Simple explanation
When the problem lives at Layer 1, this lesson covers the specific symptoms and causes to check for.
Technical explanation
- Attenuation — signal weakening over distance, worse on copper than fiber, and worse the longer/thinner/lower-quality the cable.
- Interference (EMI/crosstalk) — copper cabling is susceptible to electromagnetic interference from nearby power lines/equipment, and crosstalk between adjacent wire pairs within the same cable.
- Incorrect cable type — using a straight-through cable where a crossover is needed (largely mitigated by modern auto-MDI-X capable equipment, but still a real legacy/edge-case issue), or using the wrong category cable for the required speed.
- Damaged cables/connectors — physical wear, bent pins, or improperly terminated connectors causing intermittent or total failure.
- Duplex mismatch — one end configured for half-duplex, the other for full-duplex, causing collisions and severely degraded performance rather than total failure — a notoriously tricky issue because the link often still "works," just very poorly.
- Port status indicators — link/activity lights on a switch port are often the fastest first check: no light usually means a physical layer problem before anything else is even worth investigating.
- TX/RX reversed — transmit and receive wired backwards, common with improperly made custom cables.
- Bad SFP/transceiver — the module itself can fail independent of the cable.
Synonyms / related terms
| Term | Means | |---|---| | Attenuation | Signal weakening over distance | | Crosstalk | Interference between adjacent wire pairs | | Duplex mismatch | A subtle, performance-degrading (not connectivity-breaking) physical layer issue |
Concept Check
"A network link between two switches shows as 'up' with link lights active, but experiences unusually high collisions and poor throughput, though it doesn't fail outright." This pattern — link is up, but performance is degraded rather than absent — is a classic signature of duplex mismatch, not a simple bad cable (which would more likely cause an intermittent or fully down link rather than this specific "up but degraded" symptom pattern).
Interview-style Q&A
Q: Why check link lights before running any software-based diagnostic tool? A: "It's the fastest possible signal, and it rules out (or confirms) an entire category of problem in seconds. If there's no link light at all, there's no point spending time on ping, traceroute, or any Layer 3+ tool — the problem is physical, full stop, and no amount of software troubleshooting will fix a bad cable or a disabled port."
Memory trick
"No light, no Layer 1. Up but ugly, check Duplex." — two quick diagnostic heuristics tied to the two most common physical-layer symptom patterns.