The Reliability of Wireless Mesh Networks in Industrial
Environments
Brian Cunningham
Applications Engineer
Cooper Bussmann, Wireless Business Unit
Port Coquitlam, BC V3C 6G5
1-800-663-8806
www.cooperbussmann.com/wireless
Keywords
radio, mesh, wireless, spread spectrum, telemetry, frequency hopping, transmitter, receiver,
transceiver
Abstract
With increasing requirements on plant managers to reduce operating costs and increase safety,
wireless connectivity is an attractive alternative to buried cable and conduit. However questions
remain as to its performance in industrial settings and more so, what will happen during an
emergency when multiple alarms and signals all need to get to the control room at once? This
paper will discuss the different types of radios that are currently in use with a focus on mesh
networks and their application to process control. Spread spectrum modulation techniques of
frequency hopping, direct sequence and orthogonal frequency division multiplexing will be
compared and analyzed, and their applicability to common industrial applications reviewed.
Mesh networks will be delved into in detail covering important aspects such as network
congestion, points of failure. Radio standards and specifications will be reviewed, and the
audience educated on how to differentiate one radio’s performance from another. Range
assessments, interference mitigation and multi-path will be addressed, along with issues
surrounding the multitude of frequencies in use and the advantages and disadvantages of each.
Rounding out the discussion will be a case study of where a mesh network was successfully
applied, along with the lessons learned. The audience should leave with a solid foundation to
implement their next wireless project.
Distributed with permission of author by ISA 2011
Presented at ISA 2011; http://www.isa.org
1.0
Introduction
Wireless has significant benefits in areas of safety, cost and convenience. This paper will focus
on the safety and cost benefits, as they relate to the industrial automation field, but let’s also not
forget the convenience of garage door openers and cellular phones. In these facets, wireless is a
revolution to how things are done. New sensors can be installed to monitor the “nice to have”
parameters because the cost to install has been lowered. No longer is burying conduit and
pulling cable to a remote site the only way to gather data, to allow informed decisions on
processes to be made. Some say the industrial wireless revolution has crossed the chasm into
mainstream adoption. One writer to a column in a process control journal noted that he called
his local sales rep back recently after he received an instrument price quote, as it appeared the
usual wireless option had been missed. Every previous instrument quote had always included a
wireless option in the recent past. However the questions still remain, will it be reliable? Will it
be secure? Can I trust it? This paper will attempt to provide unbiased answers to these
questions, along with the necessary technical background to understand the technology.
2.0
Modulation Techniques and Frequencies
Fixed Frequency Radio Since the day Marconi (and others) first invented the radio, there have
been fixed frequency (FF) radios. Today, if you wish to use a FF radio, you must make an
application to the FCC (in the USA) noting your geographic area. If there are frequencies
available, the FCC will assign one to you, and you will be required to pay an annual fee. In
theory this frequency control scheme works well. However there are a limited number of
frequencies available, and particularly in dense urban areas, you may be put on a waiting list for
several years. Also, if you are a mobile user, your license may not extend into the areas your
business operates.
Next there are technical issues associated with FF radios. There are some unintentional RF
emitters such as variable frequency drives, that can generate interference on your frequency.
