Moxa ANT-WSB-ANM-05 network antenna

Preface
Wireless technologies have become
increasingly popular in industrial
automation as growing numbers of
system integrators, governmental
agencies, and industrial solution
providers continue to turn to these
solutions for their applications.
Advantages of using wireless
technologies include boosting data
transmission speed, real-time data
transmissions, remote equipment
monitoring and alerts, flexible
installation of remote equipment, and
wide coverage areas. In addition,
wireless technologies can penetrate
areas where cables are unable to
reach, saving wiring costs. By adopting
wireless technologies, industrial
applications are able to benefit from
greater versatility.
However, the completeness of data,
security of transmission, and reliability
of the wireless network are constant
concerns as wireless technologies
rely completely on the emission of
electromagnetic waves through the
air. Drawing from over 20 years of
experience, Moxa offers users the most
reliable industrial networking solutions
including Turbo Roaming™ for
seamless wireless communication, as
well as extended wireless transmission
ranges of over 10 km. In addition, our
complete selection of products for
demanding industrial environments
includes wide temperature (-40 to
75°C) models, IP67-rated protection
from water and dust, and EN50155
certification for rail traffic applications.
We hope this guidebook will provide
you with a more comprehensive
understanding of industrial wireless
technologies and serve as your best
guide to getting un-wired!
It’s time to go wireless!
Moxa Inc.
Chapter 1
Differentiating Between
Wireless Technologies
1.1 WWAN vs. WLAN vs. WPAN---------------------3
WWAN (Wireless Wide Area Network)
WLAN (Wireless Local Area Network)
WPAN (Wireless Personal Area Network)
1.2 Evolution of Cellular Networks-----------------4
3G Technologies
4G Technologies
1.3 Evolution of IEEE 802.11------------------------7
IEEE 802.11n
IEEE 802.11s
1.4 WLAN vs. Proprietary 2.4 GHz------------------9
Chapter 2
Understanding Industrial
WLAN – IEEE 802.11
2.1 IEEE 802.11 Basics--------------------------- 10
Electromagnetic Waves
Signal Power
Bandwidth, Data Rate, and Throughput
2.2 Wireless Security----------------------------- 18
A Peek at the Technology
The Evolution of Wireless Encryption
Using a Firewall as an Additional Safeguard
2.3 Antenna Theory and Selection---------------- 21
Functions of Antennas
Types of Antenna
Key Antenna Specifications
Choosing the Right Antenna for Your Project
2.4 Long Distance Wireless----------------------- 23
Application Topology
Components of the Expanded 802.11
Wireless System
Moxa’s Antennas Selection Guide------------ 30
IEEE 802.11b/g 2.4 GHz Wireless Antennas
IEEE 802.11a/b/g 2.4/5 GHz
Dual-band Antennas
IEEE 802.11a 5 GHz Wireless Antennas
Cellular Antennas
Setting Up Point-to-Point Connections
Antenna Alignment for P2P Operations
Moxa Performance Test Report
2.5 Mobile Optimization-------------------------- 35
Roaming Under Linear Movement
Roaming Speed Acceleration
Limitations of High Speed Roaming
2.6 Advanced WLAN Technologies- -------------- 37
Dual RF Redundancy
Mesh Technologies
Wireless VLAN
QoS for Video/Audio and Control
Wireless Management
2.7 Industrial Certification------------------------ 42
EN50155 Certification
ATEX/Class I Division 2
Chapter 3
Cellular Networks
3.1 Cellular Basics-------------------------------- 44
Data Service of GSM
APN in Packet Switch
3.2 Private IP Solution---------------------------- 48
Private IP vs. Public IP
Delay Time
Solution for Private IP
Moxa OnCell Central Manager
3.3 Security--------------------------------------- 50
The Virtual Private Network (VPN)
Firewall
3.4 How to Connect Serial Devices
to Cellular Networks-------------------------- 51
Traditional Modems
IP Gateways
3.5 How to Connect Ethernet Devices to
Cellular Networks----------------------------- 56
From WAN to LAN (TCP Server)
From LAN to WAN
The OnCell can be both TCP Server
and TCP Client
3.6 How to Connect I/O Devices to Cellular
Networks------------------------------------- 59
SCADA Meets Ethernet
Communication from I/O to SCADA
OPC Fundamentals
OPC and DCOM: 5 Things You Need to Know
Enhance OPC Capability for
Cellular Communications
Conclusion
Differentiating Between Wireless Technologies
Chapter 1
Differentiating Between Wireless Technologies
1.1 WWAN vs. WLAN vs. WPAN
Modem wireless technologies are developed for the growing demand in mobile data exchange. Since
demands vary depending on the application, different technologies are applied to meet specific needs.
Normally, wireless technologies are divided into three categories: WWAN, WLAN and WPAN.
WWAN (Wireless Wide Area Network)
A WWAN utilizes mobile cellular communication networks such as cellular, UMTS, GPRS, CDMA2000, GSM,
CDPD, Mobitex, HSDPA, 3G, and WiMax. All of these networks offer wide service coverage and are normally
used for citywide, nationwide, or even global digital data exchange. Cellular networks in particular are operated
by carriers such as Cingular Wireless, Vodafone, and Verizon Wireless. In cellular communication, GSM (Global
System for Mobile Communication) is the leader with over 80% market share, followed by CDMA (Code
Division Multiple Access).
The biggest issues regarding data exchange over a WWAN are the associated costs, bandwidth, and IP
management. However, as technologies improve and costs drop, WWAN is predicted to replace traditional
microwave, RF (radio frequency), and satellite communication due to its lower infrastructure costs.
NOTE: The term “cellular” is also used to refer WWAN technology in general. WWAN technologies are
discussed in detail Chapter 3.
WLAN (Wireless Local Area Network)
As suggested by its name, WLAN transmits data over a shorter distance, normally 100 meters or so. In
terms of transmission technology, WLAN uses spread-spectrum or OFDM (Orthogonal frequency-division
multiplexing) modulation technology to provide the convenience of exchanging data without the limitation of
cables.
Today’s WLANs are based on IEEE 802.11 standards and are referred to as Wi-Fi networks. The 802.11b
standard, which operates around the 2.4 GHz frequency band at 11 Mbps, was the first commercialized
wireless technology. Advances in wireless technology have made a higher transmission rate of 54 Mbps
possible with 802.11g, which also operates around 2.4 GHz, and 802.11a, which operates around the 5 GHz
frequency band. It is now very common to see dual-band Wi-Fi access points and client network adaptors that
support a mixture of 802.11a, b, and g standards. More bandwidth means that it is possible to use wireless to
replace traditional wired solutions to transmit larger data such as video.
NOTE: WLAN technologies are discussed in detail Chapter 2.
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WPAN (Wireless Personal Area Network)
A WPAN is a short-range peer-to-peer or ad hoc network built around a person’s working area. Normally the
distance is no more than 10 meters. Because of their limited transmission range, WPANs are used mainly as
cable replacement solutions for data synchronization and data transmission for personal electronic devices
such as PDAs or smart phones. Bluetooth is the most prevalent WPAN technology in use today. It allows
devices such as phones, mice, headsets, and other personal devices to connect wirelessly within a range of 10
meters. The shorter communication distances also mean lower power consumption, making Bluetooth an even
more ideal solution for short-range data transmission. Moxa will be releasing WPAN products in 2010.
1
1.2 Evolution of Cellular Networks
3G Technologies
3G refers to the third generation of telecommunication technologies that is designed to replace 2.5G (GPRS
or CDMA). The demand for 3G comes from the growing need for data transmission over wireless networks.
