5.2 Given a scenario, troubleshoot common cabling and physical interface issues.

Question 1

Incorrect cable

Answer 1

1. single mode fiber (smaller core) vs. multimode fiber (bigger core)
2. Ethernet Cable (Copper) Category 5/6/7/8
3. Shielded Twisted Pair (STP) vs. Unshielded Twisted Pair (UTP)

Question 2

Signal Degradation

Answer 2

1. Crosstalk: signal on one circuit affects another circuit; signal is “leaking” into another wire. Most likely issue with the crimp or punch-down.
2. Interference: Electromagnetic Interference (EMI)
3. Attenuation: a loss of signal as it is sent over a certain distance.

Question 3

Improper Termination

Answer 3

• Cables can foul up a perfectly good plan
• Many connectors look alike.
• Near and far pins in cables aren’t where they’re supposed to be.

Question 4

Transmitter (TX) / Receiver (RX) transposed

Answer 4

• Wiring mistake at the cable end or the punchdown block.

Question 5

What is CRC?

Answer 5

• Cyclic Redundancy Check
• This adds a frame check sequence to an Ethernet frame.
• Receive the frame, recalculate the CRC, and compare to the original.
• The non-matching CRC is an error.

Question 6

What are Runts?

Answer 6

• Frames that are less than 64 bytes (may be a result of a collision).

Question 7

What are Giants?

Answer 7

• Frames that are more than 1518 bytes (or more than the configured maximum frame size).

Question 8

What are drops?

Answer 8

• frames not transmitted or received due to contention.

Question 9

Port Status

Answer 9

1. Error disabled (some problems should be stopped in their tracks; stopping the symptom not really the problem).
2. Administratively down (the device admin has “turned off” an interface intentionally).
3. Suspended (the configuration is not compatible with current connection; set LACP on one side, but not the other side).

Question 10

What is PoE as a Hardware issue?

Answer 10

• Power over Ethernet
  1. Power budget exceeded
  2. Incorrect standard
  3. Not backwards compatible

Question 11

Transceivers as a hardware issue.

Answer 11

1. Mismatch (need to match the type of fiber/copper; need to match the wavelength)
2. Signal Strength (Each device has a sensitivity level as some devices can “hear” better than others)

Question 12

What is NEXT?

Answer 12

• Near End Crosstalk
• Interference measured at the transmitting end.

Question 13

What is FEXT?

Answer 13

• Far End Crosstalk
• Interference measured at away from the transmitter.

Question 14

What is AXT?

Answer 14

• Alien Crosstalk
• Interference from other cables.

Question 15

What is ACR?

Answer 15

• Attenuation to Crosstalk Radio
• Difference between insertion loss and NEXT