Next are illegal transmitters that briefly transmit, then shut down or move on before the FCC can
do a triangulation and order shut off and/or fine them. Finally there are multi-path conditions
that can wreak havoc on the reliability of your FF wireless system. (More on multi-path later in
this paper). In summary, FF radios are likely to work best in rural areas where there is little
likelihood of these conditions occurring. (Range to be discussed in section 3.0 of this paper)
Multi-path Conditions Multi-path is a phenomena where two radio waves leave a transmitter at
the same time but take different paths to the receiver. If the difference in the paths allows one
radio wave to arrive 180 degrees out of phase of the other radio wave, they will cancel each other
out, resulting in no reception. Conductive metals, as commonly found in tanks, piping and
supporting structures reflect radio waves very efficiently. As you can imagine, all it takes is a
forklift or truck driving into just the right spot to suddenly create a multi-path condition, and
suddenly you have lost reception. This is why FF radios are not commonly used in industrial
facilities for sensor applications, where the antennas are permanently bolted in place. Note that
for mobile voice applications, this is not an issue. This is because if the user loses reception due
to multi-path in one spot, he can simply move a few slightly in any direction, and reception
returns. We have all likely experienced multi-path with our car AM/FM radios. When
Distributed with permission of author by ISA 2011
Presented at ISA 2011; http://www.isa.org
reception fades out at a stop light, but you are clearly within the broadcast area of the station, if
by rolling forward a few feet reception picks up, a multi-path condition had occurred.
Spread Spectrum Modulation In the early 1990’s, the FCC recognized that demand for
wireless exceeded the available frequencies and made available a portion of the spectrum to
unlicensed radios. However a new set of rules were created for anyone to use this new band, and
radio manufacturers had to have their radios tested and certified before they could be imported or
operated in the US. Once FCC certification was obtained, any user anywhere in the country
could utilize these radios. The first spread spectrum band was the 900MHz band, and later a
2.4GHz band was created and later, a portion of the 5GHz spectrum set aside.
The rules the manufacturers had to adhere to were limits on the transmit power to far less than
allowed with FF radios, a modulation scheme of frequency hopping (FH), direct sequence (DS)
or orthogonal frequency division multiplexing (OFDM), among other rules. The rules were
general enough to allow for the needs of a wide range of consumer, commercial and industrial
users, yet also flexible enough to allow for tremendous innovation by the wireless community.
More on these modulation techniques later in this paper.
Frequencies Focusing on the spread spectrum bands available, the question of which band
should be uses needs addressing. All other factors being equal, the radio with the lowest
frequency will have the greatest range and object penetration ability. Comparing the two most
popular spread spectrum bands, the 900MHz and the 2.4GHz, the difference is significant: the
900MHz radio would have approximately three times greater range than the 2.4GHz radio –
again, assuming all other factors are the same. This translates to a far greater ability to penetrate
through walls, trees and buildings, to get the signal to a distant receiver. If the signal must
bounce off objects to arrive at the receiver, it should be noted that the lower the frequency, the
less energy is lost upon each reflection. For example, this would allow a 900MHz radio
connected to a gas detector a far greater chance of finding its way through a tank farm to a
control room than would a 2.4GHz radio.
Figure 1.0 Spread Spectrum Frequency Choices
A short discussion of the 2.4GHz band and its popularity is appropriate. The 2.4GHz band is
license free in most countries around the world, whereas the 900MHz band is only license free in
North America, parts of South America, Australia and New Zealand. A manufacturer of a
2.4GHz radio could then produce one model of radio, and sell it into a much wider range of
Distributed with permission of author by ISA 2011
Presented at ISA 2011; http://www.isa.org
countries than a manufacturer of a 900MHz radio could. When a radios range is not an issue,
such as for our wifi hot spots, cordless phones or rotating machinery, this is an ideal strategy.
However in North America, the long range SCADA market is dominated by 900MHz spread
spectrum and 450MHz FF radios due to their superior range and object penetration ability.
The 2.4GHz spread spectrum band and the 900MHz spread spectrum band are not the same size.
The 900MHz band is 26MHz wide whereas the 2.4GHz band is 81MHz wide. This has
implications of the number of radios that can co-exist in each band; the wider the band, the more
radios that can happily communicate without interfering with each other. Note that this depends
on how much of the band is utilized by each radio. To get higher over-the-air (OTA) throughput,
you will require a wider portion of the band. For example, only three 802.11g radios
transmitting data at 54Mbps can co-exist in the 2.4GHz band with no frequency overlap
compared to hundreds of radios using only a tiny slice of spectrum, transmitting data at 19.2kbps
OTA.
