chimikal

Reviewing Wireless Data Communications Systems

By: Paul Mercier, Phoenix Contact, Inc., Fort Worth, Texas

Wireless technology is rapidly gaining acceptance in industrial applications, and manufacturers are developing new wireless devices at an accelerating rate. With so many radio products currently on the market, choosing the correct one for an application is no easy task.

In addition, the mass availability of the internet is enabling field personnel and operations managers to stay informed of alarms, trends and reports from virtually anywhere.

The goal here is to discuss these emerging trends, and review some basic methods of developing a data gathering system for sensors, remote assets and remote control units based on private wireless networks, public wireless networks, or a “hybrid” combination of technologies.

Wireless advantages

Traditional methods of accessing field units for supervisory control and data acquisition (SCADA), machine-to-machine (M2M), automated meter reading (AMR), and sensor data transfer for asset management have primarily relied on dedicated licensed radio, trunked radio or a dedicated landline from the local telephone company.

With the widely available and proven reliability of today’s wireless solutions in the spread spectrum license-free band, digital cellular band and low-earth orbit satellite bands, many utilities and energy professionals can now quickly deploy new or additional remote data gathering units in a fraction of the time. An additional benefit is that this technology enables operators and field personnel to perform these tasks themselves if they so choose, while also allowing for a multitude of access and interface options. Mobile data applications have also been expanded by these very same technologies.

Reliability, availability and cost are important considerations in any communication network. These factors become even more critical when the application involves monitoring alarms on compressors, leak detection sensors or security surveillance devices. By contrast, the difficulty of installing and maintaining hardwire cable or leased line makes their cost very high.

Collectively, these technologies and applications solutions deliver current and proven methods of wireless data communications for overall improved efficiencies. Much of the challenge for today’s professional is applying the correct technology to fit the application requirements. The first step is to define needs and assess goals.

Define needs and goals

Today, market forces are causing a change. There is a confluence of need, desire and enabling technology. With less total manpower, there is a need to be more efficient, and with most of us being mobile workers, we have the desire to access information that is now viewed as vital to operations. Expansion of legacy systems is less attractive than before, now that enabling technological events have taken place. The internet, reductions in communications cost, reduction in sensor and end-device cost, along with less expensive host

software and shared host environments, should inspire organization to reassess operational needs.

The decision to choose a wireless device is most often made in the interest of saving money. Saving money is the culmination of saving time, restarting stalled projects, cutting expensive line items from the budget and reducing recurring expenses. Just think of the savings when you don’t have to use wire and conduit – no digging, drilling, backfilling, pulling permits, not to mention the good will incurred when you don’t have to disturb a private property owner to access your remote asset.

Think of moving wireless data from point A to point B like moving “stuff” in a box or an envelope. Some things to consider include:

• What are you moving? Sensor data or file transfers?

• How much will you bring? Bytes or Megabytes?

• Where are you moving it? Two miles or Two thousand?

• When do you need it by? Real-time, near-time or sometime?

• What route will it take? Terrestrial, celestial or subterranean?

• By what vehicle or means of transport? Public or proprietary network?

Justification

In today’s lean operating environment, having quick and secure access to the latest information is crucial. Unfortunately, until recently, companies with assets spread over wide areas have had no simple, economical way to link their sites electronically for remote monitoring. Traditionally, the options for linking these sites involved tradeoffs. Ground-based communications links such as radio or cellular only offer localized coverage. While traditional satellites covered wide areas, the cost of hardware and data transmission airtime was often prohibitively expensive.

Wireless devices are used in a wide array of applications from flow measurement, wellhead monitoring, tank-level inventory, cathodic protection, leak detection, power consumption, vibration monitoring, chart recorder replacement, and custody transfer. The possibilities are infinite as wireless data communications becomes ubiquitous and cost-effective to deploy. This is where the rubber meets the road. With so many possible applications, it becomes very important to marry the correct technology to the application. Being an informed buyer will make you that much more efficient.

Choosing a telemetry network

A telemetry network provides the communication pathway in a SCADA system. The components that make up this type of system consist of a master station or central host, data communication equipment and related topology, link media and any protocol issues, and the remote station hardware. Remember that an application can have more than one telemetry network. In some critical applications, you may want to design a back-up system or recovery procedure for your main network.

