5.7 - Troubleshooting Networks

Question 1

No network connectivity

Answer 1

• Do you have a link light?
  – Is it plugged in?
• Ping loopback (127.0.0.1)
  – Is the protocol stack working?
  – Availability and intermittent connectivity
• Ping local IP address
  – Checks local configuration, adapter, and link signal
• Ping default gateway
  – Connectivity on the local network
• Ping devices on router’s other side
  – 8.8.8.8 or 9.9.9.9

Question 2

Intermittent wireless connectivity

Answer 2

• Interference
  – Something else is using our frequency
• Signal strength
  – Transmitting signal, transmitting antenna,
  receiving antenna, etc.
• Incorrect channel
  – Usually automatic; look for manual tuning
• Bounce and latency
  – Multipath interference; flat surfaces
• Incorrect access point placement
  – Locate close to the user

Question 3

Wireless interference

Answer 3

• Predictable
  – Fluorescent lights
  – Microwave ovens
  – Cordless telephones
  – High-power sources
• Unpredictable
  – Multi-tenant building
• Measurements
  – Signal to noise ratio (SNR)
  – Performance Monitor

Question 4

Signal to noise ratio (SNR)

Answer 4

• Signal
  – What you want
• Noise
  – What you don’t want
  – Interference from other networks and devices
• You want a very large ratio
  – The same amount of signal to noise (1:1)
  would be bad

Question 5

Limited or no connectivity

Answer 5

• Windows alert in the system tray
  – “Limited or No connectivity”
  – “No Internet Access”
• Check the local IP address
  – An APIPA address will only have local connectivity
• If DHCP address is obtained, perform the ping tests
  – Local gateway, remote IP address

Question 6

Jitter

Answer 6

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 7

Poor VoIP quality

Answer 7

• High speed and low latency
  – Real-time applications are demanding
• Check the Internet connection
  – A speed test can identify slow links
• Verify the local networking equipment
  – An old router can cause significant problems
• View the network performance
  – A packet capture would be useful

Question 8

Latency

Answer 8

• 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 9

Port flapping

Answer 9

• Network interface goes up and down
  – Over and over again
• Verify the cable
  – Check the wiring
• Move between switch interfaces
  – Is the flapping associated with the switch
  interface or the device?
• Replace bad hardware or cables
  – May require additional purchases