The features of cellular networks make them particularly attractive to wireless users in comparison to IEEE
802.11 standards. Cellular has the advantages of wider coverage and the ability to stay connected in highspeed movement. To satisfy the need for data exchange over cellular networks, 3G networks were developed
to improve spectral efficiency. The improvements incorporate voice, video, and broadband wireless data
transmission all in the mobile environment.
The most commonly seen 3G systems are the Universal Mobile Telecommunication Systems (UMTS) and the
Wideband Code Division Multiple Access (WCDMA). These 3G systems are the major revenue contributors
to carriers in the past three to two years. As the technologies continue to evolve, transmission speeds have
become faster. For example, High Speed Packet Access (HSPA) offers downlink speeds that can reach 144
Mbps and 5.8 Mbps for the uplink. It is not wonder the building of 3G facilities and networks are on the rise.
Worldwide subscribers are expected to increase rapidly over the next 3 to 4 years. However, 4G technologies
are already in the works and aim to take mobile data transmission to an even higher level.
Differentiating Between Wireless Technologies
Wireless Network Coverage
HSDPA
High Speed Downlink Packet Access (HSDPA), or 3.5G, is a mobile telephony communications protocol.
It provides packet data service in WCDMA downlink. The transmission speed can reach 8–10 Mbps on a
5 MHz carrier wave, and 20 Mbps with MIMO technology. In practice, the technologies deployed include
AMC, MIMO, HARQ, fast scheduling and fast cell selection.
HSUPA
High Speed Uplink Packet Access (HSUPA), or 3.75G, was developed in response to the inadequate upload
speed of HSDPA (only 384 Kbps). The transmission speed can reach 10–15 Mbps on a 5MHz carrier wave,
28 Mbps with MIMO technology. The upload speed goes up to 5.76 Mbps, 11.5 Mbps with 3GPP Rel7
technology. With HSUPA, functions requiring massive upload bandwidth (e.g., two-way live transmission or
VoIP) can be realized.
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Differentiating Between Wireless Technologies
CDMA2000 1xEV (Evolution)
CDMA2000 1xEV is CDMA2000 1x equipped with HDR. 1xEV, in general, has two sessions:
• CDMA2000 1xEV 1st session—CDMA2000 1xEV-DO, in light of the fast data transmitted under a wireless
channel, supports downlink data speeds up to 3.1 Mbps with uplink up to 1.8 Mbps.
• CDMA2000 1xEV 2nd session—CDMA2000 1xEV-DV (Evolution-Data and Voice) supports downlink data
speeds up to 3.1 Mbps with uplink up to 1.8 Mbps. 1xEV-DV also supports 1x voice subscribers, 1xRTT
data subscribers, and high speed 1xEV-DV data subscribers to use the same wireless channel at the same
time.
4G Technologies
Fourth generation technologies made their market debut in 2009. The goal of 4G is to increase downlink speed
to 100 Mbps and uplink speed to 50 Mbps. The two major competing technologies in the 4G market are Long
Term Evolution (LTE) and WiMax sponsored by the IEEE Group.
Possible 4G Standards
WiMAX (Worldwide Interoperability for Microwave Access): Led by Intel Corporation, this is the 4G
technology with the farthest transmission range. Its highest downlink and uplink speed under mobile
communication environments can reach 75 Mbps and 50 Mbps respectively. On November 12, 2008, HTC
and Russian carrier Scartel (branded Yota) jointly launched the world’s first GSM-WiMAX integrated dualmodule mobile phone—HTC Max 4G.
UMB (Ultra Mobile Broadband): Led by Qualcomm Inc., this is the evolution standard of CDMA
technology. It has the highest transmission speed among 4G technologies currently. The highest downlink
and uplink speed under mobile communication environments can reach 288 Mbps and 75 Mbps
respectively.
LTE (Long Term Evolution): LTE is led by ETSI. Its highest downlink and uplink speed under mobile
communication environments can reach 100 Mbps and 50 Mbps respectively.
In December 2008, the Third Generation Partnership Project, also known as 3GPP, announced 3GPP
Release 8 to enhance data transmission speed in mobile networks. Release 8 standardizes the LTE and
makes it a more viable candidate for the nascent 4G standard. LTE uses both Frequency Division Depex
(FDD) and Time Division Duplex (TDD), and is able to operate on different bands ranging from 700 MHz
to 2.6 GHz. This also makes it possible to incorporate the now incompatible GSM and WCDMA and also
reduces costs.
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Despite WiMax’s current lead in commercializing its technologies, there are signs indicating that LTE is
catching up. In the past, major players like Nokia, Siemens, Motorola, Alcatel, Lucent, and Nortel showed
their support for WiMax. But starting from 2008, these players were also showing signs of interests in LTE.
Nortel had announced not to take part in Mobile WiMax. Alcatel, Lucent, and Motorola also started to
discuss LTE, announcing they will take part in both WiMax and LTE development. This has been interpreted
as an indication that WiMax development has fallen short of their expectations.
The turning point came with the abandonment of Ultra Mobile Broadband, UMB. When the leading mobile
chip provider Qualcomm announced that it will not to invest in UMB but in LTE instead, the CDMA camp
also decided to adopt LTE as its standard for next generation technologies. The unification of both CDMA
and GSM in LTE gives LTE a great advantage over WiMax.
However, LTE is not expected to dominate the market any time soon. This is because current 3G
technologies have raised HSPA+ downlink speed to 42 Mbps. With 100 Mbps possible in the near future
with HSPA, LTE will need to offer even more incentives to operators in order for it to become the industry
standard.
From the subscribers’ perspective, 4G is able to provide faster speed and satisfy more needs. The
fundamental driving force of moving mobile communications from analog to digitalization and from 2G
to 4G is the shift from wireless voice service to wireless multimedia service in subscriber needs. This has
spurred operators to adapt because they need to boost ARPU, develop new frequencies to attract more
subscribers, design more efficient spectrum use, and cut their operational costs.
In effect, 4G involves two different but overlapping concepts:
• High-speed mobile telephony system with speed as fast as ADSL’s bandwidth (10 Mbps or higher). This
concept formerly applied to wireless technologies such as Wi-Fi. It is also the vision addressed by the
successful 3G system providers presently.
• Pervasive network technology, a more abstract term often defined as wireless technology that is
“ubiquitous, ambient, and everywhere,” can involve subscribers in the system completely. Wi-Fi or the
system implemented in the future may be applied. This concept also includes Smart Radio technology
and has higher spectrum use and transmission capability. Moreover, it can also filter and transmit large
volumes of information.
Differentiating Between Wireless Technologies
4G Status
With respect to integration, 4G technologies involve more participants, technologies, industries, and
applications than just telecommunications. It can, therefore, be applied to finance, medicine, education,
transportation, and other industries. This is because the communication terminal is able to manage more
tasks, such as multimedia communications, remote control, and voice communications. If area networks,
Internet, telecommunications, radio broadcasts, and satellites are grouped together as an integrated
network in the future regardless of the terminal used, they will be able to offer complete wireless and
broadband connectivity and higher quality service. Such advancement would allow 4G technologies to
penetrate every aspect of our lives.
1
Table: 4G Technology Comparison
Technology
LTE
UMB
WiMax
Standards Setting Organization
RTSI
QCom
Intel
Original Tech.
WCDMA
CDMA2000 1xEV-DO
---
Maximum Speed
100Mbps, 50Mbps
288Mbps, 75Mbps
70Mbps, 70Mbps
Wireless Tech.
