Wired Network Troubleshooting

Question 1

Signal loss

Answer 1

• Usually gradual
  
  • Signal strength diminishes over distance
• Attenuation
  
  • Loss of intensity as signal moves through a medium
• Electrical signals through copper, light through fiber
  
  • Radio waves through the air

Question 2

Decibels (dB)

Answer 2

• Signal strength ratio measurements
  
  • One-tenth of a bel
  • Capital B for Alexander Graham Bell
• Logarithmic scale
  
  • Add and subtract losses and gains
• 3 dB = 2x the signal
• 10 dB = 10x the signal
• 20 dB = 100x the signal
• 30 db = 1000x the signal

Question 3

dB loss symptoms

Answer 3

• No connectivity
  
  • No signal!
• Intermittent connectivity
  
  • Just enough signal to sync the link
• Poor performance
  
  • Signal too weak
  • CRC errors, data corruption
• Test each connection
  
  • Test distance and signal loss

Question 4

Latency

Answer 4

• A delay between the request and the response
  
  • Waiting time
• Some latency is expected and normal
  
  • Laws of physics apply

• Examine the response times at every step along the
way
• This may require multiple measurement tools

• Packet captures can provide detailed analysis
  
  • Microsecond granularity
  • Get captures from both sides

Question 5

Jitter

Answer 5

• Most real-time media is sensitive to delay
  
  • Data should arrive at regular intervals
  • Voice communication, live video
• If you miss a packet, there’s no retransmission
  
  • There’s no time to “rewind” your phone call

• Jitter is the time between frames
• Excessive jitter can cause you to miss information,
“choppy” voice calls

Question 6

Troubleshooting excessive jitter

Answer 6

• Confirm available bandwidth
  
  • Nothing will work well if the tube is clogged
• Make sure the infrastructure is working as expected
  
  • Check queues in your switches and routers
  • No dropped frames
• Apply QoS (Quality of Service)
  
  • Prioritize real-time communication services
  • Switch, router, firewall, etc.

Question 7

Crosstalk (XT)

Answer 7

• Signal on one circuit affects another circuit
  
  • In a bad way
• Leaking of signal
  
  • You can sometimes “hear” the leak
• Measure XT with cable testers
  
  • Some training may be required

• Near End Crosstalk (NEXT)
• Interference measured at the transmitting end
(the near end)

• Far End Crosstalk (FEXT)
  
  • Interference measured away from the transmitter

Question 8

Troubleshooting crosstalk

Answer 8

• Almost always a wiring issue
  
  • Check your crimp
• Maintain your twists
  
  • The twist helps to avoid crosstalk
• Category 6A increases cable diameter
  
  • Increased distance between pairs
• Test and certify your installation
  
  • Solve problems before they are problems

Question 9

Avoiding EMI and interference

Answer 9

• Electromagnetic interference

• Cable handling
  
  • No twisting - don’t pull or stretch
  • Watch your bend radius
  • Don’t use staples, watch your cable ties

• EMI and interference with copper cables
• Avoid power cords, fluorescent lights,
electrical systems, and fire prevention components

• Test after installation
  
  • You can find most of your problems before use

Question 10

Opens and shorts

Answer 10

• A short circuit
  
  • Two connections are touching
  • Wires inside of a cable or connection
• An open circuit
  
  • A break in the connection
• Complete interruption
  
  • Can be intermittent

Question 11

Troubleshooting opens and shorts

Answer 11

• May be difficult to find
  
  • The wire has to be moved just the right way
  • Wiggle it here and there
• Replace the cable with the short or open
  
  • Difficult or impossible to repair
• Advanced troubleshooting with a TDR
  
  • Time Domain Reflectometer

Question 12

Troubleshooting pin-outs

Answer 12

• Cables can foul up a perfectly good plan
  
  • Test your cables prior to implementation
• Many connectors look alike
  
  • Do you have a good cable mapping device?
• Get a good cable person
  
  • It’s an art

Question 13

T568A and T568B termination

Answer 13

• Pin assignments from EIA/TIA-568-B standard
• Eight conductor 100-ohm balanced twisted-pair
cabling