Figure 2.0 802.11b/g Channels
Frequency Hopping One of the spread spectrum methods permitted is frequency hopping (FH).
In this modulation scheme, the transmitter and receiver are pre-programmed to know which
frequencies the system will jump to, and in which order. It appears to be random, but because it
must be known, is considered pseudo-random. FCC rules stipulate that the radio system cannot
dwell on any frequency for any significant period of time, but instead must constantly change
frequency. This technique was originally developed during World War II for the purpose of
interference avoidance, and today it continues to achieve this objective. By hopping around
interference, this type of modulation scheme is able to mitigate reliability concerns by not
depending on a single frequency.
FH radios overcome multi-path conditions because the change in frequency comes with a change
in wavelength. When one particular wavelength experiences a multi-path condition, the next
will not, due to the wavelength being shorter or longer than the last transmission.
It is important to note that FH radios are affected by interference, but only in the sense that their
overall throughput is reduced; the radio link is not lost. In order to block or jam this type of
Distributed with permission of author by ISA 2011
Presented at ISA 2011; http://www.isa.org
radio system completely, you would need to have interference spanning the entire band, covering
every single frequency that this technology uses.
Figure 3.0 Frequency Hopping Spread Spectrum
FH radios do have limitations on their OTA data rate. This is because the radio must shut down,
change frequencies, then the power amplifier must turn back on, and commence transmission.
This takes time, even in the world of microelectronics. While many FH radios have achieved
very high throughputs, this fundamental limitation means they face constraints other spread
spectrum technologies do not.
Direct Sequence A direct sequence (DS) radio takes its power, and spreads it over a wide
portion of the band such that it does not generate a significant amount of interference (to other
users) on any single frequency. On the receiver end, it uses something called processing gain to
suppress interference. The more the data is spread, the more effective the processing gain is to
suppress interference. The trade off is that the more the data is spread, the wider the receivers
filters must be set, allowing a larger opening for any potential interference to enter.
Distributed with permission of author by ISA 2011
Presented at ISA 2011; http://www.isa.org
Figure 4.0 Direct Sequence Spread Spectrum
DS radios are susceptible to multi-path conditions. However, many DS radios overcome this by
utilizing two antennas, mounted a multiple of the wavelength apart. If one antenna is in a null
spot, the other will not. While this is a simple solution when the antennas are small and
inexpensive, for longer range applications where a high gain antenna could be mounted 50 feet
up a tower, the costs add up.
DS radios have the advantage of relatively high throughput. They are the basis of the 802.11b
standard which has transmission rates of up to 11Mbps.
The operational characteristics of a DS radio are such that it will either have 100% throughput or
0%. The processing gain, which acts as a software filter, can suppress a certain amount of
interference. However once the jamming margin has been reached, throughput drops to 0%. As
interference in the operating band increases, the radio will continue to have 100% throughput,
until that critical point is reached, then all communications are lost.
Orthogonal Frequency Division Multiplexing (OFDM) OFDM radios divide the data into
numerous channels which transmit the data simultaneously. It is more efficient in its use of the
spectrum to transmit high volumes of data. As such, it is the basis of the 802.11a and g
standards, which allow transmission speeds of up to 54Mbps. The orthogonal adjective is a
description of the spacing of the channels – orthogonally spaced such that each channel does not
interfere with its neighbors.
Distributed with permission of author by ISA 2011
Presented at ISA 2011; http://www.isa.org
Figure 5.0 Orthogonal Frequency Division Multiplexing
Advantages of OFDM include better immunity to interference than DS radios and better
performance in multi-path conditions. Disadvantages include sensitivity to Doppler shift
limiting high speed mobile applications, and frequency synchronization issues requiring complex
circuitry and design.
