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
Transceiver mismatch
• 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
Reversing transmit and receive
- 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)
TX/RX reversal troubleshooting
- No connectivity
- Auto-MDIX might connect
- Try turning it on
- Locate reversal location
- Often at a punchdown
- Check your patch panel
Damaged cables
- 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)
Bottlenecks
- 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
Interface configuration problems
- Poor throughput
- Very consistent, easily reproducible
- No connectivity
- No link light
- No connectivity
- Link light and activity light
Interface configuration
- 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
VLAN mismatch
- 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
Duplex/speed match
- 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
