1. Chapter 12-WLAN Troubleshooting
Layer 2 retransmission
Coverage Considerations
Voice vs. Data
Performance
Weather

2. WLAN Troubleshooting
Similar troubleshooting techniques as compared to 802.3
Bottom up
Check physical layer first-Layer 1
Power, drivers, etc
Spectrum Analyzer to troubleshoot layer 1 with wireless
Layer 2 issues
Authentication and association
Settings on client and AP
Use a protocol Analyzer
Pg 390

3. Layer 2 retransmissions
Retransmissions mean problems
Each unicast frame is followed by an ACK
No ACK causes retransmission, no matter what the cause
Interference, not received, collision, etc
Retransmissions reduce throughput
VoIP needs smooth flow
Latency-delay is bad
Jitter-variable arrival is bad
Pg 390

4. Layer 2 retransmissions
Most apps don’t mind up to 10% retransmission rate
VoIP needs less than 2 percent
Use a protocol analyzer to check retry statistics
Possible causes
Multipath, RF interference, and low SNR are problems that exist at layer 1 yet result in layer 2 retransmissions.
hidden node, near/far, mismatched power settings, and adjacent cell interference
usually a symptom of improper WLAN design.
Pg 391

5. RF Interference Denial of service Interference
If another source is steadily sending signals, the Clear Channel Assesment (CCA), physical carrier sense, will never find the medium open, and will never send.
Interference
Other sources will cause enough noise so that signal is jumbled
CRC won’t check, no ACK sent
Pg 391

6. RF interference Narrowband interference Wideband interference
Usually won’t cause DoS for whole 2.4 Ghz band, but can affect one or more channels
Find the source with a spectrum Analyzyer
Check the frequency and then choose channels that aren’t affected
Wideband interference
If the signal can affect an entire frequency band
Jamming hardware exists
Locate the sources and remove
All band interference
FHSS causing problems with DSSS
Pg 391

7. Interference
Pg 393

8. Interference
Pg 393

9. RF interference Cordless phones Microwaves Video cameras Bluetooth
Other Networks
Pg 394

10. Multipath Caused by variable arrival time in reflected signals
Delay spread is the difference in time
Depending on phase, can cause increase or reduction in power
Causes Intersymbol Interference
If there was no RF interference evident on analyzer, could be multipath
Cannot really be “fixed”
Pg 394

11. Multipath Problems can be limited by use of antenna diversity
Also, OFDM is more resilient to multipath
802.11a and g
802.11n-clause 20 uses HT and MIMO which take advantage of multipath
Pg 394

12. Adjacent Cell interference
Adjacent channel interference
When signal is degraded by the use of an overlapping channel frequency
In 2.4 Ghz, limited number of non-overlapping channels
1, 6, Mhz of space between centers
5 channel separation
2, 7; 3, 8; 4, 9; 5, 10
Pg 396

13. Adjacent Cell interference
Good design needs overlapping coverage between cells
Must make sure that the channels don’t overlap as well
Adjacent cell interference
Pg 396

14. Adjacent Cell interference
5 Ghz range has more options
Technically each channel is non-overlapping
23 channels also gives you more space
Pg 396

15. Low SNR Signal to Noise ration is important to good reception
If background noise is too close to received signal, data can be corrupted
Difference in decibel between the received signal and background noise (noise floor)
25 dB or more is considered good
Less than 10dB is poor
Pg 397

16. Low SNR
Pg 397

17. Mismatched Power If client and AP have different power levels
If client can hear AP, but AP can’t hear client
If client power is too low, the AP won’t get a signal above noise floor
Try to keep all devices as similar power levels
Use protocol analyzer to check
Pg 398

18. Mismatched Power APs are often set to full power Antenna reciprocity
Not the best way to increase range
Use antenna gain
Antenna reciprocity
Antennas will amplify transmitted and received signals
Pg 398

19. Mismatched Power Power is often controlled by WLAN controller
Can cause problems
Common issue with VoWiFi
Pg 398

20. Near/Far Is power on Clients is too different
High power client closer to AP can drown out low power client farther away
Usually prevented by CSMA/CA
Pg 400

21. Hidden Node CSMA/CA is based on all stations hearing each other
If one client can’t hear the transmissions of another, they won’t set NAV timer to duration
Will try to access medium at same time
Causes errors/interference
Hidden Node is when a client’s transmissions are heard by the AP, but not the other clients
Interference or obstacles
Distances in cell coverage
Pg 400

22. Hidden Node
Pg 400

23. Hidden Node
Pg 400

24. Hidden Node
Pg 400

25. Hidden Node
If there is a decrease in throughput, hidden node could be cause
If one station has higher retransmission rate that others, it could be caused by hidden node
Can also use RTS/CTS
If setting station to use RTS/CTS fixes throughput, it may have been hidden node
Everyone will hear the CTS from the AP
Pg 400