Radio Standards The two emerging standards focused on industrial applications are the
ISA100 and WirelessHART. A discussion of these two standards is outside of the scope of this
paper however a few points can be mentioned. Both standards operate in the 2.4GHz band (but
they are not restricted to it), both have transmit powers well below the 1W maximum FCC
imposes to allow for battery operation and importantly, they both combine FH with DS. Using
two modulation schemes yields the best of both worlds; good interference immunity combined
with a reasonable level of throughput. Best of all, in the presence of interference,
communications will not drop off completely, only slow down.
3.0
Range and Interference
Key Specifications to Review First and foremost, ignore the range specifications manufacturers
print. There is no standard they are tested to, and to be competitive, manufacturers will generally
print how their radios perform in ideal circumstances, not the typical environment with tanks or
trees in the path, and where dozens of other radios are generating interference. The key
parameters to scour for on data sheets will be the modulation scheme as discussed in section 2.0,
transmit power, receiver sensitivity, operating frequency and OTA data rate. Carefully
reviewing these specifications will ensure you have a radio with the maximum level of
reliability.
Transmit Power This is fairly obvious, the higher the transmit power, the greater the range, all
other factors being equal. In short range applications, a high transmit power radio will yield a
high signal to noise ratio. This means the source of interference much be that much stronger to
Distributed with permission of author by ISA 2011
Presented at ISA 2011; http://www.isa.org
have any impact on communications. The disadvantage to a high transmit power radio would be
power consumption, generally only of concern with battery or solar powered systems.
Receiver Sensitivity The sensitivity of a receiver is a measure of its ability to hear a weak
signal from a distant transmitter. It is measured in dBm, and the number is negative. The lower
the number, the more sensitive the receiver. A further qualifier is the bit-error-rate (BER).
Some manufacturers will state a receiver sensitivity of -100dBm at a BER of 10-3 (1 errored bit
of 1000 transmitted), whereas another will state a sensitivity of -100dBm at a BER of 10-6 (1
errored bit of 1 million transmitted). Unfortunately there is no common standard, and some
manufacturers don’t state the receiver sensitivity at all. Lastly, some manufacturers put a packet
error rate down on their data sheets, as they transmit in packets, and either the entire packet is
valid or invalid.
Operating Frequency As discussed in section 2.0, the lower the frequency, the greater the
range and object penetration ability, all other factors being equal.
Over-the-Air (OTA) Data Rate The OTA data rate has a significant impact on a radios range.
You may have noticed this at home with a wireless laptop. Walk farther away from your
wireless router and the data rate drops. Carefully analyze the specifications for any 802.11a/b/g
router, and you will note the receiver sensitivity drops as the data rate increases. This effectively
means that to select the radio with the greatest range, you must select the one with the lowest
OTA data rate, other factors being equal. This is a fundamental rule that applies to all
modulation schemes and radios operating at all frequencies. Note that many radios are capable
of multiple OTA data rates. Often the manufacturers data sheet will only quote the receiver
sensitivity at the lowest rate, yielding the best number.
The application will likely dictate the required OTA data rate. The important point here is that
selecting a radio with an OTA data rate much higher than what you require will not provide any
advantage. Note – do account for the RF protocol’s overhead, which must be added to the user
data required throughput, to come up with the OTA throughput.
How to Determine Range This is the most common question wireless users have, so we will
discuss this thoroughly. The most effective way to determine a radios range is to power up the
exact models you intend to use, on site, holding antennas where you plan to permanently mount
them. Next measure the throughput and see if this is sufficient for your application. For modem
type products, you can measure throughput using programs like Iperf. If it is an I/O type
application, sending some contact closures to the remote site should be sufficient. View the
received signal strength. You will need a 10dB minimum difference between your measured
signal strength and the threshold of reception, as specified on the manufacturers data sheet. A
20dB difference (also called fade margin) is more conservative, and should be considered for
critical links, where foliage growth is expected to obstruct the path in the future, higher
background noise levels are anticipated in the future (due to more radios being installed) and/or
to allow for equipment degradation over time.