OFDM/MIMO/SC-FDMA
MIMO/SDMA
MIMO/SOFDMA
Schedule
2008 draft
2009
2008
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Differentiating Between Wireless Technologies
1.3 Evolution of IEEE 802.11
With the advent and development of local area networks (LAN), IEEE 802.3 has been widely adopted in many
different kinds of communication applications. The continued prevalence of wired communication has also
contributed to the growing demand for wireless communication. In 1997, IEEE released the IEEE 802.11 standards
that define the Physical Layer and Data Link Layer of TCP/IP, allowing communication based on these protocols
to be extended and used with greater flexibility. For the Physical Layer, IEEE 802.11 utilizes non-licensed ISM
(Industrial, Scientific and Medical) bands that operate between 2.4 GHz and 5 GHz. In order to make wireless
communication more prevalent and feasible, there are also task groups within IEEE designated to develop different
wireless applications.
7
IEEE 802.11
2 Mbps, 2.4 GHz band, 1997, MAC/Physical Standard
IEEE 802.11a
54 Mbps, 5 GHz band, 1999, MAC/Physical Standard
IEEE 802.11b
11 Mbps, 2.4 GHz Band, 1999, MAC/Physical Standard
IEEE 802.11c
MAC Layer Bridging to support IEEE802.1D
IEEE 802.11d
Automatic settings for different countries
IEEE 802.11e
Quality of Service (QoS)
IEEE 802.11f
IAPP, Inter-Access Point Protocol, cancelled by IEEE after February, 2006
IEEE 802.11g
54 Mbps, 2.4 GHz Band, 2003, MAC/Physical Standard
IEEE 802.11h
Support more channels on 5GHz spectrum, 2004
IEEE 802.11i
Wireless security, 2004
IEEE 802.11j
Japanese Standard upgrade, 2004
IEEE 802.11l
Reversed
IEEE 802.11m
Maintenance Standard
IEEE 802.11n
Draft now, using MIMO (Multi-input Multi Output) Technology to increase transmission
speed to 300–600Mbps
IEEE 802.11 k
Define measurement items and protocol
IEEE 802.11r
Define implementations of WLAN roaming, enables 802.11 able to be applied to mobile and
VoIP applications
IEEE 802.11s
Standard for Mesh under standard architecture
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IEEE 802.11n
In January 2004, IEEE made an announcement to form a new task force to develop new standards for the IEEE
802.11 standard. The goal of this task force was to allow wireless communication speed to reach a theoretic
number of 300 Mbps. Since the theoretic speed of this new standard, now called IEEE 802.11n, needs to reach
300 Mbps, the Physical Layer also needs to support a higher transmission speed that is at least 50 times faster
than IEEE 802.11b and 10 times faster than IEEE 802.11g. In addition to enhancing communication speed,
IEEE 802.11n also extends the communication distance to satisfy the growing needs of wireless applications.
To make this happen, IEEE 802.11n has added more specifications to the MIMO standard that allows IEEE
802.11n to be able to use multiple antennas to increase transmission speed. It also uses Alamouti coding
schemes to increase the transmission coverage.
There are two rival camps competing to dominate the IEEE 802.11n Physical Layer architecture: the WorldWide Spectrum Efficiency, which is supported by Broadcom, and TGnSync, supported by Intel and Philips.
IEEE 802.11s
Mesh STAs are individual devices using mesh services to communicate with other devices in the network.
They can also collocate with 802.11 Access Points (APs) and provide access to the mesh network to 802.11
stations (STAs), which have broad market availability. Also, mesh STAs can collocate with an 802.11 portal that
implements the role of a gateway and provides access to one or more non-802.11 network. In both cases,
802.11s provides a proxy mechanism to provide addressing support for non-mesh 802 devices, allowing endpoints to be cognizant of external addresses.
802.11s also includes mechanisms to provide deterministic network access, congestion control, and power
saving.
Table: 802.11 Standards and Date Rate
Protocol
Release
Date
Spectrum
Max. Speed
Typical Range
(indoor)
Typical Range
(outdoor)
802.11
1997
2.4–2.5 GHz
2 Mbps
---
---
802.11a
1999
5.15–5.35/5.47–5.725/
5.725–5.875 GHz
54 Mbps
30 m
---
802.11b
1999
2.4–2.5 GHz
11 Mbps
30 m
100 m
802.11g
2003
2.4–2.5 GHz
54 Mbps
30 m
100 m
802.11n
2008
2.4 GHz or 5 GHz bands
600 Mbps
50 m
125 m
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Differentiating Between Wireless Technologies
An 802.11s mesh network device is referred to as a mesh station (mesh STA). Mesh STAs form mesh links with
one another, over which mesh paths can be established using a routing protocol. 802.11s defines a default
mandatory routing protocol, or HWMP, yet allows vendors to operate using alternate protocols. HWMP is
inspired by a combination of AODV (RFC 3561[1]) and tree-based routing.
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Differentiating Between Wireless Technologies
1.4 WLAN vs. Proprietary 2.4GHz
Common usage of the WLAN limits its distance to under 100 meters. Now with Moxa’s advanced technologies, it is
also possible to extend the distance up to 10 kilometers for multi-point connections or 20 kilometers for point-topoint connections.
The IEEE 802.11 standard is designed for high-speed data transmission. However, it is also vulnerable to outside
interferences. This is unacceptable for some industrial applications where the control elements are often involved.
It is a basic control requirement that communication must not be interrupted. To meet this requirement, there are
some proprietary 2.4GHz band wireless devices that use FHSS spread spectrum technologies to meet the needs
for higher noise resistance. In summary, FHSS sacrifices throughputs and communication ranges for more stability.
Table: WLAN vs. Proprietary Wireless
Moxa
Banner
Frequency
2.4 GHz (ISM)
900 MHz (license needed)
2.4 GHz (ISM)
Standard
IEEE 802.11
Proprietary
Proprietary
Spread Spectrum
DSSS / OFDM
*a FHSS
FHSS
Throughput
22 Mbps
115200 bps
115200 bps
Distance
10 km
> 10 km
3.2 km
Communication method
Point to multiple points
Point to point
Point to point
*a: FHSS utilizes frequency hopping to avoid signal interference. Bluetooth is one example that uses this
technology. In the early days, IEEE 802.11 also used FHSS but has since adopted DSSS (Direct Sequence Spread
Spectrum) out of security concerns. 802.11a, 801.11g, and 802.11n adopt OFDM to increase their resistance to
external interferences.
About modulation and spread spectrum, please refer to Chapter 2.1
WWAN vs. WLAN vs. WPAN vs. Proprietary RF
Technologies
WWAN
WLAN
WPAN
Proprietary RF
Standard
GSM/GPRS/CDMA/
WCDMA/WiMax
IEEE802.11
Bluetooth/
ZigBee
No Standard
LAN (TCP/IP)
Point to point
Point to Point
Connection
Mode
9
Point to point (GSM)
WAN (GPRS/3G)
Communication
coverage
5 km to 30 km
100 m to 300 m
Approx. 10m
100 m to 100
km
Security
High
High
Medium
Low (not
standard)
Throughput
50 kbps to 100 Mbps
54Mbps (802.11a/g),
600 Mbps (802.11n)
115200 bps
115200 bps to
1 Mbps
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Chapter 2
Understanding Industrial WLAN – IEEE 802.11
2.1 IEEE 802.11 Basics
Wireless Communication
In a wireless environment, the communication medium is air. Radio waves carrying data propagate
from point to point through free space. Due to the characteristics of this unguided medium, wireless
communication calls for a very different set of knowledge and skills than traditional wired communication
systems. Getting the most out of your wireless environment requires a basic understanding of the
following scientific principles that govern wireless communications.
