The Reliability of Wireless Mesh Networks in Industrial Environments

1. The Reliability of Wireless Mesh Networks in Industrial Environments Brian Cunningham

2. Agenda • Modulation Techniques • Fixed Frequency Radio • Frequency Hopping, Direct Sequence and OFDM • Frequency Choices • Range and Interference • Comparing Radios • How to Determine Range • Software Propagation Studies • Dealing with Interference • Topologies and Mesh Performance • Topologies • Mesh Advantages and Disadvantages • Mesh Application Lessons

3. Fixed Frequency Radio Interference enters the bandwidth 100% 0% Percentage of signals with no collisions and errors Signal integrity drops to zero almost immediately when interference enters the bandwidth of this radio Interference outside bandwidth 5 Watts 4 3 2 1 0 Bandwidth (MHz) Allocated Freq. 457.8250 Bandwidth 25KHz wide (or 12.5KHz) Interference Increases Across Bandwidth Bandwidth (MHz) 450 457.825 470 Bandwidth (MHz)

4. Multi-pathing Original Signal Added to Reflection Tx Equals …now what if we could change frequencies…

5. Spread Spectrum Introduction • FCC allocated a portion of the 900MHz band, then later 2.4GHz and later 5GHz. • Created Rules Manufacturers Must Adhere to: • 1W of Transmit Power • FH or DS or OFDM • FCC will not referee in case of interference from others • Many other technical requirements • Manufacturers Must Submit Prototype for Testing • FCC then Certifies, and Assigns ID to Appear on Label • Radio can then be Used by Anyone, Anywhere (in the US)

6. Frequencies • Lower Frequencies: • propagate further • penetrate objects better • 900 band is 26MHz wide • 2.4GHz: • used by microwave ovens (rain fade on longer links) • is license free around the world • 2.4 band is 81MHz wide • 5.8GHz • brand new ISM band 900MHz 2.4GHz 5.8GHz

7. Direct Sequence Spread Spectrum Interference outside bandwidth Interference enters the bandwidth 100% 0% 1 Watt 0 Watt Percentage of signals with no collisions & errors Transmit Power (Watts) Interference Increases Across Bandwidth 1 Watt of power “spread” across wide bandwidth 902MHz 928MHz Bandwidth (MHz) 7

8. Frequency Hopping Interference enters the bandwidth 100% 0% Percentage of signals with no collisions & errors 1 Watt 0 Watt Interference Increases Across Bandwidth 902MHz 928MHz Bandwidth (MHz) 8

9. OFDM Interference enters the bandwidth 100% 0% 1 Watt 0 Watt Percentage of signals with no collisions & errors Transmit Power (Watts) Interference Increases Across Bandwidth 902MHz 928MHz Bandwidth (MHz)

10. Direct Sequence Vs. Frequency Hopping Vs. Orthogonal Frequency Division Multiplexing Who will Win? FREQUENCY HOPPING WAVE BANDWIDTH DIRECT SEQUENCE CHANNEL RF POWER ORTHOGONAL FREQUENCY DIVISION MULTIPLEXING FREQUENCY

11. Interpreting Radio Specifications • Ignore the range specs – there is no standard for comparison • A well designed radio link has a 20dB fade margin to allow for degrading equipment and conditions • For short range applications – this will give you the highest signal-to-noise ratio

12. Transmit Power • More power = greater range • More power = strong S/N ratio • Transmit power is only half the equation – receiver sensitivity is important • Effective radiated power can be boosted by using a high gain antenna • Does not require fancy antenna work, or critical antenna alignment • Disadvantage is power consumption – if battery or solar powered

13. Receiver Sensitivity • Spec’d in –dBm (lower number = better sensitivity) • Ask what the BER is? (bit error rate) • BER of 10^6 = 1 errored bit in 1 million • For multiple over-the-air data rates – ask what the sensitivity is for each

14. 802.11 Typical Specification • 802.11a: • -88dBm @ 6Mbps • -71dBm @ 54Mbps • 802.11b: • -95dBm @ 1Mbps • -90dBm @ 11Mbps • 802.11g: • -90dBm @ 6Mbps • -74dBm @ 54Mbps Note how the receiver sensitivity gets worse as the data rate gets higher Less time for a receiver to determine if a bit is a “0” or a “1”