Background Noise and Interference Measurement After successful measurements as
described above, use a spectrum analyzer to measure the noise floor in the area. Specifically, the
noise floor should be measured using the same antennas your radio will be connected to. This
only needs to be done at receiver or transceiver locations. Transmitters are unaffected by noise.
The received signal strength should be 10dB minimum above the noise floor.
Distributed with permission of author by ISA 2011
Presented at ISA 2011; http://www.isa.org
Some types of radios will be more affected by interference/noise floor. A modem type product
will likely have a duty cycle far higher than an I/O type radio. As such, the importance of
measuring the noise floor is much higher with high duty cycle modem type radios. For example,
a master radio modem used in a SCADA application may have a 100% duty cycle. In this
scenario, the master PLC polls each remote site (via the radio modem) immediately after
receiving an error free reply from the previous site, continuously polling all remote sites
consecutively. However an I/O type radio, used to transmit then replicate at the receiver contact
closures or 4-20mA signals, may only need 1 or 2 successful transmissions per minute.
Additionally, the occupied bandwith of the type of radio you intend to use also determines the
importance of measuring the noise floor with a spectrum analyzer. A DSSS radio can have an
occupied bandwidth of as much as 20MHz when transmitting, whereas a FHSS radio might only
occupy 30KHz. Therefore the DSSS radio application will require a cleaner spectrum; the
importance of using a spectrum analyzer to see what interference may be nearby becomes much
more critical.
Software RF Propagation Studies In some cases, antennas may have to be mounted up high to
clear obstructions and allow for curvature of the earth. It may not be possible to do a field test
temporarily holding antennas at heights of 50+ feet if no nearby structures exist. In these
circumstances a software pathloss study can be done. Note that a software prediction is not as
accurate as an on-site test, therefore antenna heights or gains may need to be adjusted after initial
installation. Propagation software requires that the specifications of the radio be entered, along
with the GPS co-ordinates of each location. The software will then predict the received signal
strength and required antenna mounting height. These software studies often function as
feasibility studies. If they require antennas to be mounted 150 feet in the air for suitable
reception, and no tower is already available, then alternate solutions like repeaters or GPRS
radios can be explored. Again, it must be emphasized that software is theoretical, and there are
many variables that can affect actual results.
Distributed with permission of author by ISA 2011
Presented at ISA 2011; http://www.isa.org
Figure 6.0 Pathloss Propagation Software
Filters One of the most important components in a radio are the filters. They block RF
interference that is outside of the operating band from reaching the low noise amplifier (LNA) of
the receiver. High quality filters will have sharp cut off characteristics ensuring that interference
adjacent to your operating frequency does not make it to the LNA. However high quality filters
are expensive and can be physically large – making them difficult to place on a circuit board.
Radios designed for high interference industrial environments should employ good quality
filters. Inexpensive radios intended for non-critical consumer use may employ low quality or no
filters at all. An external filter can be installed between the radio and the antenna, if the internal
filtering is unsuitable, however its cost can equal that of the radio.
Figure 7.0 Ideal Filter Characteristics
Figure 7.0 shows the characteristics of an ideal filter. In reality, the characteristics have rounded
corners and never perfectly vertical lines. The more expensive the filter, the closer the
resemblance to that of an ideal filter.
Distributed with permission of author by ISA 2011
Presented at ISA 2011; http://www.isa.org
Interference Mitigation Once you have determined that interference is the source of a problem
with a radio link, you need to determine if this is in-band interference or out of band interference.
If it is out of band interference, for example an old paging system operating at 929MHz, and
your radio uses the 902 to 928MHz band, you could purchase an external filter. Alternately, if
your radio can be programmed to use a lower portion of the 900MHz band, re-program the
radios. And lastly, you could change out your antennas to use high gain directional antennas,
and point them away from your source of interference.