26. Hidden Node Use RTS/CTS to diagnose. Increase power to all stations.
Use either a protocol analyzer or RTS/CTS to diagnose the hidden node problem. RTS/CTS can also be used as a temporary fix to the hidden node problem.
Increase power to all stations.
If power output is adjustable on the client side, increasing the transmission power of client stations will increase the transmission range of each station. If the transmission range of all stations is increased, the likelihood of the stations hearing each other also increases.
Remove the obstacles.
If it is determined that some sort of obstacle is preventing client stations from hearing each other, simply removing the obstacle will solve the problem. Obviously, you cannot remove a wall, but if a metal desk or fle cabinet is the obstacle, it can be moved to resolve the problem.
Move the hidden node station.
If one or two stations are in an area where they become unheard, simply moving them within transmission range of the other stations will solve the problem.
Add another access point.
The best fix for a continuous hidden problem is to add another AP. If moving the hidden nodes is not an option, adding another access point in the hidden area to provide coverage will also rectify the problem.
Pg 400

27. 802.11 Coverage considerations
Need to plan for correct coverage and capacity
Larger cells aren’t always better
Pg 404

28. Dynamic Rate Switching
Access rate is dependant on signal quality
Client stations will monitor traffic quality and switch to lower rates as needed
Use RSSI
Proprietary
There is a correlation between distance from AP and rate
Lower rates have wider coverage
Pg 405

29. Dynamic rate Switching
Pg 405

30. Dynamic Rate Switching
Recommended to turn off 1 and 2 Mbps rates
They have too wide a coverage area for faster networks
Will make the network seem too busy for other clients
Pg 405

31. Dynamic rate Switching
Pg 405

32. Roaming Like DRS, the decisions are handled by client.
Proprietary solution
Related to SNR and RSSI
Problems usually related to poor network design
Client initiates roaming with reassociation request
Pg 407

33. Roaming 15 to 25% overlap between cells is recommended
Different channels though
If too much overlap, can cause a system NOT to roam when it should
Latency is also an issue
It can take 700 ms to authenticate via 802.1x/EAP
Too long a delay for SOME traffic
VoIP
Fast Secure Roaming (FSR) is in the works
Pg 407

34. Roaming Troubleshooting roaming can be hard
May need multiple channels to be checked at once
Pg 407

35. Layer 3 Roaming Generally interrupts service
Mobile IP can prevent loss
Pg 409

36. Layer 3 Roaming
Pg 409

37. Co-Channel interference
Same channel on adjacent cell
Co-channel cooperation
Pg 409

38. Co-Channel interference
Try to arrange non-overlapping channels
Adjacent channel problems are worse than co-channel
Pg 409

39. Channel Re-use/MCA Try to arrange non-overlapping channels
Most ESS are set up this way
Try to make sure there are two cells between reused channel
Be aware of all 3 dimensions
Pg 412

40. Channel Re-use/MCA
Make sure coverage only extends one floor in either direction
Pg 415

41. Single Channel Architecture
Currently proprietary solutions
All cells use same channel
All APs uses same BSSID
Clients think the ESS is one giant Ap coverage area
Virtual AP
WLAN controller manages the clients as they roam through network
Pg 415

42. Single Channel Architecture
Pg 415

43. Single Channel Architecture
Client no longer makes roaming decision
Should provide for zero handoff time
No Co-channel interference since the SCA WLAN controller watches all traffic
Tend to turn off 1 and 2 Mbps rates
Pg 415

44. Capacity vs. coverage
The more wireless devices, the more APs and smaller the coverage area you want
Trying to limit the number of devices per AP at a time
Determine how many clients you want per AP
Then try to manage cell size
Can also co-locate to allow more clients in an area
Physical separation
Pg 418

45. Capacity vs. coverage Collocation not so helpful in MCA with reuse
Scales well in SCA
Pg 418

46. Capacity vs. coverage Oversized cells Physical environment
Watch out for full power APs
Can also cause hidden node because of power mismatch
Physical environment
Obstructions can affect the RF data
Fences, metal cabinets, safety windows
Remove obstructions when possible
Pg 421

47. Voice vs. Data Voice is more sensitive to delay
Sensitive voice data is more likely to suffer when there are design issues
Pg 421

48. Performance Transmission power rates Antenna gain Antenna type
Wavelength
Free space path loss
Physical environment
Carrier Sense Multiple Access with Collision Avoidance (CSMA/CA)
Encryption
Application use
Number of clients
Layer 2 retransmissions
Pg 423

49. Weather
Lightning
Wind
Water
Air Stratification
UV/Sun
Pg 424

50. The End