15. Range and Over-the-air Data Rate High Baud Rate Low Short Long Distance

16. Frequency and Range • Lower Frequencies: • Propagate further • Penetrate objects better (air molecules are obstructions) • Higher Frequencies: • Loses more energy after each reflection • Results in increasingly shorter ranges in non line-of-sight applications 900MHz 2.4GHz 5.8GHz

17. How to Determine Range • Use a functional radio system to test • Should be the same model you intend to install • MUST be same frequency • Should be same transmit power • Should be set to same throughput required • Sometimes antennas cannot be elevated as high as needed…

18. Pathloss Study

19. Pathloss Study

20. Range and Propagation Performance Zone Path Engineering Required Wireless Conduits up to 20 miles Common Sense Zone Success with Experience Wireless Conduits up to 1.5 miles No Worry Zone This is “Electricians’ Territory” Wireless Conduits up to 1/4 mile Circles of Success Received Signal Strength Receiver Threshold Distance

21. Interference Mitigation • Filtering! - the difference between high quality radios and the rest • Single most expensive component on the circuit board - however because we’ve already done the engineering you need some other options: • Separation! Locate the antennas at least 6 feet vertically or 10 feet horizontally away from other antennas • Use high gain (narrow beam width) directional antennas • The higher the transmit power, the greater the source of interference - but signal strength drops off exponentially with distance • The closer to our operating frequency, the less effective the filter • Switch to another frequency (band)

22. Mesh Topologies • Point-to-point • Star • Mesh • Mobil vs Fixed Applications • Mesh is the only practical method of Mobil • Mesh offers redundancy for Fixed Applications • More alternative paths = more redundancy = more reliability

23. Mesh Advantages • Automatically re-route Data via Repeaters • No predictions of which path need to be programmed • Complete freedom to roam (mobile) • If path degradation occurs due to foliage growth, or a new building constructed, re-routing takes place • If background noise levels increase, radio can re-route to a closer node

24. Mesh Disadvantages • Omni antenna use • Generally required to allow communication to nodes 360 degrees • Opens that node to interference coming at it from 360 degrees • Should use radio that employs good filters – will be expensive • Selectivity spec will determine filter quality, but rarely published in instrument world • Traffic congestion at repeater nodes • Possible bottleneck of data • Slower response time • Requires good protocol that can deal with “report by exception” • If battery powered, reduced battery life

25. Mesh Lessons Learned • Background: • Large biotech company with multiple buildings on a campus • Thousands of temperature chambers (fridges, freezers and incubators) storing research material • Research material must be kept at specific temperature • Chambers on castor wheels, moved from lab to lab, to other buildings, sometimes to a freezer farm, at will of the lead researcher in charge • Desired alarming on temperature, plus monitoring of compressor currents, door open/closed, etc. • Hardwiring just not practical

26. Mesh Lessons Learned • Dedicate some radios as repeaters • Random movement of chambers meant repeaters could not be guaranteed • Possible that some nodes could get overwhelmed with traffic • Boils down to reliability that a mesh will provide – if your repeater walks away, not so reliable • Over-the-air Diagnostics are valuable (very) • Remote configuration, diagnostics and firmware upgrades • Some chambers could not be located • Campus large requiring travel time • Some areas were off-limits or buildings locked

27. Mesh Lessons Learned • High Transmit Power makes a Mesh more Reliable (and Simpler) • 50 or 100mW of transmit power could not go through many walls – take advantage of FCC’s allowable 1W • Short range required more repeaters, roaming area smaller • Left dead zones in basements and building shadows • 2.4GHz offering had shorter range than 900MHz or other lower frequencies and interfered with Wifi • Do a Site Survey in Advance • Will catch any interference that would cause problems • Enables you to select a different frequency in advance • Shows up dead zones, allows planning for dedicated repeaters

28. Conclusion – Questions? Contact Info: Brian Cunningham Applications Engineer Port Coquitlam BC 866 713 4409 x 298 Brian.Cunningham@Cooperindustries.com