The Reliability of Wireless Mesh Networks in Industrial Environments Brian Cunningham

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 2

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

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

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) 5

Frequencies 6 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 5.8GHz 900MHz 2.4GHz

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

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

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

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

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

Typical Specification  a:  6Mbps  54Mbps  b:  1Mbps  11Mbps  g:  6Mbps  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 Baud Rate High Low ShortLong Distance Range and Over-the-air Data Rate

16  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 5.8GHz 900MHz 2.4GHz Frequency and Range

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… 17

18 Pathloss Study

19 Pathloss Study

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

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)

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 22

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 23

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 24

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 25

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 26

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 27

Conclusion – Questions? Contact Info: Brian Cunningham Applications Engineer Port Coquitlam BC x