Wireless Networking WLAN Troubleshooting Module-11

1. Wireless NetworkingWLAN TroubleshootingModule-11 Jerry Bernardini Community College of Rhode Island Wireless Networking J. Bernardini

2. Presentation Reference Material • CWNA Certified Wireless Network Administration Official Study Guide (PWO-104), David Coleman, David Westcott, 2009, Chapter-12 • CWNA Certified Wireless Network Administration Official Study Guide, Fourth Edition, Tom Carpenter, Joel Barrett • Chapter-12, page 573-602 Wireless Networking J. Bernardini

3. Troubleshooting Methodologies • A methodology is a standard way to do something and there are many a troubleshooting methodologies • Troubleshooting is the process of discovering the unknown cause of a known problem • If you know the solution to a problem you are repairing • REACT • OSI Model • Hardware/software Model • Symptom, Diagnosis, and Solution • Old System – New System • System Thinking Wireless Networking J. Bernardini

4. REACT Methodology-Part 1 • Research – Gather facts • Check documentation • Check Google and read • Spend at least 15 minutes researching • Engage – ask questions without accusing • Has any thing change in the last few days? • Don’t say “did you change anything?” • Have any strange things happened lately? • Have others experience similar problems? • Is the problem recent or has it been happening for a while? • When was the last time it worked? • Is it turned on? Wireless Networking J. Bernardini

5. REACT Methodology-Part 2 • Adjust – wrongly, many techs start here previous • Try different things to find the cause of the problem • Update firmware on AP or STA adapter • Install new drivers • Change settings or disable features • Reinstall OS or drivers • Configure • Put the system back to the original condition • Reinstall and configure settings • Take Note - Document your findings (in a journal) • Record error messages associated with problem (in a journal) • Record step-by-step what you did to locate and fix (in a journal) • Document what you have learned (in a journal) • Create and organization trouble ticket Wireless Networking J. Bernardini

6. OSI Model Methodology –Part-1 • Walk up and down the model and analyze each layer • Layer 1 – Physical Layer • Are the adapters working? • Swap out client adapters • Is it a cable issue? • Is it the wireless or wired part of the network? • RF interference? • Use a spectrum analyzer • Layer-2 • Are switches filtering traffic? • VLAN configurations? • Bridge configurations? • Use a protocol analyzer Wireless Networking J. Bernardini

7. OSI Model Methodology –Part-2 • Layer-3 • Routing tables? • AP configurations for traffic? • Test with ping , arp, ipconfig • Upper Layers • Configurations in applications? • Client software configurations? • Which applications work and which do not? • Have new applications been added? • Test with Telnet, http Wireless Networking J. Bernardini

8. Hardware/Software Methodology • Certain problems are commonly hardware and some are commonly software • Everything is working except one application------------->software • Multiple applications that use the same hardware are not functioning -----> hardware • Consider hardware problem/solution lists • Consider software problem/solution lists Wireless Networking J. Bernardini

9. Hardware Problems and Solutions Wireless Networking J. Bernardini

10. Software Problems and Solutions Wireless Networking J. Bernardini

11. Symptom, Diagnosis, and Solution • Symptoms are associated with specific problems • Symptom - Gathering information about the problem • What happen? Where did it happen? • What Technology was involved? Which users were involved? • Has it always been this way? More questions??? • Diagnosis – What is the likely cause? • Deal with possible cause at a time • Try one fix at a time and evaluate • This the path to becoming and “expert” • Solution – the potential problem fix • Try different hardware and software fixes • The more try and remember or document the quicker to “expert” Wireless Networking J. Bernardini

12. System Thinking • Process of analyzing all interdependent components • Don’t focus on the vendor you may not like focus on the problem (don’t blame Microsoft) • However, some operating systems and vendors have a history of certain kinds of problems • Ask questions that related to “what makes things work” Wireless Networking J. Bernardini

13. System Thinking Questions Wireless Networking J. Bernardini

14. Old System – New System • Old system - worked yesterday • Probably only a single problem • Configuration changes? • Added applications? • Use REACT or Symptom/Diagnosis/Solution • New System - is just being turned on • Could be multiple problems • Could be hardware, software, configurations • Break apart the system and test parts • Use system methodology Wireless Networking J. Bernardini

15. WLAN Problem Indicators • Excessive Layer-2 Retransmissions • Unicast frames must be resent due a variety of problems • Produces increased latency • Delay between frame sent and error free received frame • Produces increased jitter • Variation in latency • Produces reduced throughput • Low Throughput • End indicator of one or more problems • Retransmission can be major cause Wireless Networking J. Bernardini

