RF Interference and 802.11 Wireless LANs

1. Neil Diener CTO, Cognio Inc. October 3, 2006 RF Interference and 802.11 Wireless LANs The Experts in Enterprise Spectrum Management

2. Background • WiFi operates in Unlicensed Bands • Really an “experiment” by the FCC • No License, Little Rules • The “experiment” has been wildly successful • 200M WiFi devices this year • 3rd networking technology to hit > 100M • Soon: more unlicensed radios than cellular radios • The risks • Interference from incompatible devices • Interference from too many devices • This presentation – how to address the interference risks Unlicensed band

3. Agenda • Understanding RF Propagation • Understanding 802.11 Spectrum • Understanding interference • Recognizing and assessing spectrum problems • Resolution techniques • Beyond data… Voice and Video

4. RF Output power • dBm is measure of absolute power output • Formula: dbM = 10 log (Power in milliwatts) • An increase in 10 dBm means 10x the output power • Exs. • 0 dBm = 1 mW (Bluetooth) • 10 dBm = 10 mW • 20 dBm = 100 mW (802.11, Phones) • 30 dBm = 1 Watt (FCC Limit)

5. RF Propagation Loss • dB is a relative power measurement • Near field: 1 Meter distance results in a 40 dB loss • Every 2x increase in distance = 10 dB loss indoor (6 dB loss outdoor) • Exs. (indoor) • 2 Meters = 50 dB loss • 4 meters = 60 dB loss • 8 meters = 70 dB loss

6. RF Link Margin • 802.11 devices typically operate at receive strengths down to -85 dBm. • Received strength depends on output power of transmitter, and distance from receiver. • Ex. • AP transmits at 20 dBm • STA is 32 meters from AP • Received signal is: • +20 dBm output power -90 dB loss = -70 dBm received power

7. Agenda • Understanding RF Propagation • Understanding 802.11 Spectrum • Understanding interference • Recognizing and assessing spectrum problems • Resolution techniques • Beyond data… Voice and Video

8. 802.11 b/g 13 channels Only 3 non-overlapping channels (1,6,11) WiFi: 2.4 GHz Bands and Channels

9. A Typical Large-scale 2.4 GHz Deployment

10. 5.15-5.35 UNII Lower/Middle 31 channels 5.725-5.85 UNII Upper 13 channels Outdoor use 5.47-5.725 UNII/EMEA New band 41 channels 4.94-4.99 Public Safety 5 GHz Bands

11. Agenda • Understanding RF Propagation • Understanding 802.11 Spectrum • Understanding interference • Recognizing and assessing spectrum problems • Resolution techniques • Beyond data… Voice and Video

12. Co-Channel and Adjacent Channel Interference • Co-channel interference (CCI) comes from another AP located on the same channel • Ex. AP next door • Ex. Neighboring cell • With CSMA, effectively share the capacity • Adjacent channel interfence (ACI) comes from an AP on an overlapping channel • Ex. I am on channel 1, another AP is on channel 2 • Can be more worse than (CCI)

13. Non-WiFi Interference Sources Bluetooth Microwave ovens wireless video cameras radar outdoor microwave links Motion detectors 802.11FH 2.4/5 GHz cordless phones Wireless Game Controller fluorescent lights Wireless headphones

14. Example: Bluetooth See more hops in Max Hold See hops in Max Duty Cycle Spread Across Band Speckled pattern in Spectrogram

15. Example: Microwave Duty Cycle higher in Part of band Loud moving signal seen in Max Drifts in Frequency

16. Example: CT Cordless Phone Constant signal in Max Duty Cycle at or near 100% in two parts of band Constant signal in Swept

17. Example: DECT Phone - Disconnected Duty Cycle fairly low and spread across band Varying frequency on signals in Max Speckled appearance in Swept = Frequency Hopper

18. Example: DECT Phone - Connected Duty Cycle Increases Pulses increase in Max Speckled appearance in Swept = increases

19. Example: TDD Phone Duty Cycle constant, and high Constant signal seen in Max Constant signal seen in Swept

20. Example: Video Camera Constant Signal seen in Avg/Max Duty Cycle is 100% Constant power level stripe

21. Agenda • Understanding RF Propagation • Understanding 802.11 Spectrum • Understanding interference • Recognizing and assessing spectrum problems • Resolution techniques • Beyond data… Voice and Video

