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 Interference 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 Wideband 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 Antenna reciprocity 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. Increase power to all stations. 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 works Pg 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 there are two cells between reused channel Be aware of all 3 dimensions 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 Physical environment 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