Most network problems blamed on switches or software are actually cable faults. A split pair, a crushed run, a wiring mismatch, or a damaged patch cord can cause anything from complete link failure to a connection that drops under load. This guide gives you a systematic approach to finding the physical layer root cause.
Step 1: Read the Link LEDs
- No link light — no signal reaching the switch. Check patch cord, port, and NIC.
- Link at 10/100 Mbps instead of 1 Gbps — one or more pairs have an open or split pair. Gigabit requires all four pairs.
- Intermittent link — loose connector or cable pinched under furniture or door frames.
Step 2: Visual Inspection
Walk the run before connecting a tester. Look for: cracked plug boots, kinks, tight bends under door frames, zip ties crushing the outer jacket, and cracked or mis-punched keystone jacks.
Step 3: Cable Tester Wiremap
- Open — broken conductor. Wire cut or not contacting the termination.
- Short — two conductors touching. Usually a stray strand during termination.
- Crossed pairs — wiring standard mismatch (T568A one end, T568B the other).
- Split pair — wires from different pairs twisted together. A wiremap shows all pins connected correctly, but NEXT performance is destroyed. Requires a performance tester to catch.
Step 4: Performance Testing (Slow Speed Without Link Loss)
A wiremap pass does not guarantee performance. Common causes of speed problems that pass wiremap: split pairs (fail NEXT), excessive run length (fail insertion loss at 250 MHz), too many connections in series (TIA-568 allows max 4 per channel), and duplex mismatch between the switch and NIC.
Step 5: TDR for Long Runs
A Time Domain Reflectometer sends a pulse down the cable and measures the reflection from any impedance discontinuity. The reflection delay tells you exactly how many metres from the tester the fault is — invaluable for locating faults in in-wall runs without opening walls.
Frequently Asked Questions
Can a bad patch cord cause packet loss without link failure?
Yes. A patch cord with a cracked conductor or poorly crimped plug can maintain the Ethernet link while introducing intermittent bit errors that appear as packet loss and poor TCP throughput.
My tester shows pass but the connection is still slow. Why?
A basic wiremap only checks pin continuity, not performance. Use a certified field tester that measures insertion loss, NEXT, and return loss for a definitive answer.
How many patch cords can be in one channel?
TIA-568 permits four connections per channel: two at the work area and two at the TR. Each connection costs approximately 4 dB of insertion loss budget.