22. WiFi Protocol and Interference • WiFi uses CSMA-CA • Listen Before Talk etiquette • Many other devices don’t follow LBT • Result is “collisions” • Acknowledged Protocol • Re-transmits if no ACK received

23. IT Impact Lots of trouble calls High support costs Impact of Interference on WiFi Protocol Impact Collisions Retransmissions Rate back-off Network Impact Reduced Throughput Reduced Capacity Latency/Jitter User Impact “I can’t connect” “Why is it so slow?” “Garbled voice or video”

24. Impact of Interference on Data Throughput

25. Recognizing (and Assessing) Problems • 802.11 stats are good secondary indicators that interference is having an impact • Retries > 10% • Data Rate lower than “normal” • RF Stats are primary indicators • Channel Duty Cycle indicates a fixed-frequency device • Bursts spread around spectrum indicate a hopper • Chirping device can be radar, microwave, or jammer

26. Assessing Problem: Link Margin Again • When is an interferer loud enough to matter? • Use “link-budget analysis” Ex. • AP Transmit at 100 mW (+20 dBm) • Coverage range of AP is 16 meters • At perimeter, STA will receive: +20 dBm – 80 dB = -60 dBm • SNR = -60 dBm – [Interference] • For SNR of 19 dB:-60 – I >= 19I <= -79 dBm

27. Assessing Problems (cont) • Other thoughts on interference strength • Remember … Rx power measured with an analyzer may not be the same as received power seen by a device • Even low-power signals can cause CCA (Clear Channel Assessment) Problems: • Background Noise reduces sensitivity of chip • Weak signal may still trigger CCA • Device won’t transmit • Can cause problems with Receive • Bandwidth of interference also matters • Very narrow signal may be compensated with ECC • Typically, signals are pretty wide • Generally, most non-802.11 devices are of some concern.

28. Agenda • Understanding RF Propagation • Understanding 802.11 Spectrum • Understanding interference • Recognizing and assessing spectrum problems • Resolution techniques • Beyond data… Voice and Video

29. How to Mitigate Problems • Find and Remove Interference Device! • Shield Interference Device • Grounded shield • Change channels of AP • Ex. Microwave affecting some frequencies worse than others • Increase Tx Power of AP • Possibly use directional antenna to direct more power in desired areas. • Tx Data Rate controls • Don’t allow the lowest data rates, to avoid false back-off • Trade-off because lower data rates are more noise immune

30. Finding a Device: Using a Device Finder • Use a tool with a Device Finder • Spectrum analyzer works for some devices • Tracks power of a specific device • Best Algorithm: Divide and Conquer • Divide Space into 4 quadrants • Make measurement in each quadrant, pick strongest quadrant • Repeat if necessary, dividing that quadrant into 4…. • As you get close, room to room search • Directional Antennas can help, but not necessary

31. Finding a Device: Using a Device Finder Target Device Signal Strength History Signal Recordings

32. Agenda • Understanding RF Propagation • Understanding 802.11 Spectrum • Understanding interference • Recognizing and assessing spectrum problems • Resolution techniques • Beyond data… Voice and Video

33. VoFi: Changing the game Perception I think my WLAN is Lightly utilized So, I should be able To easily add voice Reality + But interference is eating Into my capacity So, there’s no room in the pipe for voice

34. Impact of Interference on VoFi Range

35. Quality of Service Parameters Voice/Video Application Requirements Type of Service Number of Streams Data Throughput Rate Max Packet Error Rate Max Latency Max Jitter Multiple Stream(1 HDTV/2SDTV) 3 30 Mbps 3.6* 10-5 90ms +/-10ms HDTV 1 19-24 Mbps 3.6* 10-5 90ms +/-10ms SDTV/DVD 1 3 to 7 Mbps 3.6* 10-5 90ms +/-10ms HQ Video Conf. Call 2 per 3 Mbps 3.6* 10-5 10ms +/-5ms CD Quality Audio 1 256 kbps 5.8* 10-5 100ms +/-10ms High Speed Data 1 10 Mbps 0 >100ms >100ms Med. Speed Data 1 2 Mbps 0 >100ms >100ms Low Speed Data 1 500 kbps 0 >100ms >100ms HQ Voice Calls 2 per 128 kbps 1.5* 10-3 10ms +/-5ms MQ Voice Calls 2 per 16 kbps 1.5* 10-3 30ms +/-20ms

36. Conclusions • WiFi interference exists • For reliable WiFi, you need to address interference issues. • Especially for mission critical systems • Especially for voice and video • There are excellent tools available to help you get the job done.