Topology considerations

The topology is the geometric arrangement of nodes and links that make up a network. For a SCADA system, one must choose among the topologies of either point-to-point, point-to-multipoint, or multipoint-to-multipoint (mesh) for your communication network (Figure 1).

In the PTP and PMP communication link, one station acts as a communication arbitrator (master) that controls when the other stations (slave stations) can communicate. The PMP is the main topology for SCADA and data monitoring, but a growing number of packet-based networks are making the mesh network an attractive alternative for wireless data delivery. In this architecture, there is no communication master, and any station can initiate communication with any other station to achieve a peer-to-peer link. TCP/IP based networks lend themselves to this topology.

The “big three” wireless questions still have to be:

• How far will it go?

• How fast is it? (throughput

and latency)

• How much is it?

With boxes of “stuff,” we know how to trade size, weight, speed and cost. So, what are the trade-offs for wireless? Or is it simply magic? Most salespeople want to dazzle you with distance and speed specs of their product, but make sure there are engineers backing the solution who have taken into consideration the radio frequency (RF) environment and reliability factors from the design phase up.

Network connectivity

When choosing a link media, be sure to consider such items as data transmission needs of the application, remote site and control center locations, distance between sites, available link media services, and, of course, your project budget. Be sure to consider the remote station needs. These include power (AC, solar panel and battery); environmental extremes; inputs/outputs; sleep mode; data ports; data logging; and alarm limits.

Also, consider the host gateway and the available interfaces and connectivity it supports. Do you want to a SCADA master to “talk” via OPC, DDE, ODBC and SQL? Does corporate want access to measurement data but not operational data?

A hybrid or mix of technologies can offer the best of both worlds. The cost to bring communications to a remote site using spread spectrum, cellular or low-earth orbit satellite is substantially reduced due to the relatively inexpensive hardware and the minimized amount of power needed to reach a base station or repeater. Use of “off-the-shelf” components for antennas, power supplies, and serial/Ethernet interfaces make developing and applying solutions better for the current generation of do-it-yourselfers.

Spread spectrum

It has now been 20 years since the FCC allowed spread spectrum operation to be used in the commercial sectors of radio communications. Those years have been spent in intense research and development efforts by a number of companies, and these efforts have yielded a new generation of radio systems. A significant attribute of these new radio designs is the fact that they use spectrum spreading techniques in order to share the allocated radio bands with many diverse users. The specific implementations of spread spectrum are tailored to the applications using either frequency hopping or direct sequence technologies.

A major advantage of using more spectrum than required by employing spreading techniques rather than a single, narrowband channel is the effect of being resilient to interference from noise or other radio energy. Industrial radio systems, such as wireless data collection and telemetry for oil/gas field application, often must be designed into difficult environments composed of noisy machinery, varied terrain and wide temperature extremes.

Spread spectrum technology can increase the overall reliability at the “physical layer” in these applications, but just as important is the ability to have a reliable method of insuring that the data has been delivered at the “link layer,” even if the radio channel is operating at the extremes of signal-to-noise ratios.

Building intelligent radios with packet protocols that perform data delivery acknowledgements can only happen with modern microprocessor and highly integrated technologies. The radio system software is the final frontier in modern radio network design. The major challenge to using spread spectrum is the capital cost to build the network and some minor operational maintenance expenses.

Cellular

Operators can now take advantage of the new cellular telephone network for wireless data communications. Installation of a cell-based RTU is simple, and setup is virtually automatic. Also, all communication is in a digital format, ensuring that reliable communication is available even in areas where voice cellular coverage may be marginal. The advances in packet-switching technology have made it possible to network more remote assets with greater flexibility, especially in the area of IP addressing. The most common data services that run over the primary wireless carriers’ GSM and CDMA cell technologies

is general packet radio service (GPRS) and evolution-data only (EV-DO), respectively.

Look for automatic “audit” health checks features that check the validity of communication, and notify you if any fault exists. Also, look for capability of the wireless modem to handle your specific end-device protocol. Like all good technology, make sure you are given the diagnostic tools and configuration software that will be useful in diagnosing and troubleshooting your equipment to verify correct operation and installation of the remote device. The major disadvantage here is the recurring monthly cost and lack of coverage.

Low earth orbit satellites

Companies adopting automation have had to contend with issues such as lack of existing communications infrastructure in areas with no cell tower coverage or where no licensed spectrum is available for use. In other areas, there might be saturation of spread spectrum radios causing interference, or just plain sparse network density discouraging terrestrial based wireless implementation. If you have ever driven through the gas production fields, you can appreciate the difficulty of finding a cell signal or a point of presence for plain old telephone services. Low earth orbit (LEO) technology removes many of these challenges.