To understand how energy is transferred through the air, we need to review basic electromagnetic theories.
Electromagnetic (EM) waves are formed by alternating current rapidly changing direction on a conductive
material. The rapid oscillation of electric and magnetic fields around the conductor projects electromagnetic
waves into the air (see the figure below). In order for current to be radiated into the air in the form of
electromagnetic waves, a few factors are critical, namely, the length of the conductor and frequency of the AC
current. Higher frequency reduces the requirement for conductor length.
Understanding Industrial WLAN – IEEE 802.11
Electromagnetic Waves
2
The conductors are called antennas. Antennas
transform electric energy into EM waves during
transmission and turns EM waves into electric energy
during reception. The size and length of the antenna
is directly proportional to its desired transmission/
reception frequency. As shown in the figure to the
right, electromagnetic waves are radiated from a
directional antenna in a parabolic shape.
As EM waves propagate through the air, they will experience different types of alterations as they are
intercepted by different obstacles. Obstacles in the signal path introduce the following alteration to the signals:
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Understanding Industrial WLAN – IEEE 802.11
Diffraction (Shadow Fading)
Signal strength is reduced after experiencing diffraction. Obstacles
causing diffraction usually possess sharp edges such as the edges of
buildings. When EM waves encounter an obstacle with sharp edges that
cannot be penetrated, the EM waves wrap around the obstacle to reach
the receiver.
Scattering
When EM waves encounter many small obstacles (smaller than wave
length), the EM waves scatter into many small reflective waves and
damage the main signal, causing low quality or even broken links. Such
obstacles include rough surfaces, rocks/sand/dust, tree leaves, street
lights, etc.
Reflection
When EM waves run into large obstacles such as the ground, walls,
or buildings, they reflect and change their direction and phase. If the
reflected surface is smooth, the reflected signal will likely represent the
initial signal and not be scattered.
All of the above phenomena results in multipath propagation so not all signals arrive at the receiver antenna at
the same time due to obstacles that change the signal paths. Whether you are setting up an outdoor or indoor
application, multipath can severely affect received signal quality because the delayed signals are destructive to
the main signal. The multipath issue can usually be compensated by antenna diversity at the RF level and/or by
OFDM at the baseband level.
Modulation and Spread Spectrum
The following chart categorizes different digital modulation techniques:
Digital modulation
linear
Constant envelope / nonlinear
Combined / hybrid
Spread spectrum
BPSK
BFSK
MPSK
PN
DPSK
MSK
M-ary QAM
DSSS
QPSK
GMSK
MFSK
FHSS
ODFM
π / 4 QPSK
As you can see, there are many RF modulation techniques. However, our discussion is limited only to the
techniques that pertain to the 802.11 standard, namely FHSS, DSSS, and OFDM.
FHSS (Frequency Hopping Spread Spectrum)
This modulation technique is one of the techniques
used in spread spectrum signal transmission. It is also
known as Frequency-Hopping Code Division Multiple
Access (FH-CDMA). Spread spectrum enables a signal
to be transmitted across a frequency band that is much
wider than the minimum bandwidth required by the
information signal. The transmitter “spreads” the energy,
originally concentrated in narrowband, across a number
of frequency band channels on a wider electromagnetic
spectrum. Some of the advantages include:
- Improved privacy
- Decreased narrowband interference
- Increased signal capacity
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DSSS (Direct Sequence Spread Spectrum)
DSSS divides a stream of information to be transmitted into small pieces, each of which is allocated to
a frequency channel across the spectrum. DSSS generates a redundant bit pattern for each bit to be
transmitted. This bit pattern is called a chip (or chipping code). Even if one or more bits in the chip are
damaged during transmission, statistical techniques embedded in the radio can recover the original data
without the need for retransmission. Direct sequence spread spectrum is also known as direct sequence
code division multiple access (DS-CDMA). This modulation technique is officially accepted and used by the
IEEE 802.11b and IEEE 802.11g standards.
Signal Level
Channel 2
2417
2
2422
3
2427 2432
4
5
2437
6
Channel 10
2442 2447
7
8
2452
9
2457
10
2
2462 2467 2472
11
12
13
2477
14
Frequency
(MHz)
OFDM (Orthogonal Frequency Division Multiplexing)
OFDM is a modulation scheme that divides a single digital signal across 1,000 or more signal carriers
simultaneously. The signals are sent at right angles (orthogonal) to each other so they do not interfere with
each other. OFDM has the ability to overcome multi-path effects by using multiple carriers to transmit the
same signal. OFDM is commonly used in IEEE 802.11a and 802.11g standards. Non/near line-of-sight
associations can be achieved using the OFDM technique.
The following table summarizes the modulation techniques:
Modulation Technique
DHSS
FHSS
OFDM
Narrowband
Interference
Less resistance
(22 MHz wide contiguous
bands)
More resistance
(79 MHz wide contiguous
bands)
Much less
(multicarrier
modulation)
Interference
susceptibility
Medium
High
Low
Collocation
Less
More
Uses several parallel
sub-carriers
Compatibility
802.11b (WiFi Alliance)
None
802.11a, 802.11g
Implementation Cost
Comparatively Less
Comparatively more
High
Throughput
5 – 6 Mbps
2 Mbps for 802.11
25 Mbps
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Understanding Industrial WLAN – IEEE 802.11
2400 2412
1
Channel 6
12
Understanding Industrial WLAN – IEEE 802.11
Lastly, let’s use the 802.11g standard as an example for how the transmission type and modulation scheme
corresponds to each data rate:
802.11g Data Rate
(Mbps)
Transmission
Type
Modulation
Scheme
54
OFDM
64 QAM
48
OFDM
64 QAM
36
OFDM
16 QAM
24
OFDM
16 QAM
18
OFDM
QPSK1
12
OFDM
*a QPSK
11
DSSS
CCK2
*a QPSK: Quadrature Phase Shift Keying
9
OFDM
BPSK3
6
OFDM
*b CCK: Complementary Code Keying
5.5
DSSS
BPSK
*b CCK
2
DSSS
1
DSSS
*c BPSK: Bi-phase Shift Keying
QPSK
c
* BPSK
ISM and Licensed Band
The FCC (Federal Communications Commission) regulates the usable frequency bands and the maximum
allowable power in these frequency bands for the United States. WLAN devices are allowed to use the ISM
(Industrial/Scientific/Medical) band by the FCC. The ISM band consists of 3 different sub-bands: 902 MHz, 2.4
GHz and 5.8 GHz. The FCC has also further defined the UNII (Unlicensed National Information Infrastructure)
band for WLAN usage. The following diagram shows the spectrum overview of the ISM and UNII bands.
Figure: ISM and UMI Bands
Advantages and Disadvantages of Using Unlicensed Bands
ISM and UNII are both un-licensed bands which means anyone can transmit in these bands without a
license from the FCC. It is the opening of these un-licensed bands that has allowed the WLAN business to
grow in small businesses and homes. The freedom of these license-free bands also means a great number
of un-licensed users may share the bandwidth with you.
Our discussion only includes the 2.4 GHz ISM band and 5 GHz UNII band because these 2 frequency bands
are the most commonly used in WLAN applications.
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2.4 GHz ISM Band
As 802.11b/g is the most commonly used WLAN standard today, the 2.4 GHz ISM band is supported by
almost every country worldwide. Not every country supports the same channels in the 2.4 GHz ISM band,
so you need to make sure the wireless AP matches the standard used by your country. The following chart
shows channels supported in the 2.4 GHz ISM band for different countries/continents.
