Copper Wiring Troubleshooting Facts

Question 1

Electromagnetic interference (EMI) & Radio Frequency Interference (FRI)

Answer 1

• External signals that interfere with normal network communications

Question 2

Common sources of EMI/RFI

Answer 2

• Nearby generators
• motors ( such as elevator motors)
• radio transmitters
• welders
• transformers
• fluorescent lighting

Question 3

How to protect against EMI/FRI

Answer 3

• Use fiber instead of copper (fiber immune to EMI/RFI)
• Use shielded twisted pair cables (they have a metal foil that encloses all of the wires. some might include a drain wire (a bare wire in the cable that absorbs EMI/RFI)
• Avoid installing cables near EMI/RFI sources

Question 4

Crosstalk

Answer 4

• interference that is caused by signals within the twisted pairs of wires.
• Ex. current flow on one wire causing a current flow on an adjacent wire

Question 5

Crosstalk Facts

Answer 5

• Twisting of wires into pairs helps reduce crosstalk between wires
• Each pair of wires is twisted at a different rate to reduce crosstalk between pairs
• Crosstalk is often introduced within connectors, where the twists are removed to add the connector. Crosstalk can also occur where wires are crushed or where the plastic coating is worn

Question 6

Name three forms of crosstalk

Answer 6

• Near end crosstalk (NEXT)
• Far end crosstalk (FEXT)
• Alien crosstalk

Question 7

Near end crosstalk (NEXT)

Answer 7

• Is measured on the same end as the transmitter
• Ex. when a signal is sent on one wire, near end crosstalk measures the interference on an adjacent wire at the same connector end

Question 8

Far end crosstalk (FEXT)

Answer 8

• Measured on the opposite end from the transmitter
• Ex. when a signal is sent on one wire, far end crosstalk measures the interference on the adjacent wire at the opposite connector end

Question 9

Alien crosstalk

Answer 9

• Is introduced from adjacent, parallel cables

* Ex. a signal sent on wire causes interference on a wire that is within a separate twisted pair cable bundle

Question 10

Attenuation

Answer 10

• the loss of signal strength from one end of a cable to the other. This is also know a dB loss

Question 11

Attenuation Facts

Answer 11

• The longer the cable, the more attenuation. For this reason it I important never to exceed the maximum cable length defined by the networking architecture
• Cables at higher temperature experience more attenuation than cables at a lower temperature
• A repeater regenerates the signal and removes the effects of attenuation

Question 12

Open impedance mismatch (echo)

Answer 12

• the measure of resistance within the transmission medium

Question 13

Open impedance mismatch Facts

Answer 13

• Impedance is measured in ohms
• All cables must have the same impedance rating. The impedance rating for the cable must match the impedance of the transmitting device
• Impedance is mostly a factor in coaxial cables used for networking

Question 14

Open impedance mismatch Facts 2

Answer 14

• When signals move from a cable wit one impedance rating to a cable with another rating, some of the signal is reflected back t the transmitter, distorting the signal. With video (cable TV) impedance mismatch is manifested as ghosting of the image
• Cable distance does not affect the impedance of the cable

Question 15

Shorts

Answer 15

• Occurs when electrical signals take a path other than the intended path.
• In twisted pair wiring a short means that a signal sent on one wire arrives on a different wire.
• Shorts occur wen tow wires touch this can be caused by worn wire jackets, crushed wires, or a metal objects piercing two or more wires

Question 16

Open Circuit

Answer 16

• When a cut in the wire prevents the original signal from reaching the end of the wire
• Open circuit different from a short in that the signal stops ( electricity cannot flow because the path is disconnected)

Question 17

Miswired

Answer 17

• Caused by incorrect wire positons on both connectors

Question 18

Wiring Problems (Miswired)

Answer 18

• Reverse connection
• Wiremapping
• split pair

Question 19

Reverse connection

Answer 19

• when a cable is wired using on standard on one end and another standard on the other end, creating a crossover cable
• While this might be intentional, it can cause problems when a crossover cable is used instead of a straight through cable

Question 20

Wiremapping

Answer 20

• Refers to the matching of a wire with a pin in one end with the same pin on the other end
• Ex and error in the wiremapping results when the wire a pin 1 connects to pin 4

Question 21

Split pair

Answer 21

• condition is when a single wire in two different pairs is reversed a both ends
• Ex. if instead of he solid green wire, the solid brown wire is matched with the green/white wire in pins 1 and 2. with a split pair configuration, the cable might still work especially if it is short but it could introduce crosstalk

Question 22

Incorrect termination

Answer 22

• Occurs when an incompatible or incorrect connector is used. This can result in reduced performance or complete connection lost

Question 23

Bad connector

Answer 23

• is a damaged connector that is casing connectivity issues.

* Ex a broken locking ab on a RJ45 connector can cause intermittent connection problems