5
The Task of Local Area Networks in Industry
Industrial local area networks (LANs) are used to establish communication
between computers, programmable logic controllers (PLCs), and a variety of
industrial devices such as I/O modules, motor starters, sensors, actuators,
valves, and so on. LANs can have several control points governing functions
cooperatively or all functions can be controlled from a central location.
Many industrial facilities lack the network coverage needed to operate
effectively. Most have isolated islands of automation. That is, various
operations of the manufacturing or processing plant are automated, but the
whole is not integrated. The left hand does not know what the right hand
is doing. Coordinating this type of a manufacturing enterprise is highly
labor-intensive.
The challenge is to get all of these components to communicate together
and function as a system. To do this they must first, of course, be
connected. The typical industrial LAN will have everything interconnected
with wire cable.
The second challenge to coordinating the network is that all components
must use the same protocol (understand the same electronic code coming
from the computers, PLCs, etc.) and run on the same buses. The problem is
that in any industrial facility, there will already be a variety of protocols in
place – protocols such as Ethernet, ProfiBus, Modbus, DeviceNet and dozens
of others – with the corresponding hardware buses on which they run.
Replacing the equipment that runs on these protocols and buses could be
prohibitively expensive.
So we have two major problems to contend with in networking the
industrial enterprise: wire and protocols/buses.
6
The Limitations of Wire Networks
Figure 1. Typical Hard-Wired Industrial Network
Valve Island
Pushbutton
Cluster
Message Display
Pneumatic Manifold
Block I/O
Sensor
Allen Bradley
Other
Devices
Motor
Controller
AB
User InterfaceController
7
While LANs that are totally interconnected by wire are generally reliable,
they do have their limitations:
• Physical limitations and problems – Wires break; bad connections can
cause ‘standing waves,’ which degrade performance. In addition, wires
pick up electronic noise.
• Cost of installation and maintenance – Designing and installing
wiring is usually a time-consuming and costly task. Maintaining
outdoor wiring usually puts technicians up on a pole or down in a hole
just to access the wire. And then, locating the problem can be
extremely difficult.
• Protocol incompatibility – Existing systems in industrial facilities are
frequently tied to incompatible protocols. Connecting these systems
and getting them to “talk” to one another is a difficult task at best.
• Lack of mobility and adaptability – When your products or processes
change, your production facilities must change. Wiring must be
replaced or re-routed to accommodate the changes. This can be a huge
headache and expense.
• Distance and space limitations – The longer the wire, the more
susceptible it becomes to electrical noise and the more difficult it will
be to locate problems when they occur. Wire can be damaged, crimped
and cut in hard-to-find places. If wire must run outdoors, it is difficult
to protect it from the extremes of weather. It is also not practical to
run wires in places where they could be exposed to extreme
temperatures, get in the way of moving machinery, etc.
• Wire logistics – In sophisticated configurations, the complexity of the
wiring can be overwhelming. The insides of panels begin to look like
spaghetti bowls. This means a slower set-up time and longer repair
delays.
8
What is a WLAN?
A Wireless Local Area Network (WLAN) is a LAN
that uses transceivers that exchange radio signals
to substitute for some or all of the wires.
(Transceivers are radios that can both send and
receive). WLANs do not typically replace wired
LANs. They allow you to expand the LAN to places
where wire is inconvenient, cost-prohibitive,
or ineffective.
There are two parts to a wireless LAN: the access
point transceiver and the remote client
transceivers (see Figure 2). The access point is the
stationary transceiver that attaches to the main
wired LAN. The remote client transceivers link the
remote parts of the LAN to the main LAN via radio
waves. The main LAN and the remote parts of the
LAN have no physical contact, but from the point
of view of all the computers, equipment, sensors
and actuators, they operate exactly as if they were
one large, hard-wired LAN.
(Note that there are also wireless modems today
that provide a partial networking solution. They
require special software, programming, and a direct
hookup to the PLC. Because of the difficulty of
installation and operation, these wireless modems
are an inferior solution.)
The advantage of adding WLANs is in increased
flexibility, mobility and the ability to reconfigure.
Equipment no longer has to be anchored to a fixed
spot. The user no longer has to work from a
stationary workspace. Parts of the WLAN can be located across spaces that
are impossible to bridge by wire.
Protocol “Gateways” and “Bridges”
In addition, the transceivers of a WLAN can form protocol “gateways”
and “bridges.” A gateway works like this: The access point transceiver
communicates in one protocol with the main LAN while the remote
transceiver communicates with the remote LAN in another. The WLAN
OpenLine
3rd Party
Hardware
Operator Workstation
9
OpenLine RTU System
Manufacturing Enterprise Network
Plant Information Network
SCADA
Workstation
Engineering
Development
Workstation
EZCom WLAN Transceiver
3rd Party RTU
OpenLine
OpenLine
HMI
Alarm
Trending
OpenLine
RS-232 Operator
Interface
Sensors and Actuators
Sensors and Actuators
MicroDAC
DacNet
EZCom WLAN
Receiver
Device Network
Sensors and
Actuators
Mainframe
Enterprise
Software
Computers &
Servers
In a WLAN, remote components are connected to remote client transceivers which send and
receive signals to and from the access point transceiver on the main LAN.
transceivers do all the work of making the previously incompatible
equipment speak to one another. Protocol “gateways” among as many as
fourteen different industrial protocols are in the works. (See Figure 10).
Protocol bridges are used to link to LANs that use the same or very
similar protocols. They do not require as much “translation” functionality
as gateways.
