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

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• 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

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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

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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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BaudRate

High

Low

Short LongDistance

Range and Over-the-air Data Rate

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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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Pathloss Study

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Pathloss Study

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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

Brian.Cunningham@Cooperindustries.com

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