Channel Number
Center Frequency
USA
EU, M. East, Asia
Japan
1
2.412
Y
Y
Y
2
2.417
Y
Y
Y
3
2.422
Y
Y
Y
4
2.427
Y
Y
Y
5
2.432
Y
Y
Y
6
2.437
Y
Y
Y
7
2.442
Y
Y
Y
8
2.447
Y
Y
Y
2.452
Y
Y
Y
2.457
Y
Y
Y
11
2.462
Y
Y
Y
12
2.462
Y
Y
13
2.472
Y
Y
14
2.484
Y *DSSS only
The FCC opened the frequency band between 2.4 to 2.5 GHz, and the IEEE uses 2.400 to 2.4835 GHz. The
minor mismatch is to provide a buffer to prevent power from leaking into the forbidden band.
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Understanding Industrial WLAN – IEEE 802.11
9
10
2
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Understanding Industrial WLAN – IEEE 802.11
UNII Band
The 5 GHz UNII band consists of 3 parts, each 100 MHz wide. The 802.11a standard uses this band. Each
part of the UNII band includes 4 non-overlapping channels with 5 MHz of guard band between them. The
FCC states that the lower band (UNII-1) can only be used indoors, the middle band (UNII-2) can be used
indoors or outdoors, and the higher band (UNII-3) should only be used outdoors. Since UNII-1 and UNII-2
can be used indoors, the maximum number of non-overlapping channels in an indoor environment is 8. See
below for channels supported in the 5 GHz UNII band for different countries.
Channel ID
Frequency (MHz)
USA
EU, M. East, Asia
Japan
36
5180
V
V
V
40
5200
V
V
V
44
5220
V
V
V
48
5240
V
V
V
52
5260
V
V
V
56
5280
V
V
V
60
5300
V
V
V
64
5320
V
V
V
100
5500
V
V
V
104
5520
V
V
V
108
5540
V
V
V
112
5560
V
V
V
116
5580
V
V
V
120
5600
V
V
V
124
5620
V
V
V
128
5640
V
V
V
132
5660
V
V
V
136
5680
V
V
V
140
5700
V
V
V
149
5745
V
153
5765
V
157
5785
V
161
5805
V
165
5825
V
Signal Power
Radio signals are transmitted with a certain power level. Power is measured in watts. However, a watt is a
rather large amount of power in WLAN. Therefore, power is usually measured in milliwatts (mW), which is onethousandth of a watt. A typical wireless AP transmits between 30 to 100 mW of power, and about 50 mW for
wireless adaptors (clients). Certain applications will require higher transmit (Tx) power and may attempt to use
power boosters or customized high power modules to amplify the transmit power. However, such attempts
may cause the system to exceed the radio emission regulations (i.e., FCC regulations) of one’s country so take
caution during high power operation.
dB, dBm, dBi
Power measured in mW is hard on the math when we are dealing with extremely small power levels at the
receiver end. Therefore, instead of using absolute values (milliwatts) we often convert them into dBm. The
unit of dBm is a logarithmic representation of mW. The conversions are as follows:
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The following table shows some common conversion values between
dBm and mW:
dBm
Watt
dBm
Watt
+40dBm
10W
+12dBm
16mW
+30dBm
1W
+9dBm
8mW
+20dBm
100mW
+6dBm
4mW
+10dBm
10mW
+3dBm
2mW
0dBm
1mW
0dBm
1mW
-10dBm
100uW
-3dBm
500uW
-20dBm
10uW
-6dBm
250uW
-30dBm
1uW
-9dBm
125uW
-40dBm
100nW
-12dBm
62.5uW
In dealing with antenna gain specifications, the gain factor is often represented by “dBi”. The “i” stands for
“isotropic”, which means the gain is relative to an isotropic radiator (i.e., a radiating sphere in space). This
ideal radiation is impossible to realize but
its pattern is the reference for all realizable
antennas. The gain of a passive antenna is
measured by how effectively the antennas
can focus the energy (how narrow is the
antenna angle), rather than the actual boost
in transition power. Therefore, the narrower
the antenna angle, the higher the antenna
gain. The diagram below shows the antenna
angles of a high and low gain antenna.
Transmit Power and Received Sensitivity
When a radio signal is being transmitted through the air, it will experience a great loss in signal strength
caused by attenuation introduced by free space. Therefore, when evaluating a wireless system, one needs
to be aware of the signal power level at the transmitter end and at the receiver end. The signal power
received cannot be so weak as to break the communication link, or too strong as to saturate the receiver’s
amplifiers.
Understanding Industrial WLAN – IEEE 802.11
The dB is a unit of relative quantity, which means it is merely a multiplication factor used to represent the gain
or loss of signal power. A useful rule of thumb is an addition or subtraction of 3 dB is equivalent to a multiple of
2 or 0.5. An addition or subtraction of 10 dB is equivalent to a multiple of 10 or 0.1.
2
These concerns call for estimating the “power budget” of a wireless system. By making a power budget
estimation, you will have an idea of how far you can extend your wireless link without losing communication.
Please note that the following calculations are pure theoretical estimations that are not meant to guarantee
communication distance. There are many other factors involved that will affect transmission distance.
Pt : Transmit Power
Gt, Gr: Antenna Gain
Pr: Sensitivity
f is the frequency in mHz, pt and pr in dBm, and gt and gr in dBi, which are easier to obtain from product
specifications. To get the effective range d in km, all we have to do is plug in the values for pt, pr, gt, gr, and
f.
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The receiver’s sensitivity is the minimum power level the receiver can accept to process the received data. The
specified sensitivity is not the power detected by the receiving antenna but the power present as the receiver
module. An important point to note from the above equation is that as frequency increases, the effective
distance decreases. Therefore, the 802.11a (5 GHz) standard will yield a shorter communication distance than
802.11b/g (2.4 GHz). Users who wish to communicate long distances should therefore select 802.11b/g as
their operating standard.
Bandwidth, Data Rate, and Throughput
Usually when “bandwidth” is mentioned, it means one of two things:
1. The actual width of a frequency band measured in Hz (Hertz); the effective bandwidth would be the
frequency band that is actually carrying data.
2. The maximum data rate available (bits per second) in a communication link.
The former is the technically correct definition of bandwidth. For example, the 802.11b/g standards operate
between 2.4 GHz and 2.4835 GHz, giving a total effective bandwidth of 83.5 MHz with a channel bandwidth of
22 MHz.
The data rate of a particular wireless standard is the maximum data transfer speed (bit per second) the
communication link can achieve, such as 54 Mbps for 802.11g. Please note that this is the specified transfer
rate for raw data. The WLAN protocol packages the user data with layers of headers and trailers with
inter-packet gaps in between the packets. For example, TCP communication requires the receiving end to
acknowledge the received data by sending ACK packets back to the receiver. Therefore, the actual user
data rate will be lower than the specified data rate because user data is only a portion of the raw data being
transmitted via the wireless media. The actual user data rate is called the “throughput” of the wireless link.
Typically, we can expect the throughput to be about half of the specified data rate (i.e., throughput = 25 Mbps
when data rate = 54 Mbps).
The following figure is an example of throughput measurements as signal attenuation increases (curves
correspond to different noise immunity settings):
As you can see, when the signal is too strong (low attenuation) or too weak (high attenuation), the overall
throughput dips bellow the optimum value.
