Transcript Title of presentation goes here

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
Interference
enters
the bandwidth
100%
5
Watts
Percentage
of signals
with no
collisions
and errors
Interference
outside
bandwidth
4
Signal integrity drops
to zero almost
immediately when
interference enters the
bandwidth of this radio
0%
Allocated Freq.
3
Bandwidth (MHz)
Interference Increases Across Bandwidth
457.8250
Bandwidth 25KHz wide
(or 12.5KHz)
2
1
Bandwidth
(MHz)
0
450
457.825
Bandwidth (MHz)
470
3
Multi-pathing
Original
Signal
Tx
…now what if we could
change frequencies…
Added to
Reflection
Equal
s
4
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
• 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
6
Direct Sequence Spread Spectrum
Interference
enters
the bandwidth
100%
1 Watt
Percentage
of signals
with no
collisions &
errors
Interference
outside
bandwidth
Transmit
Power
(Watts)
0%
Interference Increases Across Bandwidth
1 Watt of power “spread”
across wide bandwidth
0 Watt
902MHz
928MHz
Bandwidth (MHz)
7
7
Frequency Hopping
Interference
enters
100%
the bandwidth
Percentage
of signals
with no
collisions &
errors
1 Watt
0%
Interference Increases Across Bandwidth
0 Watt
902MHz
928MHz
Bandwidth (MHz)
8
8
OFDM
100%
1 Watt
Interference
enters
the
bandwidth
Percentage
of signals
with no
collisions &
errors
Transmit
Power
(Watts)
0%
Interference Increases Across Bandwidth
0 Watt
902MHz
Bandwidth (MHz)
928MHz
9
Who will Win?
Direct Sequence Vs. Frequency Hopping Vs. Orthogonal Frequency
Division Multiplexing
FREQUENCY
HOPPING
WAVE
BANDWIDTH
DIRECT
SEQUENCE
CHANNEL
RF
POWER
ORTHOGONAL
FREQUENCY
DIVISION
MULTIPLEXING
FREQUENCY
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
11
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
12
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
13
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”
14
Range and Over-the-air Data Rate
High
Baud
Rate
Low
Short
Distance
Long
15
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
16
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
Pathloss Study
18
Pathloss Study
19
Range and Propagation
Received Signal Strength
Circles of Success
Performance Zone
Path Engineering Required
Wireless Conduits up to 20 miles
Receiver
Threshold
Distance
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
20
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)
21
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 reroute 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
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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
27
Conclusion – Questions?
Contact Info:
Brian Cunningham
Applications Engineer
Port Coquitlam BC
866 713 4409 x 298
[email protected]
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