If the interference is in-band, such as a wireless internet service provider that operates from 902
to 912MHz, and your radio also uses 900MHz band, your options are limited. First, see if the
radios can be re-programmed to use the upper portion of the band. Next, see if the antennas can
be re-located several feet above or below the interferers antenna. Antenna separation of just a
few feet provides a significant attenuation of an interferers signal. If omni directional antennas
are being used, if possible, switch to yagi directional antennas to make them “blind” to sources
of interference outside of the direction they are aimed. If the radios are programmable, increase
the number of retries, in the hopes that some transmissions will make it through, providing a
functional radio link. One solution implemented involved switching the radio type from modems
to I/O radios. Modems generally require a 50-90% success rate of transmissions, whereas I/O
radios require a success rate of 10-20%. Finally, switch out the radios for ones using another
frequency, but measure the spectrum there first, to avoid repeating the same problem.
4.0
Topologies and Mesh Performance
Topologies There are three common topologies encountered in SCADA applications; point-topoint, star and mesh. Point-to-point networks involve only two radios and are generally straight
forward. Star networks normally involve one master radio in the center, and all others must link
to it. Star networks can involve repeaters, used to relay data to the master when distances or
obstructions prevent a reliable direct path. Mesh networks involve no specific master radio, but
instead send data from one node to another, with the ability to re-route data through another node
(or repeater), if the primary path becomes unavailable. In most real world industrial SCADA
installations, mesh networks usually do have a master, and more or less resemble star networks
extensively utilizing repeaters.
SCADA Polling Methods There are two commonly used SCADA data gathering techniques;
consecutive polling and report by exception. Polling remote sites consecutively ensures that only
one radio transmits at any given time, however all communications must be co-ordinated by the
master radio. Report by exception allows urgent data to get transmitted, however is not
deterministic so should not be used on very fast processes. This is changing, however, with very
high throughputs now possible. The issue of what happens when multiple remote sites wish to
all report their exception at the same time, is to be discussed later.
Mesh Types This section is to differentiate between fixed and mobile mesh networks and their
differing needs. Fixed mesh networks are where each node is permanently bolted down, and will
never move. In this application, the mesh radio offers the advantage of providing a redundant
radio path, via other nodes, to the master or desired end point. This is useful where obstructions
may block a path, seasonal foliage growth may reduce reception in the summer only, or for
critical links, where the data absolutely must arrive.
Distributed with permission of author by ISA 2011
Presented at ISA 2011; http://www.isa.org
Mobile applications are where the mesh network provides a significant advantage. Nodes
moving around may pass out of range of one repeater node, and require another repeater node to
pass data to the intended recipient (master). Having a mesh network of radios will also extend
the size of an area where nodes may roam. As long as they can maintain an RF link to at least
one other node, they can successfully get data to the master.
Mesh Advantages As discussed above, the big advantage of mesh networks is the ability to
automatically re-route data via repeater nodes to the final destination. Paths and routes do not
need to be programmed into each radio. In the case of mobile applications, a mesh offers
complete freedom to roam within the area of mesh coverage. No predictions need to be made of
where a radio may be located, and which repeaters should be made available. In many mobile
applications, a mesh network is the only solution that meets a users requirement for complete
flexibility of roaming.
A good mesh radio will offer the user the ability to program the minimum received signal
strength and or quality of service before switching over to another node. A dedicated repeater
node would typically have more stringent RF signal parameters than one in outlying areas of the
mesh.
If there is no AC/DC power source available and the mesh radios must operate off batteries, the
transmit power is often much lower to ensure a reasonable battery life. This makes mesh radios
mandatory, as the associated shorter range means the signal may not be able to make it directly
to the control room. Leading into the discussion of disadvantages, is the well documented
problem of repeater nodes draining their batteries quickly due to a high duty cycle – when
functioning as a repeater for many remote nodes.