LEOs are ideal for transmitting sensitive or proprietary data. Information is moved in small digital packets that are difficult to intercept, and private data can be encoded within and made secure to prevent unauthorized use. The systems use acknowledgements from a SCADA host or workstation to remote sites for complete, end-to-end guaranteed data delivery. The advantages of satellite are largely about coverage. The disadvantages, including hardware and service costs, along with slower data throughput and latency, have made this solution less acceptable for many control or data intensive applications.

Data delivery services

With so many solutions and no single-source manufacturer, some companies are seizing the opportunity to provide data delivery services utilizing a hybrid of spread spectrum and UHF radio technology for the last mile(s), and then “back-hauling” that information via a satellite and/or cellular network access. This data can be delivered in a number of formats, depending on needs. For example, one service allows a secure Web-based client to view the operation of the site over the Internet using a password-protected file server. Data can also be moved from the file into relational databases, and the service can usually support integration with legacy operations and accounting systems.

Some industry observers have noted that as companies seek to enhance their enterprise computing capabilities, there is a need to incorporate data from an increasing number of remote points, such as sensors, machines and meters. The data collected from these field devices can range from a few bits to megabytes, and may represent the company’s cash register or reveal trouble with a critical piece of equipment.

M2M data acquisition and delivery service provides a cost-effective alternative to the manual collection of data and to the creation of an automated collection system. It eliminates the need to build, operate, and manage the telecommunication and computer network yourself. It is sometimes less expensive for clients to gather data with a third- party service rather than collecting the data manually, since field assets are just as likely to be installed on mountaintops as they are on rooftops, making human access difficult. Further, a data collection service differs greatly from traditional telecommunication alternatives which only provide transport, which is a very small portion of a fully managed end-to-end solution (Figure 2).

A standard offering from a data delivery company should inc-lude ruggedized, industrial remote gateway units which connect to equipment in the field; transport of your company’s data via one of several available telecommunication technologies; 24 x 7 network management and help desk support; data archival; notification in the event of a field alarm; a web-based network visibility tool; plus the ability for clients to retrieve data and generate reports.

The author

Paul Mercier is the National Wireless Specialist for Phoenix Contact USA, and has been involved with development and deployment of wireless data systems for over 15 years. He has helped pioneer spread spectrum into the oil/gas, utility and water/wastewater industries, and is a former ENTELEC Silver Scribe Award winner in 1998 for his paper on “Unlicensed Radio in a Changing Regulatory Environment.”

Acknowledgment

Based on a paper presented at ENTELEC Conference and Exposition, April 19-21, 2006, Houston, Texas.

