Wireless Networking WLAN Troubleshooting Module-11 Jerry Bernardini Community College of Rhode Island 6/17/20151Wireless Networking J. Bernardini

Wireless Networking

WLAN TroubleshootingModule-11

Jerry BernardiniCommunity College of Rhode Island

04/18/23 1Wireless Networking J. Bernardini

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

04/18/23 Wireless Networking J. Bernardini 2

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


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?


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


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


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


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


Hardware Problems and Solutions


Software Problems and Solutions


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”


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”


System Thinking Questions


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


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


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


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


Abnormally High Retransmission Count


All-Band and Narrowband interference

• All-band or wideband – signals that cover all or a large portion of a band of frequencies.

Freq.

Am

plitu

de

Channels 1 - 11

WidebandInterference

NarrowbandInterference

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



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

RF Spectrum Analyzers


2.0 5.0Frequency, f (GHz)

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

Laptop RF Spectrum Analyzers


•Fluke•Cisco•Other•Cognio (now owned by Cisco)

Throughput Possible Solutions


System Throughput• 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


1

5

11 Mbps

2 Mbps

100 MbpsWeakest Link = 2 MbpsHalf Duplex 45% x ( 2Mbps)System Throughput 900 kbps

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


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


Channel Interference

• Adjacent Channel interference - Signal impairment to one frequency/channel due to the presence of another signal on a nearby frequency/channel.

Freq.

Amp.

1 2 3 4

Wide Band

ChannelGuardBand

OverlapNo Guard Band

Co-Location WLAN

CH-1 CH-11

CH-6

Freq.

Am

plitu

de

CH-1 CH-11CH-6

OverlappingSidebands

3-Channel System

Recommend thesame manufacture

for the WLAN devices.

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


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


Multipath Problems• Multipath, or more accurately, multipath propagation - The

phenomenon that results in radio signals reaching the receiving antenna by two or more paths.

12 3

4

Metal Warehouse

APAP1

3

2

Possible Effects•Increased signal amplitude at receiver•Decreased signal amplitude at receiver•Data corruption•Signal nullification•Could increase throughput using MIMO (802.11n)

Multipath Wave forms

APAP13

2

Distorted upfadeResultant waveform

Distorted downfadeResultant waveform

Am

plitu

de

Time

A

B

DS

DS = Delay Spread 10 to 300 nsec

Antenna Diversity Solution to Multipath• 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

AccessPoint

Reflective Surface

1 2

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.

Narrowband

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

Freq.

Am

plitu

de

Channel - 3

NarrowbandInterference

ALL-Band interference

2.4 GHzPhone

MicrowaveOven

BluetoothDevices

FluorescentLighting

Hidden Node

Wireless Client

AccessPoints

21Wireless Client

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


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

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

1

3

Near Field Far Field

Natural Interference

• Interference caused by nature.– Smog, Fog, Rain, Snow– Wind– Lightning– Solar

Radome

Lightning ArrestorGrid Antenna

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)

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).

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.

Wind Load MPH lb.

(Based on worst case air temp. -30°F. Windload would be 18% less at +60°F.)

40 6.3

60 14.2

80 25.2

100 39.4

120 56.6

Stratification

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

Range Considerations

• Transmission Power and Frequency• Antenna Type and Placement• Environment

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


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


Handheld Protocol Analyzers

• PDA-Based – Convenient – Low processing power and small screen– Medium cost ($1000)– PocketPC based applications– WiFi PPC, CommView, MiniStumbler, TamoSoft

• Integrated – Specialized– Powerful – Full featured– High cost ($10,000 to $25,000)– Fluke OptiView Integrated Network Analyzer


Laptop Protocol Analyzers

• Large choice of applications– Wireshark (PC and Linux)– Wildpackets Omnipeek– AirMagnet– Others

• Wireless adapter Driver Issues– Applications only work with certain adapters– Obtain application first– Download the needed drivers

• Adapter Choices– PCMCIA– ExpressCard– USB

04/18/23Wireless Networking J. Bernardini 52

http://en.wikipedia.org/wiki/Image:Wireshark_Icon.svg

WiFi Hopper and Netstumbler –Almost free tools


Website: http://wifihopper.com/overview.html

http://wifihopper.com/overview.html

Wireless Intrusion Detection Systems

• Cisco


Intrusion Detection System


Wireless Security Articles

• http://www.windowsecurity.com/articles_tutorials/intrusion_detection/


http://www.windowsecurity.com/articles_tutorials/intrusion_detection/

http://www.windowsecurity.com/articles_tutorials/intrusion_detection/

Distributed RF Spectrum Analyzers


Spectrum analyzer showing narrowband RF interference


Interference

Throughput Testing Tools


Documents

Wireless Networking WLAN Troubleshooting Module-11 Jerry Bernardini Community College of Rhode Island 6/17/20151Wireless Networking J. Bernardini