10 Network Troubleshooting Tips
Industrial networks can experience communication difficulties, and Adrian Young, senior customer support engineer with Fluke Networks, helps correct them. Here, Young offers concrete advice to avoid network communication difficulties and provides 10 troubleshooting tips for when cabling trouble occurs with either copper (twisted pair) or fiber optic networks.
Copper, twisted pair
1. Use the appropriate test equipment to collect information from the network. If the test fails, run the diagnostics. Using advanced diagnostics, like Fourier analysis, one can isolate whether the challenge is within a connection or the cable. In tough situations, a customer support engineer can analyze the report information and let you know if it suggests a connection or cable issue. For instance, twisted pair cables need to be twisted right up to the M12 connector to avoid crosstalk issues.
2. Check for return loss. In 100BASE-TX and ODVA EtherNet/IP Ethernet protocols, return loss can come from either connectors or cable. A typical, $100 continuity wire map tester won’t identify return loss issues. Use of a qualification or certification tester will. In one example, the return loss measurement indicated 2.6 m of bad cable in a cable run at 34.4 m from a connector.
3. Ensure the cable stays dry. One might think cable resists water, but moisture can get in at connection points, he says. Fluke Networks receives two or three calls a week due to cable with moisture damage. In one example, rain filled a conduit under a resort concrete floor. The Return loss trace is unique in this situation, and a high definition time domain reflectometer (HDTDR) trace can locate the water in a cable. In one example, it was 18.6 m from the tester location. Note that:
Cable cannot be dried out, but must be replaced after conduit is dried;
Blowing out conduit with air may not work;
Drawing a water-absorbent cloth (ShamWow or similar) through the conduit works well; and
Note gel-filled Cat5e is NOT allowed for in-building use.
4. Consider insertion losses. Patch cables can have up to 50% more insertion loss than solid cable. A cable run of 100 m may not be reliable, if it uses many patch cords. ANSI/TIA-1005 standard says stay within a 90 m (295 ft) run of solid cable. Then, up to 10 m (33 ft) of equipment and user patch cords can be used.
5. Remember: higher temperature can decrease reliability. As temperature increases so does insertion loss: There’s a 0.4% increase in loss per degree C. The standard doesn’t allow changes in the test limits. The network design must account for that.
6. Installation materials may appear to affect cabling performance. Lubricants used for pulling wire through conduit can create failures when testing immediately. Dramatic failures on insertion loss only are a lubricant issue. The HDTDR trace will look fine. With time, the lubricant will cure, and the insertion loss will improve.
7. Look at dc resistance. DTX Cable Analyzer can compare measurements with limits found in 100Base-TX IEEE 802.3 for example; dc resistance isn’t a requirement, and although the value is recorded, there is no PASS/FAIL criteria.
Note: In one troubleshooting example, readings showed 25.1 Ohms versus 6.7 Ohms. The higher value caused significant network errors, due to an M12 connection to a patch cord, causing a problem for the connected robots in the automotive industry.
Fiber optic cable
8. Check the fiber connectors. Most communication challenges are not in the cable, but in the connectors or termination points. For example:
Field polishing fiber-optic cable can create many errors. Field termination and polishing can vary widely in quality, depending on installer skills. Over polishing can create an air gap causing errors on the network.
Better: Pre-polished factor terminated connectors;
Best: Factory terminated connectors have less than 50 nm reflectance; and
Reflectance measurements aren’t required in the standard, but -35 dB is good value to start.
Remember, just putting a hand into the back of a panel can crack a fiber cable; a visual fault locator can locate a break or bend.
9. Clean carefully. Ethernet fiber requires careful cleaning and inspection for top performance. A simple fiber scope shows the fiber end face. A fiber video microscope can show how bad a fiber end face is.
Among common cleaning errors is use of isopropyl alcohol less than 98%, which can leave residue and dirt;
Hybrid cleaners evaporate more quickly, are more aggressive for cleaning, and have antistatic properties; and
Dirty connectors can create cross contamination with mated connectors.
10. Be careful not to add loss during network design. OM3 fiber can support 10GBASE-S to 300 m, and new OM4 fiber can go 550 m, but do the math, because performance requirements can vary widely depending on the application.
Young gave this advice at a Nov. 11, 2009, session at Automation Fair in Anaheim, CA, an event sponsored by Rockwell Automation.