Mesh Disadvantages Mesh networks have few disadvantages, and it is the authors opinion that
the industrial wireless future will heavily feature mesh technology. However one disadvantage is
the requirement of the antenna to be of omni or wide beam yagi. By using an antenna that can
pick up signals from many directions, you expose the radio to interference it may not otherwise
see. This implies a more heavy reliance on good quality filters, which can be expensive, and
sometimes cost pressures may compromise this critical component in the radios design phase.
An additional disadvantage of a mesh network combined with a report by exception
communications strategy, is what happens when several nodes all need to report values at the
same time. Congestion can occur if there are only 1 or 2 repeater nodes connecting dozens of
remote points. A slower response occurs, therefore it is necessary to calculate the worst case
response time, and ensure there is sufficient bandwidth to get data to the master within an
acceptable timeframe. Lastly, the protocol should acknowledge replies, so a radio knows to
continue broadcasting its data if not received.
Mesh Application Lessons A large biotech company stored research material in freezers,
fridges and incubators. These temperature critical samples must be stored while FDA
certification is obtained or as part of on-going research. Temperature records were required, and
alarming on high/low temperatures so the samples could be moved to another
freezer/fridge/incubator in the event of equipment failure. These temperature chambers were on
castor wheels, and moved to different labs and to different buildings. A wireless mesh network
Distributed with permission of author by ISA 2011
Presented at ISA 2011; http://www.isa.org
was the only practical solution that met the unrestricted mobile needs of this organization. In
deploying the system, several lessons were learned:
1) Dedicate some radios as repeaters. Due to the mobile nature of the application, it could
not be guaranteed that there would always be a temperature chamber with a mesh radio in
a given area to act as a repeater. The client required certainty. Dedicated repeaters in the
center of each building provided this. In addition, one radio could end up as a repeater
for as many as a hundred nodes. In a power outage, many nodes could transmit at the
same time with data congestion occuring on a single repeater radio. The dedicated
repeaters reduced this to a level in-line with the throughput of the radios.
2) Over-the-air diagnostics are your friend. Being able to view a remote radios
configuration and diagnostics saves a walk/drive to that remote site. In addition,
sometimes the chambers could not be located, or access to a particular building
unavailable. The time savings associated with this cannot be understated.
3) High transmit power makes a mesh network more reliable and simpler. In a campus like
setting, radios with 50 or 100mW of transmit power would not provide sufficient range.
As a result, more dedicated repeaters are required and the chambers would lose
communications when moved to basement areas or between buildings. If background
noise levels increase, having a large fade margin, by virtue of the high transmit power,
allows the network to continue operating. Related to range, a radio system operating at
the lowest available frequency will have the greatest range and ability to penetrate walls.
A previous wireless supplier in this facility found this out the hard way.
4) Do a site survey in advance. The site survey measured received signal strength
throughout the campus. As such, where and how many dedicated repeaters could be
planned and dead zones with no reception eliminated. The survey should also include a
spectrum analysis, to show how much interference from existing wireless systems is
present.
There were many other lessons also learned on this project. The ones above were the most
important. On a non-wireless note, another lesson learned would be to hire an experienced
systems integrator who lives close by (if possible). It will save your company many airfares and
hotel charges, and your family will appreciate you being home each evening.
5.0
Conclusion
Wireless technology has benefitted from tremendous innovation in the past decades improving
its suitability to industrial use. A radio system that changes or uses multiple frequencies will be
more reliable. Range specifications on data sheets present only a guideline, other parameters
should be analyzed to compare different manufacturers. Site surveys are mandatory for all but
the shortest radio links. Antenna work, filters and spectrum analysis are the keys to resolving
interference problems. Mesh networks provide the ultimate flexibility for mobile applications
where unrestricted freedom to roam is required. Learn from the experiences of past installations,
this can save considerable time and labor.
Distributed with permission of author by ISA 2011
Presented at ISA 2011; http://www.isa.org