Let’s look at some commonly accepted rules, standards and codes and the reasons behind them. Each rule is followed by some reasons. It’s your job to pick the real reason why the rule exists. Correct answers and explanations are in red.
1. Structured cabling standards are based on a maximum cable length of 100 meters.
A. It was an early attempt at metrification in the U.S.
B. An AT&T study of customer sites showed that was long enough.
C. That’s as far as early Ethernet or Token Ring could run on telephone wire.
D. IBM decided that was long enough.
AT&T’s 1982 survey of 79 businesses with 10,000 cable runs showed 99.9 percent of all runs were under 300 feet (~91m)
2. Never look into the end of a fiber because it will burn your retinas.
A. All fiber optic systems are hazardous to your eyes.
B. Fiber focuses optical power into your eyeball.
C. Your retina is more sensitive to IR light used in fiber optics.
D. Fiber was mistakenly equated with collimated laser light.
Only a few fiber optic systems are hazardous the way regular lasers can be, but caution requires treating all fiber optic systems as potential hazards.
3. Always properly ground any fiber optic cable with metallic conductors.
A. Fiber optic cable is sensitive to ground loops.
B. It reduces noise in the cables.
C. Any conductive cables can be hazardous.
D. Metallic sheaths are high resistance and require grounding.
Fiber is non-conductive and immune to ground loops or electrical noise, but any conductors in a fiber optic cable can be hazardous if energized.
4. Standards require testing fiber optics with a light source and power meter (LSPM), not an OTDR.
A. LSPM tests the fiber in the same manner as a system transmits light.
B. LSPM is much cheaper than the OTDR.
C. More installers have LSPM than OTDRs.
D. OTDRs give too much info. for most installers.
It’s the only way to test the fiber in the manner in which it will be used by a transmission system.
5. The TIA-568 standard allows fiber optic connector loss to be up to 0.75 dB.
A. It was written in the early 1990s when all fiber optic connectors had losses ~0.75 dB.
B. That loss is still marginal for prepolished/splice connectors.
C. Connector loss is not a big factor in short cables.
D. Manufacturers want a higher loss number to cover all connector types.
Typical adhesive/polish connectors generally have <0.3 dB loss and even prepolished/splice connectors are <0.5 dB if properly installed, but some new multifiber ribbon connectors are pushing the 0.75 dB limit.
6. The TIA-568 standard calls for testing cabling with a “Certification Tester.”
A. This guarantees the cable performance with specific networks.
B. This tests performance to the standard, not any specific application.
C. Certification testers are used to verify the standards and vice versa.
D. Certification testers test cabling faster than other ones
Since structured cabling standards are written to define cabling performance only to allow network designers to design any products to operate on that cabling, testing needs to be done to the standard, not any specific application.
7. Maintain the twists in each pair of UTP cable to within ½ inch (13 mm) of the termination.
A. It’s an arbitrary length specified by component manufacturers.
B. It makes for a neater installation.
C. It reduces crosstalk, return loss and most other termination problems.
D. Tools are designed to work properly with twisted wires only.
The twists in UTP cabling are the secret to its performance, so keeping those twists close to the termination is necessary to maintaining cabling performance.
8. Don’t pull UTP cable at more than 25 lbs. tension or kink it during installation.
A. It can change the twist rates of the pairs and affect performance.
B. It stretches the wires and increases resistance.
C. The cable jacket may wrinkle or tear.
D. Irregular pulling affects the delay skew more than anything else.
As above, maintaining the twist rates in the wire pairs is important to overall cable performance, so pulling cables must not damage the pairs.
9. The performance grade of any UTP link can be no higher than the lowest category-rated component in the link.
A. Connectors are more important than cable.
B. Cable is more important than connectors.
C. The rating of each component in the link affects the total performance.
D. Termination processes depend on the category rating of the components.
UTP cabling components (terminations, cable and even patchcords) are designed to work together to maintain cabling performance. If any component is rated lower than the others, it will reduce the performance of that link to its level.
10. Use coax cable with the maximum amount of shield coverage.
A. It simplifies termination with F connectors.
B. It reduces signal emission.
C. It reduces the cable’s sensitivity to interference.
D. It reduces signal emission and sensitivity to interference.
The quality of the shield on coax cable determines how well it does it’s job. Shielding the cable from external interference and reducing emissions from the cable.
HAYES is a VDV writer and educator and the president of The Fiber Optic Association. Find him at www.jimhayes.com.