Chapter 12-WLAN Troubleshooting

1
Chapter 12-WLAN Troubleshooting

• Layer 2 retransmission
• 802.11 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

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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

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Layer 2 retransmissions

• Most apps dont 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.

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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 wont check, no ACK sent

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RF interference

• Narrowband interference
• Usually wont 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
  arent 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

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Interference
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Interference
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RF interference

• Cordless phones
• Microwaves
• Video cameras
• Bluetooth
• Other Networks

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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

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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

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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

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Adjacent Cell interference

• Good design needs overlapping coverage between
  cells
• Must make sure that the channels dont overlap as
  well
• Adjacent cell interference

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Adjacent Cell interference

• 5 Ghz range has more options
• Technically each channel is non-overlapping
• 23 channels also gives you more space

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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

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Low SNR
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Mismatched Power

• If client and AP have different power levels
• If client can hear AP, but AP cant hear client
• If client power is too low, the AP wont get a
  signal above noise floor
• Try to keep all devices as similar power levels
• Use protocol analyzer to check

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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

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Mismatched Power

• Power is often controlled by WLAN controller
• Can cause problems
• Common issue with VoWiFi

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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

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Hidden Node

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

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Hidden Node
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Hidden Node
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Hidden Node
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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

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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.

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802.11 Coverage considerations

• Need to plan for correct coverage and capacity
• Larger cells arent always better

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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

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Dynamic rate Switching
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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

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Dynamic rate Switching
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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

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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

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Roaming

• Troubleshooting roaming can be hard
• May need multiple channels to be checked at once

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Layer 3 Roaming

• Generally interrupts service
• Mobile IP can prevent loss

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Layer 3 Roaming
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Co-Channel interference

• Same channel on adjacent cell
• Co-channel cooperation

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Co-Channel interference

• Try to arrange non-overlapping channels
• Adjacent channel problems are worse than
  co-channel

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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

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Channel Re-use/MCA

• Make sure coverage only extends one floor in
  either direction

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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

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Single Channel Architecture
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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

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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

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Capacity vs. coverage

• Collocation not so helpful in MCA with reuse
• Scales well in SCA

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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

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Voice vs. Data

• Voice is more sensitive to delay
• Sensitive voice data is more likely to suffer
  when there are design issues

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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

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Weather

• Lightning
• Wind
• Water
• Air Stratification
• UV/Sun

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The End