Chapter 12-WLAN Troubleshooting

• Layer 2 retransmission

• 802.11 Coverage Considerations

• Voice vs. Data

• Performance

• Weather

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

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

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

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

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

Interference

Pg 393

Interference

Pg 393

RF interference• Cordless phones

• Microwaves

• Video cameras

• Bluetooth

• Other Networks

Pg 394

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

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

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

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

Adjacent Cell interference• 5 Ghz range has more options

– Technically each channel is non-overlapping– 23 channels also gives you more space

Pg 396

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

Low SNR

Pg 397

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

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

Mismatched Power• Power is often controlled by WLAN

controller– Can cause problems

• Common issue with VoWiFi

Pg 398

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

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

Hidden Node

Pg 400

Hidden Node

Pg 400

Hidden Node

Pg 400

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

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

802.11 Coverage considerations• Need to plan for correct coverage and

capacity– Larger cells aren’t always better

Pg 404

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

Dynamic rate Switching

Pg 405

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

Dynamic rate Switching

Pg 405

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

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 worksPg 407

Roaming• Troubleshooting roaming can be hard

• May need multiple channels to be checked at once

Pg 407

Layer 3 Roaming• Generally interrupts service

– Mobile IP can prevent loss

Pg 409

Layer 3 Roaming

Pg 409

Co-Channel interference• Same channel on adjacent cell

– Co-channel cooperation

Pg 409

Co-Channel interference• Try to arrange non-overlapping channels

• Adjacent channel problems are worse than co-channel

Pg 409

Channel Re-use/MCA• Try to arrange non-overlapping channels

• Most ESS are set up this way

• Try to make sure thereare two cells betweenreused channel

• Be aware of all 3dimensions

Pg 412

Channel Re-use/MCA• Make sure coverage only extends one floor

in either direction

Pg 415

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

Single Channel Architecture

Pg 415

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

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

Capacity vs. coverage• Collocation not so helpful in MCA with

reuse

• Scales well in SCA

Pg 418

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

Voice vs. Data• Voice is more sensitive to delay

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

Pg 421

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

Weather• Lightning

• Wind

• Water

• Air Stratification

• UV/Sun

Pg 424

The End