CHEMISTRY TEST

Purpose of the
GRE Subject Tests
The GRE Subject Tests are designed to help graduate
school admission committees and fellowship sponsors
assess the qualifications of applicants in specific fields
of study. The tests also provide you with an assessment
of your own qualifications.
Scores on the tests are intended to indicate knowledge
of the subject matter emphasized in many undergraduate
programs as preparation for graduate study.
Because past achievement is usually a good indicator of
future performance, the scores are helpful in predicting
success in graduate study. Because the tests are standardized,
the test scores permit comparison of students from
different institutions with different undergraduate
programs. For some Subject Tests, subscores are provided
in addition to the total score; these subscores indicate
the strengths and weaknesses of your preparation, and
they may help you plan future studies.
The GRE Board recommends that scores on the
Subject Tests be considered in conjunction with other
relevant information about applicants. Because numerous
factors influence success in graduate school, reliance
on a single measure to predict success is not advisable.
Other indicators of competence typically include undergraduate
transcripts showing courses taken and grades
earned, letters of recommendation, and GRE General
Test scores. For information about the appropriate use
of GRE scores, write to GRE Program, Educational
Testing Service, Mail Stop 57-L, Princeton, NJ 08541,
or visit our Web site at www.gre.org/codelst.html.
Development of the
Subject Tests
Each new edition of a Subject Test is developed by a
committee of examiners composed of professors in the
subject who are on undergraduate and graduate faculties
in different types of institutions and in different regions
of the United States and Canada. In selecting members
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of the appropriate professional associations in the subject.
The content and scope of each test are specified and
reviewed periodically by the committee of examiners.
Test questions are written by the committee and by
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and by subject-matter specialists at ETS. All questions
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and revised as necessary. The accepted questions are
assembled into a test in accordance with the content
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Subject-matter and measurement specialists on the
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appropriate balance of people in different groups and
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it is not possible for a single test to cover all the
material you may have studied. The examiners, therefore,
select questions that test the basic knowledge and
skills most important for successful graduate study in
the particular field. The committee keeps the test
up-to-date by regularly developing new editions and
revising existing editions. In this way, the test content
changes steadily but gradually, much like most curricula.
In addition, curriculum surveys are conducted
periodically to ensure that the content of a test
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curriculum.
After a new edition of a Subject Test is first administered,
examinees’ responses to each test question are
analyzed in a variety of ways to determine whether
each question functioned as expected. These analyses
may reveal that a question is ambiguous, requires
knowledge beyond the scope of the test, or is inappropriate
for the total group or a particular subgroup of
examinees taking the test. Answers to such questions
are not used in computing scores.
Following this analysis, the new test edition is
equated to an existing test edition. In the equating
process, statistical methods are used to assess the
difficulty of the new test. Then scores are adjusted so
that examinees who took a difficult edition of the test
are not penalized, and examinees who took an easier
edition of the test do not have an advantage. Variations
in the number of questions in the different
editions of the test are also taken into account in
this process.
Scores on the Subject Tests are reported as threedigit
scaled scores with the third digit always zero.
The maximum possible range for all Subject Test total
scores is from 200 to 990. The actual range of scores for
a particular Subject Test, however, may be smaller. The
maximum possible range of Subject Test subscores is
20 to 99; however, the actual range of subscores for
any test or test edition may be smaller than 20 to 99.
Subject Test score interpretive information is provided
in Interpreting Your GRE Scores, which you will receive
with your GRE score report, and on the GRE Web site
at www.gre.org/codelst.html.
Content of the Chemistry Test
The test consists of about 136 multiple-choice questions.
A periodic table is printed in the test booklet as
well as a table of information (see page 10) presenting
various physical constants and a few conversion factors
among SI units. Whenever necessary, additional values
of physical constants are printed with the text of the
question. Test questions are constructed to simplify
mathematical manipulations. As a result, neither
calculators nor tables of logarithms are needed. If the
solution to a problem requires the use of logarithms,
the necessary values are included with the question.
The content of the test emphasizes the four fields
into which chemistry has been traditionally divided
and some interrelationships among the fields. Because
of these interrelationships, individual questions may
test more than one field of chemistry. Some examinees
may associate a particular question with one field,
whereas other examinees may have encountered the
same material in a different field. For example, the
knowledge necessary to answer some questions classified
as testing organic chemistry may well have been
acquired in analytical chemistry courses by some
examinees. Consequently, the emphases of the four
fields indicated in the following outline of material
covered by the test should not be considered definitive.
I. ANALYTICAL CHEMISTRY — 15%
A. Data Acquisition and Use of Statistics —
Errors, statistical considerations
B. Solutions and Standardization —
Concentration terms, primary standards
C. Homogeneous Equilibria — Acid-base,
oxidation-reduction, complexometry
D. Heterogeneous Equilibria — Gravimetric
analysis, solubility, precipitation titrations,
chemical separations
E. Instrumental Methods — Electrochemical
methods, spectroscopic methods,
chromatographic methods, thermal
methods, calibration of instruments

