Size the run before you pull it: shielding, power, loss. Then prove it and look it up.
The code reads X/YTP: the shield around all cables, then the shield around each pair. Choose the code to see where it belongs.
Four naked twisted pairs in a jacket. Right for homes and clean offices. Cheapest, thinnest, easiest to terminate. It loses to motors, VFDs, fluorescent ballasts and 200-cable bundles. That is when you pay for foil.
Grounding rule: bond shielded cable at one end, usually the patch panel. Ground both ends and you build a ground-loop antenna that performs worse than unshielded cable.
Heat is the enemy: keep PoE bundles at 24 cables or fewer, prefer 23 AWG Cat 6A, never bury bundles in insulation.
Eight questions. No partial credit in the field.
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Lower is thicker. 23 AWG beats 24 AWG for PoE heat and attenuation. Cheap CCA (copper-clad aluminium) is banned from the standards, so buy pure copper.
Near-end crosstalk: a pair shouting into its neighbour. Alien (AXT): neighbouring cables shouting in. Cat 6A thickness is mostly alien-crosstalk armour.
The permanent link is the in-wall 90 m. The channel is the link plus 10 m of patch cords. Certify the right one for the warranty.
Ports that detect straight against crossover and adapt. Universal since gigabit, so crossover cables are now exam trivia.
Keystones (punch-down) for solid in-wall cable. For plugs on solid: staggered or feed-through RJ45s. Standard plugs are tuned for stranded.
Labour dwarfs cable cost. The second drop carries a phone, access point, HDMI extender or redundancy, for about 5 dollars of copper while the walls are open.
OS is Optical Single-mode (9 µm core, yellow, kilometres). OM is Optical Multimode (50 µm core, aqua or lime, hundreds of metres). A bigger OM number means more modal bandwidth, not a different size.
UPC (blue, flat) for datacom. APC (green, 8-degree angle) kills back-reflection for PON, RF video and long-haul. Never mate green to blue.
Logarithmic: 3 dB means half the light is gone. A 0.5 dB dirty connector eats about 11 percent of the signal. Two of them plus margin can sink an SR link.
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