standards are what make equipment from different vendors interoperate. Interoperability has limits worth respecting: fiber optics and twisted-pair copper are not interchangeable, and mixing devices rated for different speeds means the link negotiates down to what both ends support.
Collisions on a shared cable#
Shared half-duplex Ethernet has the same contention problem as Wi-Fi, and 802.3 solves it with carrier-sense multiple access with collision detection (CSMA/CD):
- A device listens for a quiet moment on the cable and waits if it is busy.
- Devices sharing the cable transmit by taking turns.
- If two transmit simultaneously, the collision corrupts both frames.
- Both back off for a random interval before retrying, which makes a second collision between the same pair unlikely.
The region of the network where collisions are possible is the collision domain. Switched full-duplex links, which is nearly all modern wired Ethernet, give each port its own collision domain and remove the contention entirely — CSMA/CD is legacy behavior on those links, not active mechanism.
Ethernet standards#
| Standard | Max speed | Medium | Nominal max distance |
|---|---|---|---|
| 10BASE2 | 10 Mbps | Thin coaxial | 185 m |
| 10BASE-T | 10 Mbps | Cat 3 twisted pair | 100 m |
| 10BASE-F | 10 Mbps | Multimode fiber | 2,000 m |
| 100BASE-TX | 100 Mbps | Cat 5 twisted pair | 100 m |
| 1000BASE-T | 1 Gbps | Cat 5e/6 twisted pair | 100 m |
| 1000BASE-SX | 1 Gbps | Multimode fiber | 220–550 m |
| 1000BASE-LX | 1 Gbps | Single-mode fiber | 5,000 m |
| 10GBASE-T | 10 Gbps | Cat 6a/7 twisted pair | 100 m |
| 10GBASE-SR | 10 Gbps | Multimode fiber | 300–400 m |
| 10GBASE-LR | 10 Gbps | Single-mode fiber | 10 km |
| 40GBASE-LR4 | 40 Gbps | Single-mode fiber | 10 km |
| 100GBASE-LR4 | 100 Gbps | Single-mode fiber | 10 km |
The twisted-pair distance limit of 100 m is a property of the whole channel, including patch cords and terminations, not just the run of cable in the wall. Fiber distances vary with the grade of fiber and the optic driving it; the figures above are planning numbers.
Fiber Ethernet and OM grades#
Fiber Ethernet transmits data as pulses of light through glass. Light travels further before needing regeneration than an electrical signal in copper, and fiber is immune to electromagnetic interference (EMI), which makes it the reliable choice near motors, fluorescent lighting, and heavy power runs.
Multimode fiber is graded by optical multimode (OM) classes, which describe the reach a given fiber supports at a given speed:
| Grade | Description |
|---|---|
| OM1 | 62.5 µm core, LED sources. Roughly 1 Gbps to 300 m, 10 Gbps to 33 m |
| OM2 | 50 µm core. Roughly 1 Gbps to 600 m, 10 Gbps to 82 m |
| OM3 | Laser-optimized. 10 Gbps to 300 m, 40/100 Gbps to 100 m |
| OM4 | Enhanced OM3. 10 Gbps to 550 m, 40/100 Gbps to 150 m |
| OM5 | Wideband, supports shortwave wavelength division multiplexing (SWDM) |
Treat these as nominal. The reach a link actually achieves depends on the fiber’s modal bandwidth, the wavelength, the transceiver, and the loss budget of the installed run.
Single-mode and multimode fiber#
Single-mode fiber has a narrow core, around 9 µm, that carries light along essentially one path. Without multiple paths there is no modal dispersion, so it holds a signal over long distances. Telecoms and Internet service providers (ISPs) use it for backhaul and long-haul links.
Multimode fiber has a wider core that admits several light paths at once. Those paths arrive at slightly different times — modal dispersion — which limits usable distance. It is the practical choice inside buildings, campuses, and data centers.
The usual “single-mode costs more” shorthand is about the optics, not the glass. Single-mode transceivers need tighter alignment and more precise light sources, and that is where the cost difference lives.
Direct attach copper#
Copper is simpler than fiber over short distances: no optics to align, less power per link, and more tolerance for rough handling. Direct attach copper (DAC) cables use twinaxial (“twinax”) construction — two insulated conductors inside a shield — with the transceiver ends permanently attached. They are common for top-of-rack switch connections a few meters long.
Coaxial cable#
Coaxial cable carries a signal on a single center conductor inside a shield. It predates twinax in LAN use, reaches further than twinax but at lower speeds, and is more durable and easier to work with than fiber. It survives today in cable broadband and video distribution rather than in LANs.
Plenum cabling#
A plenum space is a void used to move air for heating, ventilation, and air conditioning (HVAC) — typically above a drop ceiling or below a raised floor. Cable run through one has to be plenum-rated: flame-retardant jacketing that produces little smoke and low toxicity when it burns, so a fire is not distributed through the building’s air handling. Plenum cable costs more than standard cable, and where it is required, it is required by fire and building code rather than by preference.
Study-note shortcuts worth correcting#
- CSMA/CD detects collisions; CSMA/CA avoids them. A wired station can hear the cable while transmitting. On a modern full-duplex switched link there are no collisions to detect at all, which is why CSMA/CD is effectively history.
- Category ratings describe the cable and the whole channel. A Cat 6A patch lead does not rescue a Cat 5e run in the wall, and connectors count.
- Fiber is immune to EMI, not indifferent to everything. Bend radius, contaminated connector endfaces, and accumulated loss all break fiber links.
- Single-mode is not simply “the fast one.” It carries further because the core is narrow enough to admit one path, which is a distance property before it is a speed one.
- Plenum rating is a fire-code requirement, not an upgrade. Running non-plenum cable in a plenum space is a code violation regardless of how well the link performs.
Suggested practice: read the media in your own links#
On a network you own:
- List your interfaces with
ip -br link, then runethtool <interface>on a wired one. Record the negotiated speed and duplex, and compare them with what the port and cable are rated for. - Check
ip -s linkfor error and drop counters on that interface. A link can negotiate successfully and still be marginal. - Read the jacket printing on a patch cable. Note its category and its fire rating (CM, CMR, or CMP), then find where it is run and decide whether the rating matches the space.
- Swap a suspect cable for a known-good one of the same category and re-check
ethtooland the error counters. This is the cheapest layer-1 test there is. - If you have fiber, identify the connector type and whether the run is single-mode or multimode from the jacket colour and printing, then check it against the optic in the transceiver. A mismatch links up and performs badly.
This shows what your equipment negotiated. It does not show why — signal quality and cable condition need dedicated tools to measure rather than infer.
Related pages#
- Transmission media — the overview, and how copper and glass compare with radio.
- Wireless media — the shared-channel problem a cable does not have.
- Transceivers and connectors — the modules and plugs that attach a device to these media.
- The OSI model — where physical signalling sits relative to framing and addressing.
- Network appliances — the switches and routers terminating these links.
Sources and further reading#
This page was edited from my own study notes, taken from Ian Neil’s CompTIA Network+ certification guide, and checked against the primary sources:
- IEEE 802.3 Ethernet Working Group — the wired Ethernet standards, port types, and media specifications.
- TIA-568 structured cabling — the category definitions and channel requirements, referenced rather than reproduced.
Structured cabling grades (ISO/IEC 11801, TIA-568) and plenum fire ratings (NEC Article 800 and the UL listings behind it) come from standards bodies that sell their documents rather than publishing them openly. For a specific cable or optic, the vendor datasheet and the applicable local code are the references that matter.