Wired Network Troubleshooting Flashcards
Signal loss
- 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
Decibels (dB)
- 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
dB loss symptoms
- 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
Latency
- 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
Jitter
- 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
Troubleshooting excessive jitter
- 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.
Crosstalk (XT)
- 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
Troubleshooting crosstalk
- 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
Avoiding EMI and interference
• 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
Opens and shorts
- 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
Troubleshooting opens and shorts
- 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
Troubleshooting pin-outs
- 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
T568A and T568B termination
• 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
Incorrect cable type
- 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
Troubleshooting interfaces
- Interface errors
- May indicate bad cable or hardware problem
- Verify configurations
- Speed, duplex, VLAN, etc.
- Verify two-way traffic
- End-to-end connectivity
