Ethernet cables: copper vs fiber

Copper: electricity over twisted pairs

The cable plugged into your laptop, game console or office desk is almost always copper. Inside the plastic jacket are eight thin wires, twisted together in four pairs. Data travels as tiny changes in voltage along those wires.

The twisting isn't decoration: it helps each pair cancel out some of the electrical noise around it. Copper cables are cheap, flexible and easy to plug in with the familiar RJ45 connector. The catch is distance: standard twisted-pair Ethernet, such as 1000BASE-T, is rated for runs of up to 100 meters.

Fiber: light in a strand of glass

A fiber optic cable has a core of glass about as thin as a human hair. At one end, a laser (or, on some short links, an LED-like light source) flashes on and off very fast: light means one value, dark means another. The light stays inside the glass by bouncing off its walls, an effect called total internal reflection, until it reaches a light detector at the other end.

Why fiber goes farther

Light in glass fades much more slowly than an electrical signal in copper, so fiber reaches much farther. Multimode fiber, with a wider core, typically covers hundreds of meters, for example across a building. Single-mode fiber, with a very narrow core, reaches 10 kilometers and beyond, between buildings and cities.

And because fiber carries light rather than electricity, electrical noise from motors, power lines or fluorescent lights can't interfere with it. That makes it a good fit for factories and long runs next to power cables.

Which one goes where

• Short runs to a desk, a printer or a wall jack: copper. It's cheaper and simpler.
• Long runs between floors, buildings or cities, or anywhere electrically noisy: fiber.

Many networks use both: copper from each desk to a nearby switch, and fiber linking those switches back to the core of the network.

Try it yourself

Look at the cable going into your computer or router. If the plug is a wide plastic clip with eight gold contacts, it's copper twisted pair (RJ45). Check the jacket too: most are printed with the category, such as CAT5e or CAT6. If your internet service is "fiber to the home", the box on your wall where the provider's line comes in takes a thin fiber, while the cables inside your home are usually still copper.

Good to know

• 100 m is the standard limit for twisted-pair Ethernet, such as 1000BASE-T.
• Fiber reach depends on the type: multimode is hundreds of meters, single-mode reaches 10 km and beyond.
• Fiber guides light by total internal reflection.
• Many modern optics use lasers; some multimode links use VCSELs or LEDs.

For CCNA learners

This lines up with CCNA 200-301 exam topic 1.3, compare physical interface and cabling types: know that copper twisted pair is limited to about 100 m, that multimode fiber suits shorter runs and single-mode fiber long distances, and that fiber is immune to electromagnetic interference.