Troubleshooting physical networks
Cable limitations
- Cat5 (100BASE-TX): 100 Mbps, 100 m
- Cat5e (1000BASE-T): 1 Gbps, 100 m
- Cat6 (1000BASE-T / 10GBASE-T): 1 Gbps at 100 m, 10 Gbps at 55 m
- Cat6a/7 (10GBASE-T): 10 Gbps, 100 m
- Cat8 (40GBASE-T): 40 Gbps, 30 m
- Coax: 100 Mbps, 500 m
- Twinax: 10 Gbps at 5 m (100 Gbps at 7 m on newer)
- MMF (1000BASE-SX / 10GBASE-SR): 1 Gbps at 220-500 m, 10 Gbps at 400 m
- SMF (1000BASE-LX / 10GBASE-LR): 1 Gbps at 5 km, 10 Gbps at 10 km
Shielded costs more but fights EMI (fiber is truly immune). Plenum cable goes in horizontal air spaces and carries a high fire rating; riser runs vertically between floors in non-plenum areas.
Special cables: rollover/console (null-modem to a router console for out-of-band), crossover (device-to-device without a switch). PoE needs Cat5e minimum.
Signal issues
Attenuation: the signal weakens over distance (twisted pair around 100 m, coax around 500 m; fiber goes far but dirty or cheap connectors hurt). The factors: distance, frequency, environmental noise, physical surroundings. The fix: the right cable, shorter runs, amplifiers and repeaters.
Interference comes from neighboring cables in the same frequency band; use higher-category cable and don’t run data beside power.
dB loss quantifies deterioration (voltage on copper, light on fiber). The tools: a cable certifier (attenuation and dB on copper), fiber light meter, spectrum analyzer (interference), cable analyzer.
Copper faults
Incorrect pinouts happen at the patch panel (check 568B color order on the punchdown), the wall jack or keystone, or the RJ-45 (pins 1-8, clip down, left to right). Verify with a cable tester or wire map; re-punch or re-crimp to fix.
Bad ports: test NICs and switch ports with a loopback plug and software; replace what fails.
An open is a break in the wire; a short is two wires touching. Rewire the connector or replace the damaged cable.
Fiber faults
Wrong transceiver type (they must match; most are hot-pluggable), reversed transmit/receive (swap the pair), and dirty connectors. Tiny dust blocks light: dry-clean with a lint-free cloth in one direction, wet-clean with 91%+ isopropanol for fingerprints. A light meter’s dB reading tells you when it’s clean.
Ethernet issues
NIC LEDs: activity light off means no link, solid means link, blinking means traffic. The speed light: off is 10 Mbps, orange 100 Mbps, green 1 Gbps.
Duplex mismatch (one side full, one half) is the classic: packet loss without jitter, high receive errors, runts. Fix it with autonegotiation on both ends or matching manual config; switches want full duplex.
Interface counters and port states
- CRC errors: the integrity check fails (noise, physical faults).
- Runts: under minimum size (collisions, a bad NIC).
- Giants: over max size (misconfig or device malfunction).
- Drops: buffers full, traffic beyond capacity.
Port states: error disabled (auto-shutdown from an error or policy violation), administratively down (deliberate), suspended (protocol or policy violation).
PoE issues
PoE (802.3af) delivers up to 15.4 W; PoE+ (802.3at) up to 30 W. Some power is lost in the run.
Power budget exceeded means total device demand beats the switch’s supply: shed devices or upgrade. Incorrect standard means the device and switch mismatch: replace the switch or add a matching PoE injector. The symptoms either way: random restarts, erratic behavior, dead devices.