Throughput can be measured with various throughput measuring tools. One of the free throughput measuring
tools available is Jperf, downloadable here: http://sourceforge.net/projects/iperf
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2.2 Wireless Security
If you’re new to wireless, the first thing you should realize is that the signals you send and receive from a nearby
access point are easily intercepted by anyone in the vicinity who has a wireless card and a computer. The purpose
of WLAN security techniques is to render the connection unusable and the data unreadable by anyone but you and
the person (or machine) you’re communicating with.
Although most people do not need in-depth knowledge of WLAN security, understanding the basics can make it
easier for you to find the right product for your application. For example, one of the most basic questions you can
ask is whether or not a product supports WPA and/or WPA2. But why should you care? Most wireless products
available on the market today support WEP. Even though WEP may protect your data from the casual passerby,
it still leaves you vulnerable to attack from someone with some basic network knowledge and some time on their
hands, as we point out in the next section.
A Peek at the Technology
Authentication
The 802.1X standard dictates how authentication on wired and wireless LANs is carried out. 802.1X
authentication uses port-based access control, which means that the various entities involved in the
authentication process gain access to each other’s resources by connecting through “ports.” In effect, the
authentication procedure involves placing a “guard” at each port to prevent unauthorized users from gaining
access to protected data.
The 802.1X authentication procedure involves three basic players:
• The supplicant is the client (PC
or laptop computer, for example)
who would like to gain access to
network resources through the
wireless network.
• The authenticator, which is
usually an access point (AP) for a
wireless network, plays the role of
gatekeeper.
• The authentication server,
which connects to the AP over
a wired network, handles the
authentication procedure. More
often than not, a RADIUS server is used.
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Understanding Industrial WLAN – IEEE 802.11
There are two basic aspects to wireless security: authentication and encryption. Simply put, a system uses
authentication to check a user’s credentials and determine if the user should be given access to the data and
resources provided by the protected network. Encryption, on the other hand, encodes the data so that anyone
who does not have the secret “key” will not be able to read the data.
In effect, the authenticator and authentication server work as a team to verify the identity of the supplicant.
The authentication server also takes responsibility for computing the “keys” that the encryption algorithm
will use. Although the details of authentication may be complex, the overall procedure is easy to describe:
STEP 1: The Authenticator relays authentication messages between the WLAN and the Ethernet.
STEP 2: The Authentication Server and Supplicant establish a secure tunnel that is used to pass encrypted
messages.
STEP 3: The Authenticator performs the authentication check based on the agreed upon method (TLS,
PEAP-MSCHAP-V2, TTL, etc.).
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Encryption
The science of encryption or, in more down-to-earth terms, the making and breaking of codes, is one of the
most crucial aspects of WLAN technology. This is because the radio waves used to transmit data packets
between your computer and the wireless access point can pass through walls, floors, and other barriers.
People who use laptops that have a wireless LAN card will know this first-hand, since it is often possible to
pick up signals from wireless access points located in nearby apartments. Using a password to restrict entry
to your network may not provide enough protection, since a reasonably clever person can still intercept your
data packets. In fact, if the person intercepting the wireless data is more than reasonably clever, he or she
may also be able to download and read the contents of the packets.
As illustrated in the schematic below, wireless encryption has evolved from WEP, which was released in
1999, to the 802.11i standard, more commonly referred to as WPA2.
The Evolution of Wireless Encryption
WPA2
WPA2 is the second generation of WPA. The primary difference between WPA and WPA2 is the technology
used for data encryption. WPA uses Temporal Key Integrity Protocol (TKIP) for data encryption, whereas WPA2
uses Advanced Encryption Standard (AES), a stronger encryption technology suitable for industries that require
highly secure networks.
WPA
Wi-Fi Protected Access (WPA) is a stronger security method that was created in response to the flaws
discovered in WEP. It was intended as an intermediate measure until further 802.11i security measures were
developed. When implemented with authentication methods such as RADIUS, WPA is considered secure
enough for all but the most sensitive enterprise applications. For most home and small business use, an
effective level of security can be obtained by using WPA with a pre-shared key (PSK) that is shared by all users.
802.1X
802.1X is an authentication method that prevents unauthorized users from entering the network. It is used with
WPA to form a complete WLAN security system. On many wireless systems, users either log into individual
access points, or can freely enter the wireless network but cannot get further without additional authentication.
802.1X makes users authenticate to the wireless network itself, not an individual AP or another other level like a
VPN. This is more secure, as unauthorized traffic can be denied right at the AP.
WEP
Wired Equivalent Privacy (WEP) provides a basic level of security to prevent unauthorized access to the
network and protect wireless data. Static shared keys (fixed length alphanumeric/hexadecimal strings) are
used to encrypt data and are manually distributed to all wireless stations that want to use the wireless network.
WEP has been found to have serious flaws and is not recommended for networks that require a high level of
security. For more robust wireless security, most access points support Wi-Fi Protected Access (WPA or WPA2)
for improved data encryption and user authentication.
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Using a Firewall as an Additional Safeguard
One of the most basic aspects of maintaining the security of your network involves using a firewall to filter out
unwanted traffic. To protect a private LAN from unwanted traffic originating outside the LAN, firewall software
often runs on a gateway that connects the LAN to the Internet. The firewall is configured to filter out traffic
based on various characteristics of the incoming packets, such as IP address, MAC address, type of protocol,
etc.
Even if your private LAN does not connect to a public network, once you allow access to the LAN through a
wireless AP, you open the network to possible external attack. As an added safeguard, some manufacturers
include firewall software on the access point to filter out traffic accessing the network through the AP. For
example, Moxa’s AWK-3121 supports the latest encryption technology (WEP, WAP, WAP2) and allows system
managers to filter traffic by MAC address, IP, as well as TCP/UDP filtering options.
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Understanding Industrial WLAN – IEEE 802.11
2.3 Antenna Theory and Selection
Choosing the right antenna after a site survey is a small but important factor when planning a wireless project. The
purpose of this section is to explain what an antenna is and how to choose the right antenna to help build a reliable
wireless network.
Functions of Antennas
An antenna is a transducer that is designed to transmit or receive electromagnetic waves. It is like a converter
that converts electromagnetic waves and electrical currents back and forth. Different wireless devices use
different antennas to operate in different frequencies and to achieve, for example, a desired range. The most
important parameter of an antenna is its working frequency. For example, a 2.4GHz antenna is too weak to use
in IEEE 802.11a communication and the data rate will fall back to a very low level or even drop to ground zero.
Types of Antenna
There are two basic types of antennas, omni-directional and directional. The two types are categorized by the
direction in which they beam radio signals. Omni-directional antennas are designed to radiate signals equally in
all directions. Use this type of antenna if you need to transmit from a central node, such as an access point, to
users scattered all around the area.
Directional antennas provide a more focused signal than omni-directional antennas. Signals are typically
transmitted in an oval-shaped pattern with a beam width of only a few degrees. With higher gain, directional
antennas can also be used outdoors to extend point-to-point links over a longer transmission distance, or to
form a point-to-multipoint network.
Key Antenna Specifications
Connector types
Before you purchase an antenna for your wireless device, you should check the type of antenna connector
that your device uses. You will need to buy an antenna with a matching connector. There are several types
of antenna connectors, including MCX, TNC, N-type, SMA, and RP-SMA (RP stands for “reverse polarity” or
“reverse ping”). On WLAN devices, the most commonly used antenna connector is PRSMA and N-type for
IEEE 802.11 wireless applications.
N-type(male)
N-type(female)
RP-SMA(female) RP-SMA(male)
SMA(female)
SMA(male)
Half-Power Beam Width (HPBW)
This parameter is measured from the antenna’s radiation pattern, and refers to the beam width at which the
antenna’s radiation drops to half of its peak value. It also refers to the antenna’s effective coverage area.
