Cables and connectors: copper, RJ45 and coax
Bits need something to travel on. Open up an Ethernet cable to find four twisted pairs of copper wire, see why the twist matters, meet the RJ45 plug and its two wiring orders, learn what Cat5e, Cat6 and Cat6a mean and why a run stops at 100 metres, then compare coax, USB-C and HDMI.
5 quick questions
Pick an answer to see why. Your score saves with your progress.
01Why are the wires in an Ethernet cable twisted in pairs?
Each pair carries the signal as a difference between its two wires. Noise adds almost equally to both, so subtracting one from the other removes it.
02What is the difference between T568A and T568B?
Only the orange and green pairs change places. Use the same order at both ends and the cable works either way.
03You need 10 Gb/s over an 80 m run. Which cable category should you choose?
Cat5e is rated for 1 Gb/s, and Cat6 reaches 10 Gb/s only to about 55 m. Cat6a carries 10 Gb/s for the full 100 m.
04What is the longest a single twisted-pair Ethernet run (channel) may be?
100 m in total: up to 90 m of cable in the wall plus 10 m of patch cords. For longer distances you add a switch or use fibre.
05Which layer of a coax cable keeps outside noise away from the signal?
The shield wraps all the way round the insulator and the centre wire, keeping noise out and the signal in.
Glossary
- Twisted pair
- Two insulated copper wires twisted around each other. An Ethernet cable has four of them.
- RJ45 (8P8C)
- The clear 8-pin plug on an Ethernet cable. 8P8C means 8 positions, 8 contacts.
- T568A / T568B
- The two standard orders for which wire colour goes on which pin of an RJ45 plug.
- Cable category
- A rating such as Cat5e, Cat6 or Cat6a that says how fast a cable can carry data, and how far.
- Patch cord
- A short cable with a plug on each end, for example from a laptop to a wall socket.
- PoE
- Power over Ethernet: sending electrical power along the same cable as the data.
- Coaxial cable
- A round cable with a centre wire, an insulator, a metal shield and a jacket, used for cable TV and internet.
- F-connector
- The screw-on connector used for coax; the coax centre wire itself is its pin.
Notes and sources
- Signal speed uses the same rule of thumb as "Bits on the wire": about 2/3 of the speed of light, 5.00 µs per km; real twisted pair is about 0.64-0.70 c.
- Categories follow ANSI/TIA-568.2-D. 10GBASE-T (IEEE 802.3an) reaches about 55 m on Cat6 (TIA TSB-155) and 100 m on Cat6a.
- PoE figures are per port: power sent by the switch, then the smaller amount guaranteed at the device after cable losses (IEEE 802.3af/at/bt).
- Strictly, RJ45 is an old telephone jack name; the Ethernet plug is an 8P8C modular connector (IEC 60603-7) wired to TIA-568 T568A or T568B.
- The plug, the cut-open pairs and the coax are drawn magnified; beads stand for the signal and are slowed down billions of times.
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