sysop@hub:~/explainers/cable-guide$
cable-guide
Which cable for 10 Gbit, why the new access point reboots at the end of a long run, and what the colours on a fibre mean. The tables to look things up, one drawing to wire a plug, and a calculator for voltage drop.
copper: category, speed, distance
| Category | Speed at 100 m | Also | Typical use |
|---|---|---|---|
| Cat 5e | 1 Gbit | 2.5 Gbit usually | existing buildings |
| Cat 6 | 1 Gbit | 10 Gbit up to about 55 m, 5 Gbit at 100 m | offices built 2005 to 2015 |
| Cat 6A | 10 Gbit | PoE up to 90 W without trouble | new installations |
| Cat 7 / 7A | 10 Gbit | sold as cable; the link is certified as 6A with RJ45 | common in German-speaking countries |
| Cat 8 | 25 and 40 Gbit, only to 30 m | inside a data centre row |
- The category is printed on the jacket. A link is as good as its worst part: Cat 6A cable with Cat 5e sockets is a Cat 5e link.
- 100 m means 90 m of fixed cable plus 10 m of patch cables in total.
- CCA (copper-clad aluminium) is not a category. It breaks, heats up under PoE and fails certification. Buy solid copper for fixed runs.
- Stranded cable for patch cords, solid cable in the wall. Each needs its own kind of plug.
- New installations: Cat 6A. It carries 10 Gbit and the high PoE classes without getting warm in bundles.
wiring an RJ45 plug
- Untwist as little as possible, about 1 cm. The twist is what makes the cable work.
- The jacket must reach into the plug, under the strain relief.
- Test every hand-made cable. A split pair passes a simple continuity check and fails at gigabit.
- Crossover cables are history: every port since gigabit sorts it out itself.
fibre: types and colours
| Type | Jacket | Core | Reach |
|---|---|---|---|
| OM1, OM2 | orange | 62.5 or 50 µm multimode | 1 Gbit: 275 to 550 m. Old. |
| OM3 | aqua | 50 µm multimode | 10 Gbit: 300 m |
| OM4 | aqua or violet | 50 µm multimode | 10 Gbit: 400 m, 40 and 100 Gbit: 150 m |
| OM5 | lime green | 50 µm multimode | like OM4, more wavelengths |
| OS2 | yellow | 9 µm single-mode | 10 km and far more, any speed |
- Multimode and single-mode do not mix: the transceivers on both ends and the fibre between must match.
- Connector colour: blue is single-mode with a flat polish (UPC), green is angled (APC). Never plug green into blue: it damages both.
- LC is the small duplex plug on nearly every SFP. SC is the larger square one. MPO carries 8 or 12 fibres for 40 and 100 Gbit.
- Do not look into a fibre or a port. Keep the dust caps on: one grain of dust is a bad link.
- Bend gently: no tighter than a coffee mug.
DAC and AOC: the short links in the rack
| Cable | What it is | Length | Use it when |
|---|---|---|---|
| DAC, passive | copper cable with SFP+ or QSFP plugs fixed on both ends | up to 3 to 5 m | server to switch in the same rack: cheapest, least power, lowest delay |
| DAC, active | the same with amplifiers in the plugs | up to 7 to 10 m | the next rack |
| AOC | fibre with transceivers fixed on both ends | up to 30 m and more | thin and light where copper is too stiff |
| Transceivers and patch fibre | separate parts | any | everything longer, and whenever the two ends are different makers |
- Switches check the vendor code of what you plug in. A cable coded for one maker may be refused by another: order it coded for both ends.
voltage drop
A long, thin cable loses voltage on the way. PoE devices at the end of 90 m, door stations and 12 V cameras are where it shows: the device restarts under load.
rules for every cable
- Label both ends, the same on both, before the cable goes in.
- Data cables cross power cables at a right angle and do not run alongside them.
- Hook-and-loop ties, hand tight. A cable tie pulled hard changes the cable's geometry and its speed.
- Leave a little slack at both ends. The next person has to re-terminate it once.
- Outdoor runs need outdoor cable, and copper between two buildings needs surge protection, or better: fibre.
- Patch cables in three or four lengths and one colour per purpose beat a drawer of random ones.
Speeds and distances are those of the standards; a good installation often does more, a bad one less. The voltage drop is worked out for copper at 20 °C with the current the device draws at the lowered voltage, without connector losses: treat the result as the best case. PoE standards expect at most 12.5 ohms per pair loop on 100 m. Related: network hardware guide, PoE budget calculator, connector identifier, switch port finder, the technician's toolkit, rack planner.