Once you get outside the half-power beam width, the signal typically drops off very quickly. A very high-gain
antenna has a very narrow angled half-power beam width, which makes the directionality high as well.
Antenna Polarity
Polarization refers to the direction in which the electromagnetic field lines point as energy radiates away
from the antenna. The simplest and most common type is linear polarization. When power is sent from
transmitter to receiver, only that portion of the beam with the same polarization can be received. An
improper antenna installation may decrease performance.
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Frequency
Different wireless applications use different frequencies to achieve their purposes. To make sure wireless
devices work as expected, users need to choose the right antenna with the right frequency. For example
using a 5GHz IEEE 802.11a application with a 2.4GHz antenna can weaken or even completely wipe out the
signal.
Choosing the Right Antenna for Your Project
In addition to the key antenna specifications outlined above, there are some very simple tips you can use to
choose the right antenna for your wireless project.
For a fixed point-to-point connection, we recommend choosing a directional antenna. Rather than
broadcasting their signals linearly, directional antennas form a Fresnel Zone (a spherical expansion of the signal
waves) and increase signal strength. The increased signal strength ensures smoother data transmission and
connection.
A few applications require special types of antennas, such as a leakage antenna for collecting data along rail
tracks. These are very special cases and the deployment and infrastructure costs can be very high.
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Understanding Industrial WLAN – IEEE 802.11
When facing an application that requires constant changing of locations, omni-directional antennas make
a better choice. An omni-directional antenna emits waves equally in all directions so it is easier for moving
objects with constantly changing angles and positions to receive signals.
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Understanding Industrial WLAN – IEEE 802.11
2.4 Long Distance Wireless
Wireless transmissions today are based upon the IEEE 802.11 protocol stacks. By modifying these stacks, wireless
solution providers can optimize them for long-range, point-to-point applications. It has also been used in the
developing world to link communities separated by difficult geography with little or no connectivity options. In
this section, we will introduce long distance wireless technology and list the components required to extend your
wireless range.
Application Topology
Wireless Link (AP-Client Mode)
By setting up a wireless link between the
AP and Client, several buildings in an
extensive corporate campus can be easily
integrated into the company network.
AP-Client connections can also be
used to provide Internet access in areas
where cabling would be too expensive or
impractical to install. A good LOS (Line
of Sight) is required between the AP
and Client devices. Distances of several
kilometers can be bridged by this type of
wireless link.
Ethernet
Access Point
Mobile Device
AP Client
AP Client
PLC
Panel PC
AP-Client Operation
Wireless Distribution System (WDS)
AP-Client Operation
Wireless Distribution System (WDS) is a special type of wireless link. This mode allows several buildings
on a company to be connected to the central office. The central AP is configured the partner MAC (Media
Access Control) the remote WDS AP is configured the central AP MAC (Media Access Control). The wireless
link will reduce half bandwidth (due to use the same frequency channel) when add more one AP under chain
connection.
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Wireless Bridge System (Dual RF)
Moxa’s proprietary Wireless Bridge System (Dual RF) allows several buildings on a corporate campus to be
connected to the central office. The central AP is configured as the “master” device and the remote client
stations as “slave” devices. The wireless link will not reduce the bandwidth (to due to the use of Dual RF and
isolation of the overlap frequency channel) but will extend the wireless range.
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Understanding Industrial WLAN – IEEE 802.11
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Understanding Industrial WLAN – IEEE 802.11
Components of the Expanded 802.11 Wireless System
Expanded 802.11 wireless systems consist of the following components, some of which are optional.
Access Points
Moxa supplies 802.11a/b/g/n (802.11n will be implemented in the future, the technology works by using
multiple antennas to target one or more sources to increase transmission power and throughput) wireless
AP/Bridge/Client devices to extend the wireless range.
IEEE 802.11a is a modified version of the IEEE 802.11 standard and was approved in 1999. IEEE 802.11a
adopts the same standards as IEEE 802.11 and operates in the 5 GHz band. It uses 52 Orthogonal
Frequency Division Multiplexing (OFDM) waves and has a maximum capacity of 54 Mbps. This has already
satisfied the standard requirement of network communication which needs around 20 Mbps of bandwidth.
It is also possible to drop the communication speed to 48, 36, 24, 18, 12, 9 or even 6 Mbps. IEEE 802.11a
has 12 parallel channels, among them 8 of which are used for indoor communications and 4 for point-topoint communications. IEEE 802.11b is not inter-operable with IEEE 802.11a unless the communication
devices support both standards. IEEE 802.11a has the advantage of less interference than IEEE 802.11b
as IEEE 802.11b’s 2.4 GHz band is widely used. However, the high frequency also has some downsides.
IEEE 802.11a has a much narrower coverage, so it needs more access points. This also means that signals
can not be transmitted as far as IEEE 802.11b because it is much easier for signals to be absorbed by
surrounding objects.
Parameter Tuning
Wireless devices have traditionally been limited in range due to the inherent design of the 802.11 standard.
802.11 protocol uses acknowledge for each received frame. If an acknowledgement is not received, the
frame is re-transmitted. By default the maximum distance between transmitter and receiver is 1 mile (1.6
km). On longer distances the delay will force retransmissions so Moxa has tuned our Wireless product to
support long-range deployments using wireless 802.11.
Moxa Wireless Products are now enhanced with the ability to automatically adjust parameters such as slot
time, ACK time-out, and CTS time-out to fine tune the wireless device for optimal performance and achieve
a longer range.
Environmental Conditions:
Two factors are considered as below:
• 2.4GHz interference: There are literally hundreds of other sources of interference that aggregate into
a formidable obstacle to enabling long range use in occupied areas: microwave ovens, baby monitors,
wireless cameras, remote car starters, wireless phones, and Bluetooth products.
• Landscape interface: Obstacles are among the biggest problems when setting up a long-range wireless
application. Trees and forests degrade the microwave signal, and rolling hills make it difficult to establish
line-of-sight propagation. In a city, buildings will impact integrity, speed and connectivity. Steel frames
partly reflect radio signals, and concrete or plaster walls absorb microwave signals significantly, but sheet
metal in walls or roofs may efficiently reflect wireless signals, causing an almost total loss of signal.
Power Amplifier
Moxa supplies RF devices with 63/200/800 mW and boosters to extend your wireless range.
For example, if you have a 18 dBm (63 mW) device and replace Moxa’s 200 mW RF device, you can
increase 18 dBm (63 mW) to 23 dBm (200 mW); if you replace Moxa’s 800 mW RF device, you can increase
18 dBm (63 mW) to 29 dBm (800 mW). Based on our experience, you can increase the range by using
Moxa’s RF devices or boosters.
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External Antennas
Moxa’s 802.11a/b/g/n wireless AP/bridge/client devices are supplied with a low gain antenna. However,
for many of the long range applications, additional external antennas are necessary to extend the wireless
range. The following sections contain a brief description of the two types of antennas:
• Omni-directional antennas transmit horizontally with equal power in all directions. They have very limited
vertical spread, which determines the antenna gain. Antennas of this type are typically located in the
center of open spaces or larger offices to provide even coverage to all clients.
2
Fiberglass Antenna
Dipole Antenna
• Uni-directional antennas have beams with narrow horizontal and vertical angles. Uni-directional antennas
are mainly used on rooftops or masts for establishing point-to-point links that interconnect areas of a
network that are separated by a distance.
E-Plane
H-Plane
Understanding Industrial WLAN – IEEE 802.11
H-Plane
E-Plane
Directional / Panel
Antenna
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Understanding Industrial WLAN – IEEE 802.11
Setting Up Point-to-Point Connections
This sector introduces the basic principles involved in designing point-to-point links and provides tips on
aligning the antennas.