16. Retransmissions Effects • Increased latency • Increased jitter • Reduced throughput • Negative effect on applications • Latency produces echoes for VoIP • Jitter produces choppy audio for VoWiFi phones • Most data applications can tolerate 10% retransmission • Time sensitive applications require less than5% retransmission Wireless Networking J. Bernardini

17. Causes of Layer-2 Retransmissions • Multipath Signals • RF Interference • Low Signal to Noise Ratio SNR) • Hidden Node Stations • Near/Far Nodes • AP and Client Power Mismatching • Adjacent Cell Interference Wireless Networking J. Bernardini

18. Abnormally High Retransmission Count Wireless Networking J. Bernardini

19. All-Band and Narrowband interference • All-band or wideband – signals that cover all or a large portion of a band of frequencies. Narrowband Interference Wideband Interference Amplitude Freq. Channels 1 - 11

20. Eliminating Interference • Walk around with a spectrum analyzer to determine the interference frequencies • Locate the source and remove if possible • Modify source-frequency band, amplitude • Change channels • Limit coverage area Wireless Networking J. Bernardini

21. Spectrum Analyzers Scan radio frequency spectrum and provides graphical display of results Typically measure signal-to-noise ratio Single-frequency analyzers measure signal-to-noise ratio at specified frequency Helpful in identifying interference problems Thus, helps properly position/orient AP Costly $500 $50,000 Wireless Networking J. Bernardini

22. 5.0 Frequency, f (GHz) 2.0 RF Spectrum Analyzers Wireless Networking J. Bernardini

23. Handheld RF Spectrum Analyzers –Expensive Frequency range: 100kHz - 6 GHz. Internal preamplifier with frequency range from 100 kHz - 6 GHz. Displayed average noise level typ. -135 dBm (RBW 100 Hz). 0.5 dB typ. level accuracy up to 6 GHz. Resolution bandwidths 100 Hz - 1 MHz, 1 and 3 steps. Wide range of detectors; sample, max/min peak, auto peak, RMS. Cost: $13,500 Wireless Networking J. Bernardini

24. Laptop RF Spectrum Analyzers • Fluke • Cisco • Other • Cognio (now owned by Cisco) Wireless Networking J. Bernardini

25. Throughput Possible Solutions Wireless Networking J. Bernardini

26. System Throughput 11 Mbps 1 • System Throughput - The actual number of bits, characters, or data blocks passing through a data communication system, or portion of that system in a given time. • Bits per second less communication overhead • 802.11b 11 Mbps throughput is 5 Mbps • 802.11g 54 Mbps throughput id 22 Mbps Weakest Link = 2 Mbps Half Duplex  45% x ( 2Mbps) System Throughput  900 kbps 100 Mbps 2 Mbps 5 Wireless Networking J. Bernardini

27. Throughput PHY Limitations • 802.11b (DSSS) - 11 Mbps • 802.11a (OFDM) - 54 Mbps • 802.11g (OFDM ERP) - 54 Mbps • 80211n (ERP , Greenfield, protections) – 200 Mbps • Mixed modes reduce maximum potential throughput • Wired-Side Bandwidth • 10 Mbps Ethernet may not be faster enough for 10 users • 100 to 1000 Mbps for multiple APs Wireless Networking J. Bernardini

28. Co-Channel and Adjacent-Channel Interference • More than one WLAN attempting to coexist in the same RF coverage area, on the same channel or a channel that is too close • System throughput is effected by this problem • Non-overlapping channels are supposed to be 1, 6, 11 • Adjacent-Channel interference • RF-power from channel-1 could be in channel-6 and channel-6 in channel-11 • Caused by APs that too close • APs and adapters with excessive power levels Wireless Networking J. Bernardini

29. Channel Interference • Adjacent Channel interference - Signal impairment to one frequency/channel due to the presence of another signal on a nearby frequency/channel. Amp. 1 3 2 4 Channel No Guard Band Overlap GuardBand Freq. Wide Band

30. Co-Location WLAN 3-Channel System Amplitude CH-1 CH-6 CH-11 Freq. Overlapping Sidebands CH-1 CH-11 Recommend the same manufacturefor the WLAN devices. CH-6

31. Interference Testing and Solutions • Move channels to 1 and 11 • Adjust power levels • Relocate APs • Change antenna • Test for retransmission and throughput with one AP off • Transmit on Channel-11 and count captured frames on Channels- 8,7,6 (beacon frames could be used for test) • Adjust AP power and repeat frame count test Wireless Networking J. Bernardini