Figure 2. Wireless Industrial Network
10
Advantages of WLANs
Impossible Wiring Problems Solved – How do you maintain electrical
contact over the heat of a blast furnace? Across a burning desert? A frozen
tundra? A busy street? With a WLAN, wiring isn’t necessary. Radio waves
pass easily through heat, cold, traffic, and the flames of a furnace.
Long-Distance Capabilities – Interconnecting production or processes in a
large facility or a network of facilities can use up many miles of wire. The
transceivers of today’s WLANs have ranges of 5 to 15 miles and can be
extended almost limitlessly with the use of repeaters.
Flexibility – With WLANs, changes to production line or process
configuration can be made quickly – without closing it down for lengthy
periods for costly and time-consuming rewiring.
Reduced Wiring Costs – With wireless components connecting key parts of
the LAN, there is less wire to install and to maintain.
Mobility – Workers on the go can use portable terminals to send inventory,
production, or shipping and receiving information to a central data-
collecting computer.
Noise Resistance – The new WLANs using spread spectrum technology are
impervious to industrial electrical noise.
Reliability – The new wireless communications are actually more reliable
than wire. They are virtually jam-proof.
No Service Provider Needed – No hidden costs. You do not require a license
or a service provider (as with cellular phones, ESMR mobile radios, and
pagers) to operate on the radio wavelength WLANs use.
Data Security – It is extremely unlikely that the electronic signals on your
industrial WLAN could be readable by anyone anyway – but because of
spread spectrum technology (see page 14), the transmission is virtually
impossible to intercept by unauthorized “listeners.” WLAN-transmitted
processes cannot be jammed or intercepted by the competition.
11
Would Your Operation Benefit from a WLAN?
The growth in the use of WLANs is increasing rapidly as the technology
improves and prices become more competitive. Many industries, or sectors
of industries, already accept wireless technology as the norm. This includes
many water and wastewater treatment plants, oil and gas facilities,
electrical utilities, irrigation systems and more.
Other uses, such as facility and machine maintenance, are beginning to
experience rapid growth. And with the latest WLAN technology the way is
finally open for all kinds of manufacturing and processing applications,
particularly in data acquisition and control networks. The field is poised for
an explosion of applications.
(For more details on applications, see the section Current and Emerging
Applications, page 21.)
12
Spread Spectrum Technology
The basic spread spectrum technology that makes wireless local area
networks possible has been around for a long time. The United States
military developed spread spectrum radio during World War II as a way to
send radio signals that resisted jamming and were hard to intercept.
Spread Spectrum refers to a class of modulation techniques
characterized by wide frequency spectra. A true spread spectrum signal
meets two criteria:
1 The transmitted signal bandwidth is much wider than the information
bandwidth (see figure 3). Instead of a narrow, specific frequency like
that used by an FM radio station, the signal is much wider than is
actually necessary for the information being transmitted. The actual data
being transmitted is modulated across the wide waveband and sounds
like noise to unauthorized receivers.
2 Some pattern or code other than the data being transmitted determines
the actual transmitted bandwidth. It is the use of these codes that
allows the authorized receiver to pick out the needed information from
the signal. The width of the signal and the structure of the code allow
the data being transmitted to be understood even if parts of the signal
were to be blocked by electrical noise.
Because spread spectrum is immune to interference in this way, it’s a
natural for industrial network applications. The FCC has set aside three
bands for commercial spread spectrum use: 900 MHz, 2.4 GHz, and 5.7 GHz.
Products operating in these bands are operating where very little industrial
noise is present. No FCC site licensing is required in the ISM band.
13
Figure 3. Spread Spectrum
Spread Spectrum takes its name from the wide bandwidth it uses. The wide bandwidth is part of
what makes it immune to interference. Compare to the narrow FM band shown in the center of
the graph.
CW SIGNAL
AMPLITUDE
SPREAD SIGNAL
AMPLITUDE
FREQUENCY (MHz)
2.43 2.44 2.45 2.46 2.47
(dBm)
18
15
12
9
6
3
0
(dBm)
1.2
1.0
0.8
0.6
0.4
0.2
0
14
Selecting the Right Spread Spectrum System
Of the various spread spectrum systems that have been adapted for
commercial use, the two most commonly used are:
Direct sequence spread spectrum (DSSS) systems encode the data to
be transmitted by using a seemingly random sequence of binary values.
This is called a pseudo-random noise (PN) code. The combined digital data
and PN are scrambled and spread over a fixed range of the frequency band.
Because the PN code has a frequency bandwidth much higher than the
bandwidth of the data, the transmitted signal will have a spectrum that is
nearly the same as the wideband PN signal.
On the other end, a receiver correlator picks up the signal. This SS
correlator is ‘tuned’ so it only responds to signals that are encoded
with the specific PN code. The correlator filters out all the garbage
and extracts the needed coded information. This allows several sets of
transceivers to operate using different codes in the same geographical
area without interfering with each other. This is called Code Division
Multiple Access (CDMA).
Frequency hopping spread spectrum (FHSS) means the signal is spread over
a wide band by transmitting for a short burst and then ‘hopping’ to another
frequency. The order of the hops depends on the code sequence.
Two key elements are needed for FHSS systems to function. First the
hopping pattern must be known to the receiver. Second, the radio
designated as “master” must provide the synchronization so that other
radios using the same pattern can follow and hop at the same time.
Although different, both FHSS and DSSS products are well-suited to
industrial applications due to their noise immunity and ruggedness.
Grayhill EZCom wireless includes products that use FHSS and products that
use DSSS spread spectrum.
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