chapter 12-wlan troubleshooting layer 2 retransmission 802.11 coverage considerations voice vs. data...
Post on 18-Dec-2015
223 Views
Preview:
TRANSCRIPT
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
top related