Copper, fiber and their limits
Copper twisted-pair (Cat5e/Cat6/Cat6a) is cheap and easy but limited to about 100 m per run and is vulnerable to electrical interference. Fiber carries light, so it goes far (hundreds of metres to many kilometres) and ignores electrical noise - used for long runs, between buildings and for high-speed backbones. Single-mode fiber goes furthest; multimode suits shorter data-centre links.
Speed, duplex and autonegotiation
Two ends of a link must agree on speed and duplex. Auto-negotiation usually handles this, but if one side is hard-set and the other auto, you get a duplex mismatch: the link 'works' but is slow, with late collisions and CRC errors piling up. Fix: set both ends the same (or both to auto).
Transceivers and connectors
Switch uplinks often use pluggable optics (SFP/SFP+/QSFP) matched to the fiber type and distance. Common connectors: RJ45 for copper Ethernet, LC/SC for fiber, and you must match the transceiver to the cable and the wavelength on both ends or the link stays down.
PoE and physical troubleshooting
Power over Ethernet delivers power and data on one cable to APs, phones and cameras. The switch has a total power budget; ask for more than it can supply and devices brown out. When something is 'dead', start at Layer 1: check the cable, the connector, link lights, the right port, the speed/duplex, and error counters before blaming IP or DNS.