Chapter 12-WLAN Troubleshooting

1. Chapter 12-WLAN Troubleshooting • Layer 2 retransmission • 802.11 Coverage Considerations • Voice vs. Data • Performance • Weather

2. Exam Essentials • Explain the causes and effect of Layer 2 retransmissions. • Understand that layer 2 retransmissions can be caused by multipath, hidden nodes, mismatched power settings, RF interference, low SNR, near/far problems, and adjacent cell interference. Layer 2 retransmissions affect throughput, latency, and jitter. • Define dynamic rate switching. • Understand the process of stations shifting between data rates. Know that dynamic rate switching is also referred to as dynamic rate shifting, adaptive rate selection, and automatic rate selection. Explain why disabling the two lower 802.11b/g data rates is often recommended.

3. Exam Essentials • Explain the various aspects of roaming. • Understand that roaming is proprietary in nature. Know the variables that client stations may use when initiating the roaming process. Understand the importance of proper coverage cell overlap. Describe latency issues that can occur with roaming. Understand why crossing layer 3 boundaries can cause problems and what solutions might exist. • Define the differences between adjacent channel interference and co-channel interference. • Understand the negative effects of both adjacent cell interference and co-channel interference. Explain why channel reuse patterns minimize the problems. Know what to consider when designing channel reuse patterns at both 2.4 GHz and 5 GHz in a multiple channel architecture.

4. Exam Essentials • Explain the differences between MCA and SCA wireless LAN design. • Understand that MCA uses cell sizing to meet capacity needs, whereas SCA uses channel stacking to meet capacity needs. Explain the virtual BSSID and other aspects of an SCA design. • Identify the various types of interference. • Know the differences between all-band, narrowband, wideband, physical, and intersymbol interference. Understand that a spectrum analyzer is your best interference-troubleshooting tool

5. Exam Essentials • Explain the hidden node problem. • Identify all the potential causes of the hidden node problem. Explain how to troubleshoot hidden nodes as well as how to fx the hidden node problem. • Define the near/far problem. • Explain what causes near/far and how the problem can be rectifed. • Identify performance variables. • Explain all the variables that affect both the range of RF coverage and the throughput that can result within a basic service set. • Understand the consequences of weather conditions. • Explain the problems that might arise due to water conditions, wind, lightning, and air stratification. Explain how these problems might be solved.

6. 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

7. 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

8. 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

9. RF Interference • Denial of service • 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

10. RF interference • Narrowband 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

11. Interference Pg 393

12. Interference Pg 393

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

14. 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

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

16. 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, 11-25 Mhz of space between centers • 5 channel separation • 2, 7; 3, 8; 4, 9; 5, 10 Pg 396

17. 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

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

19. 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

20. Low SNR Pg 397

21. 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

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

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

24. 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

25. 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

26. Hidden Node Pg 400

27. Hidden Node Pg 400

28. Hidden Node Pg 400

29. 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

30. Hidden Node • Use RTS/CTS to diagnose. • 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

31. 802.11 Coverage considerations • Need to plan for correct coverage and capacity • Larger cells aren’t always better Pg 404

32. 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

33. Dynamic rate Switching Pg 405

34. 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

35. Dynamic rate Switching Pg 405

36. 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

37. 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

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

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

40. Layer 3 Roaming Pg 409

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

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

43. 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 betweenreused channel • Be aware of all 3dimensions Pg 412

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

45. 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

46. Single Channel Architecture Pg 415

47. 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

48. 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

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

50. Capacity vs. coverage • Oversized cells • 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