• T568A and T568B are different pin assignments
for 8P8C connectors
• Assigns the T568A pin-out to horizontal cabling

• Many organizations traditionally use 568B
  
  • Difficult to change in mid-stream

• You can’t terminate one side of the cable with 568A
and the other with 568B
• It won’t be a straight-through cable

Question 14

Incorrect cable type

Answer 14

• Excessive physical errors, CRC errors
  
  • Check your layer 1 first
• Check the outside of the cable
  
  • Usually printed on the outside
  • May also have length marks printed
• Confirm the cable specifications with a TDR
  
  • Advanced cable tester can identify damaged cables

Question 15

Troubleshooting interfaces

Answer 15

• Interface errors
  
  • May indicate bad cable or hardware problem
• Verify configurations
  
  • Speed, duplex, VLAN, etc.
• Verify two-way traffic
  
  • End-to-end connectivity

Question 16

Transceiver mismatch

Answer 16

• Transceivers have to match the fiber
• Single mode transceiver connects to single mode
fiber

• Transceiver needs to match the wavelength
  
  • 850nm, 1310nm, etc.
• Use the correct transceivers and optical fiber
  
  • Check the entire link
• Signal loss
  
  • Dropped frames, missing frames

Question 17

Reversing transmit and receive

Answer 17

• Wiring mistake
  
  • Cable ends
  • Punchdowns
• Easy to find with a wire map
  
  • 1-3, 2-6, 3-1, 6-2
  • Simple to identify

• Some network interfaces will automatically correct
(Auto-MDIX)

Question 18

TX/RX reversal troubleshooting

Answer 18

• No connectivity
  
  • Auto-MDIX might connect
  • Try turning it on
• Locate reversal location
  
  • Often at a punchdown
  • Check your patch panel

Question 19

Damaged cables

Answer 19

• Copper cables are pretty rugged
  
  • But they aren’t indestructible
• Cables can be out in the open
  
  • Stepped on, folded between a table and wall
• Check your physical layer
  
  • Cables should not be bent or folded
  • Check for any bent pins on the device
• It’s difficult to see inside of the cable
  
  • Check your TDR, replace the cable (if possible)

Question 20

Bottlenecks

Answer 20

• There’s never just one performance metric
  
  • A series of technologies working together

• I/O bus, CPU speed, storage access speed,
network throughput, etc.
• One of these can slow all of the others down

• You must monitor all of them to find the slowest one
  
  • This may be more difficult than you might expect

Question 21

Interface configuration problems

Answer 21

• Poor throughput
  
  • Very consistent, easily reproducible
• No connectivity
  
  • No link light
• No connectivity
  
  • Link light and activity light

Question 22

Interface configuration

Answer 22

• Auto vs. Manual configuration
  
  • Personal preference
• Light status
  
  • No light, no connection
• Speed
  
  • Must be identical on both sides
• Duplex
  
  • If mismatched, speed will suffer

Question 23

VLAN mismatch

Answer 23

• Switch is configured with the incorrect VLAN
  
  • Configured per switch interface

• Link light, but no surfing
• A DHCP IP address may not be on the correct
subnet
• Manually IP addressing won’t work at all

• Check the switch configuration for VLAN
configuration
• Each port should have a VLAN setting
• VLAN 1 is usually the default

Question 24

Duplex/speed match

Answer 24

• Speed and duplex
  
  • Speed: 10 / 100 /1,000 / Auto
  • Duplex: Half / Full / Auto

• Incorrect speed
• Many switch configurations will auto-negotiate
speed
• Less than expected throughput

• Incorrect duplex
• Again, the switch may auto-negotiate
• Needs to match on both sides
• A mismatch will cause significant slowdowns
• Increase in Late Collisions may indicate a duplex
mismatch