Cable Testing: Wiremap, TDR, and Reading What the Tester Says
Before you start testing your network cables, it's important to understand what each type of tester can do and how to interpret its results. This guide covers continuity/wiremap testers, cable qualifiers, and certifiers, as well as common issues that might affect your cable performance.
Continuity/Wiremap Testers ($10-30)
Continuity/wiremap testers are the simplest tools for basic cable testing. They check if all 8 wires are properly connected to their corresponding pins on both ends of a cable.
How It Works
- Remote Unit: Clips onto the far end of the cable.
- LED Patterns:
- All LEDs Light in Order (1-8): Passes continuity test.
- One LED Dark: Indicates an open pair, which is the most common fault.
- Two LEDs Out of Order: Shows a miswired or crossed pair (usually a 568A/B mixup).
- Simultaneous Wrong Pairs: Suggests a split pair, where wires from different twisted pairs are swapped.
Common Issues
- Untwisted Wires: If the wires are untwisted too far back (>13mm), it can fail Cat6 standards.
- Damaged Cable: Staples, sharp bends, or chair wheels can damage the cable.
- Split Pairs: Hand-punched jacks can cause split pairs.
- CCA Cable: Copper-clad aluminum cables have high resistance and may not support Power over Ethernet (PoE).
Cable Qualifiers ($100-400)
Cable qualifiers go beyond continuity testing by evaluating the signal quality of your cable.
How It Works
- Time Domain Reflectometry (TDR): Sends a pulse down the cable, measures reflections to determine distance and faults.
- Length Measurement: Measures the length of the cable accurately, which is useful for pinpointing issues inside walls or conduits.
Certifiers ($1000s)
Certifiers like the Fluke DSX are used by installers to ensure cables meet specific standards. They test various parameters including insertion loss and crosstalk.
How It Works
- Insertion Loss: Measures signal strength at each pin.
- NEXT Crosstalk: Tests for interference between pairs of wires.
- Return Loss: Ensures that signals are not reflected back along the cable.
Common Issues in Real Life
- Bad Terminations: Untwisting wires too far can cause issues, especially with Cat6 standards.
- Damaged Cable: Physical damage from staples or sharp bends is common.
- Split Pairs: Hand-punched jacks often cause split pairs.
- CCA Cable: Copper-clad aluminum cables have high resistance and may not support PoE.
Free Tests Before Any Tester
Before using a tester, you can perform some basic checks:
- Link Light: Ensure the link is up (Layer 1).
- Negotiated Speed: Check if the port is negotiating full speed.
- Swap-the-Cable Test: Swap cables to quickly identify faulty ones.
When It's Not the Cable
If you have a working link light, full speed negotiation, and incrementing errors on switch counters, it might be interference or a dying port/SFP. Always check these counters before recrimping your cable.
By understanding these tests and common issues, you can ensure that your network cables are functioning properly and efficiently.