F. Environmental Applications
G. Radiochemical Methods — Detectors,
applications
II. INORGANIC CHEMISTRY — 25%
A. General Chemistry — Periodic trends,
oxidation states, nuclear chemistry
B. Ionic Substances — Lattice geometries,
lattice energies, ionic radii and radius/
ratio effects
C. Covalent Molecular Substances — Lewis
diagrams, molecular point groups,
VSEPR concept, valence bond description
and hybridization, molecular orbital
description, bond energies, covalent and
van der Waals radii of the elements,
intermolecular forces
D. Metals and Semiconductors — Structure,
band theory, physical and chemical
consequences of band theory
E. Concepts of Acids and Bases — Brønsted-
Lowry approaches, Lewis theory, solvent
system approaches
F. Chemistry of the Main Group Elements —
Electronic structures, occurrences and
recovery, physical and chemical properties
of the elements and their compounds
G. Chemistry of the Transition Elements —
Electronic structures, occurrences and
recovery, physical and chemical properties
of the elements and their compounds, coordination
chemistry
H. Special Topics — Organometallic chemistry,
catalysis, bioinorganic chemistry, applied
solid-state chemistry, environmental
chemistry
III. ORGANIC CHEMISTRY — 30%
A. Structure, Bonding, and Nomenclature —
Lewis structures, orbital hybridization,
configuration and stereochemical notation,
conformational analysis, systematic IUPAC
nomenclature, spectroscopy (IR and 1H and
13 C NMR)
B. Functional Groups — Preparation, reactions,
and interconversions of alkanes, alkenes,
alkynes, dienes, alkyl halides, alcohols,
ethers, epoxides, sulfides, thiols, aromatic
compounds, aldehydes, ketones, carboxylic
acids and their derivatives, amines
C. Reaction Mechanisms — Nucleophilic
displacements and addition, nucleophilic
aromatic substitution, electrophilic
additions, electrophilic aromatic
substitutions, eliminations, Diels-Alder
and other cycloadditions
D. Reactive Intermediates — Chemistry and
nature of carbocations, carbanions,
free radicals, carbenes, benzynes, enols
E. Organometallics — Preparation and reactions
of Grignard and organolithium reagents,
lithium organocuprates, and other modern
main group and transition metal reagents
and catalysts
F. Special Topics — Resonance, molecular
orbital theory, catalysis, acid-base theory,
carbon acidity, aromaticity, antiaromaticity,
macromolecules, lipids, amino acids, peptides,
carbohydrates, nucleic acids, terpenes,
asymmetric synthesis, orbital symmetry,
polymers
IV. PHYSICAL CHEMISTRY — 30%
A. Thermodynamics — First, second, and
third laws, thermochemistry, ideal and
real gases and solutions, Gibbs and Helmholtz
energy, chemical potential, chemical
equilibria, phase equilibria, colligative
properties, statistical thermodynamics
B. Quantum Chemistry and Applications
to Spectroscopy — Classical experiments,
principles of quantum mechanics,
atomic and molecular structure, molecular
spectroscopy
C. Dynamics — Experimental and theoretical
chemical kinetics, solution and liquid
dynamics, photochemistry

Preparing for a Subject Test
GRE Subject Test questions are designed to measure
skills and knowledge gained over a long period of time.
Although you might increase your scores to some
extent through preparation a few weeks or months
before you take the test, last-minute cramming is
unlikely to be of further help. The following information
may be helpful.
_ A general review of your college courses is
probably the best preparation for the test. However,
the test covers a broad range of subject
matter, and no one is expected to be familiar
with the content of every question.
_ Use this practice book to become familiar with
the types of questions in the GRE Chemistry Test,
paying special attention to the directions. If you
thoroughly understand the directions before you
take the test, you will have more time during the
test to focus on the questions themselves.
Test-Taking Strategies
The questions in the practice test in this book illustrate
the types of multiple-choice questions in the test.
When you take the test, you will mark your answers
on a separate machine-scorable answer sheet. Total
testing time is two hours and fifty minutes; there are
no separately timed sections. Following are some
general test-taking strategies you may want to consider.
_ Read the test directions carefully, and work as
rapidly as you can without being careless. For
each question, choose the best answer from the
available options.
_ All questions are of equal value; do not waste
time pondering individual questions you find
extremely difficult or unfamiliar.
_ You may want to work through the test quite
rapidly, first answering only the questions about
which you feel confident, then going back and
answering questions that require more thought,
and concluding with the most difficult questions
if there is time.
_ If you decide to change an answer, make sure you
completely erase it and fill in the oval corresponding
to your desired answer.
_ Questions for which you mark no answer or more
than one answer are not counted in scoring.
_ As a correction for haphazard guessing, onefourth
of the number of questions you answer
incorrectly is subtracted from the number of
questions you answer correctly. It is improbable
that mere guessing will improve your score
significantly; it may even lower your score.
If, however, you are not certain of the correct
answer but have some knowledge of the question
and are able to eliminate one or more of the
answer choices, your chance of getting the right
answer is improved, and it may be to your advantage
to answer the question.
_ Record all answers on your answer sheet.
Answers recorded in your test book will not
be counted.
_ Do not wait until the last five minutes of a
testing session to record answers on your
answer sheet.