32. RF Noise, Noise Floor, RF Interference • RF Noise are signals generated by RF systems other than for the intended communications • RF Noise may corrupt frames • Noise Floor is the background level of RF noise • Signal-to-Noise Ratio is the ratio of desired signal to background noise • 40 db SNR means the signal is 40 db higher than the noise (10,000 times higher) • Narrow and Wide band interference can corrupt frames • Can be detected by checking for CRC errors a with frame analyzer • Check for retransmissions • Solutions could be reduced data rate or small frames • Use the fragment threshold to control frame size Wireless Networking J. Bernardini

33. Multipath Problems • Multipath, or more accurately, multipath propagation - The phenomenon that results in radio signals reaching the receiving antenna by two or more paths. 2 • Possible Effects • Increased signal amplitude at receiver • Decreased signal amplitude at receiver • Data corruption • Signal nullification • Could increase throughput using MIMO (802.11n) AP 1 4 3 3 2 1 Metal Warehouse

34. Multipath Wave forms 2 AP A 3 1 Distorted upfade Resultant waveform Amplitude B Distorted downfade Resultant waveform DS Time DS = Delay Spread  10 to 300 nsec

35. Antenna Diversity Solution to Multipath Reflective Surface • Antenna Diversity – using two or more antennas at physically different locations to improve the reception or transmission of an RF signal. • In a small building with many reflections antenna diversity may not help • AP will select the antenna with the best signal Client 1 2 Access Point

36. Multipath Troubleshooting and Solutions • Can not observe directly • Symptoms • Links that should work based upon link budget calculations • Dead spots in FR coverage during survey • High retransmissions • RF noise floor when transceivers are off • Use a site survey to check for holes/nulls in the illumination field. • Use specialized test systems that can check for the possibility of multipath within a wireless local area network or link. • If possible try moving the AP and or Client. • Use an antenna diversity system.

37. Narrowband • Narrowband - refers to a signal which occupies only a small amountof space on the radio spectrum, the opposite of broadband or wideband. Narrowband Interference Amplitude Freq. Channel - 3

38. ALL-Band interference Microwave Oven 2.4 GHz Phone Fluorescent Lighting Bluetooth Devices

39. Hidden Node 1 2 Wireless Client Wireless Client Access Points

40. Hidden Node Detection and Solution • Symptoms • Increase corruption • Increased retransmission • Use protocol analyzer near to the AP and count corrupted frames • Use protocol analyzer near to the STA and count retransmission frames • Solutions • Use RTS/RTS • Increase STA power • Remove obstacles and move STA if possible • APs and STA at same IEEE 802.11h power level Wireless Networking J. Bernardini

41. Near/Far Problem • Near/Far Effect – A condition where the AP cannot hear a client because it is being masked by other clients due to: • Client has low transmit power • Client is a great distance from the AP • Fading problems. • Symptoms • STA can not contact AP • STA has low throughput • STA adapter looks like it failed • Intermittent AP connection • Testing • Capture retransmissions and corruption frames from STA close to AP

42. Near/Far Solutions • Increase distant STA output power • Decrease close STA output power • Move remote STA closer to AP • Move AP closer to distant STA • Install another AP closer to distant STA 2 3 1 Far Field Near Field

43. Natural Interference • Interference caused by nature. • Smog, Fog, Rain, Snow • Wind • Lightning • Solar Radome Lightning Arrestor Grid Antenna

44. Weather Effects • In the 2.4 GHz band there are minimal effects to the signal absorption due to Fog – Snow. • In the 5.0 GHz band the absorption effects start to increase on the signal. • Thunder storms, ice, heavy rain and hail can reduce the quality of RF link • Snow can accumulate on building and trees and effect the Fresnel zone • Snow , ice and heavy wind can misalign antennas (grid antennas are better in wind)

45. Lightning • Lightning interference - is sporadic and the primary concern is for survival of the equipment. It can also change the signal path by changing the air (heated by the lightning energy to above 50,000ºF).

46. Wind • Wind has no direct impact on a wireless signal. However wind may have a strong indirect effect by moving the antenna. Wind loads are specified for various outside antennas.

47. Stratification • Fog or ionosphere layering. This can cause a bending of the wavefront due to diffraction.

48. Range Considerations • Transmission Power and Frequency • Antenna Type and Placement • Environment

49. VoWLAN Issues • A new and growing technology that requires QoS • Latency (less than 150 ms)and throughput are important • Today use the same vendor for all equipment • Common Problems • Dropped calls during roaming • Dropped call when staying within a BSS • Calls not going through to target Wireless Networking J. Bernardini

50. WLAN Analysis Systems • Handheld Protocol Analyzers • Laptop Protocol Analyzers • Basic Protocol Analyzers • Handheld RF Spectrum Analyzers • Laptop RF Spectrum Analyzers • Wireless Intrusion Detection Systems • Wireless Intrusion Prevention Systems • Distributed RF Spectrum Analyzers Wireless Networking J. Bernardini