The Reliability of Wireless Mesh Networks in Industrial Environments Brian Cunningham.
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Transcript of 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
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Fixed Frequency Radio
3
Interference outside
bandwidth
Allocated Freq.
457.8250
Bandwidth 25KHz wide(or 12.5KHz)
Bandwidth (MHz)
5 Watts
4
3
2
1
0
450 457.825 470
Bandwidth (MHz)
Bandwidth (MHz)
Interference entersthe 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
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Reflection
Original Signal
Added to
Equals
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)
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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
8 8
902MHz 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
TransmitPower
(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
FREQUENCYHOPPING
WAVE
DIRECTSEQUENCE
CHANNEL
ORTHOGONALFREQUENCY
DIVISIONMULTIPLEXING
BANDWIDTH
RFPOWER
FREQUENCY
Who will Win?
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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
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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
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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
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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”
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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…
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Distance
Re
ce
ive
d S
ign
al
Str
en
gth
ReceiverThreshold
No Worry ZoneThis is “Electricians’ Territory” Wireless Conduits up to 1/4 mile
Common Sense ZoneSuccess with ExperienceWireless Conduits up to 1.5 miles
Performance Zone Path Engineering RequiredWireless Conduits up to 20 miles
Circles of Success
Range and Propagation
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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
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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
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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
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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
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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
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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
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Conclusion – Questions?
Contact Info:
Brian Cunningham
Applications Engineer
Port Coquitlam BC
866 713 4409 x 298
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