The following basic questions must be answered when designing long range wireless links:
• What antennas are required for the desired application?
• How must the antennas be positioned to ensure a problem-free connection?
• What performance characteristics do the antennas need to ensure sufficient data throughput within the
legal limits?
MOXA Antenna Calculator
You can use the Moxa Antenna Calculator to calculate the
output power of the access points as well as the achievable
distances and data rates. The program can be used from our
website at www.moxa.com/Event/IW/2009/Industrial_Wireless_
Typology/index.htm
After selecting your components (access points, antennas,
cable, etc.) the calculator works out the data rates, ranges,
and the antenna gain settings that have to be entered into the
access point.
Positioning the Antennas
Antennas do not broadcast their signals linearly, but within an angle that depends on the model in question.
The spherical expansion of the signal waves results in amplification of or interference to the effective power
output at certain intervals of the connection between the transmitter and receiver.
The Fresnel Zone must remain free from obstruction in order to ensure that the maximum level of output
from the transmitting antenna reaches the receiving antenna. Any obstructing element protruding into this
zone will significantly impair the effective signal power. The object not only screens off a portion of the
Fresnel Zone, but the resulting reflections also lead to a significant reduction in signal reception.
The concept of RF LOS is based on a parabolic free space zone called the Fresnel Zone that will ensure a
more stable link if at least 60% of the vertical radius is kept obstacle free. The figure below shows how to
estimate the Fresnel Zone of a wireless link where r (meter) is the vertical radius of the oval, d (km) is the
distance between the transceivers, and f (GHz) is the radio frequency.
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To ensure that the Fresnel Zone remains unobstructed, the height of the antennas must exceed that of the
highest obstruction by this radius. The figure below shows the full height of the antenna mast.
Antenna Gains
Antenna Alignment for P2P Operations
When there is not enough obstacle-free Fresnel Zone available, you may need to relocate the wireless devices
or elevate the antennas to clear more Fresnel Zone space (see figure below). In long distance applications,
Earth Bulge may need to be taken into consideration as an obstacle.
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Understanding Industrial WLAN – IEEE 802.11
The gain of each antenna specifies its directionality. In general, the lower the gain, the more evenly
distributed in all directions the radiation will be. High gain antennas, on the other hand, emit radiation in a
more specific direction. The gain defines its power gain or directive gain in terms of the ratio of the intensity,
or power per unit surface. In general, when we choose an antenna, the longer the transmission distance, the
higher the antenna gain must be. At the same time, we must sacrifice omni-directional coverage.
28
Understanding Industrial WLAN – IEEE 802.11
The precise alignment of the antennas is of considerable importance in establishing long range wireless
connections. The more central the receiving antenna is located in the “ideal line” of the transmitting antenna,
the better the actual performance and the effective bandwidth are. If the receiving antenna is outside of this
ideal area, however, significant losses in performance will result.
The current signal quality over a long range wireless connection can be displayed on the device’s LEDs or in
the Moxa monitor in order to help find the best possible alignment for the antennas. The more LED indicators
means the stronger the connection.
In the Moxa monitor, the
connection quality display is
opened with the context menu
once signal monitoring has
commenced. The P2P dialog
displays the absolute values for
the current signal strength and the
maximum value upon starting the
measurement. The development of
the signal strength over time and
the maximum value are displayed.
Initially only one of the two
antennas should be adjusted until
a maximum value is achieved. This
first antenna is then fixed and the
second antenna is then adjusted
to attain the best signal quality.
29
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2009 Industrial Wireless Guidebook
Moxa’s Antennas Selection Guide
IEEE 802.11b/g 2.4 GHz Wireless Antennas
ANT-WSB-ANF-09
ANT-WSB-PNF-12
ANT-WSB-PNF-18
E-Plane
E-Plane
E-Plane
E-Plane
H-Plane
H-Plane
H-Plane
H-Plane
Frequency
Range
2.4 to 2.5 GHz
2.4 to 2.5 GHz
2.4 to 2.5 GHz
2.4 to 2.5 GHz
Antenna Type
λ/4 Dipole
Omni-directional
Directional, Panel
Directional, Panel
Typical Antenna
Gain
5 dBi
9 dBi
12 dBi
18 dBi
Impedance
50±5 ohms
50±5 ohms
50±5 ohms
50±5 ohms
Polarization
Linear
Vertical
Linear
Linear
Linear
HPBW/
Horizontal
360°
360°
50°
30°
HPBW/Vertical
---
10°
30°
20°
V.S.W.R.
2.0
1 : 1.3 Max.
1 : 1.5 Max.
1 : 1.5 Max.
Power Handling
---
15 W Max.
10 W Max.
15 W Max.
Connector(s)
Operating
Temperature
RP-SMA (male)
N-type (female)
N-type (female)
N-type (female)
-40 to 80°C
-40 to 80°C
-40 to 80°C
-40 to 80°C
IP Rating
---
IP65
IP65
IP65
Antenna Profile
---
420 mm length
215 x 90 x 30 mm
270 x 205 x 15 mm
Weight
300 g
430 g
560 g
1020 g
Antenna
Patterns
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Understanding Industrial WLAN – IEEE 802.11
ANT-WSB-AHRM-051.5m
Product Name
30
Understanding Industrial WLAN – IEEE 802.11
IEEE 802.11a/b/g 2.4/5 GHz Dual-band Antennas
Product Name
ANT-WDB-ANF-0609
E-Plane (2.4 GHz)
E-Plane (5 GHz)
ANT-WDB-PNF-1518
E-Plane (2.4 GHz)
E-Plane (5 GHz)
H-Plane (2.4 GHz)
H-Plane (5 GHz)
Antenna Patterns
H-Plane (2.4 GHz)
31
H-Plane (5 GHz)
Frequency Range
2.4 to 2.5 / 5.1 to 5.9 GHz
2.4 to 2.5 / 5.1 to 5.9 GHz
Antenna Type
Omni-directional
Directional, Panel
Typical Antenna
Gain
6/9 dBi
15/18 dBi
Impedance
50±5 ohms
50±5 ohms
Polarization Linear
Linear
Linear
HPBW/Horizontal
360°
50/10°
HPBW/Vertical
10/8°
30/10°
V.S.W.R.
1 : 1.5 Max.
1 : 1.5 Max.
Power Handling
10 W Max.
20 W Max.
Connector(s)
Operating
Temperature
N-type (female)
N-type (female)
-40 to 80°C
-40 to 80°C
IP Rating
IP65
IP65
Antenna Profile
260 mm length
270 x 205 x 15 mm
Weight
155 g
1020 g
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IEEE 802.11a 5 GHz Wireless Antennas
Product Name
ANT-WSB5-ANF-12
E-Plane
ANT-WSB5-PNF-18
E-Plane
2
H-Plane
H-Plane
Frequency Range
5.1 to 5.9 GHz
5.1 to 5.9 GHz
Antenna Type
Omni-directional
Directional, Panel
Typical Antenna
Gain
12 dBi
18 dBi
Impedance
50±5 ohms
50±5 ohms
Polarization Linear
Linear
Linear
HPBW/Horizontal
360°
10°
HPBW/Vertical
6°
10°
V.S.W.R.
1 : 1.3 Max.
1 : 1.5 Max.
Power Handling
10 W Max.
10 W Max.
Connector(s)
Operating
Temperature
N-type (female)
N-type (female)
-40 to 80°C
-40 to 80°C
IP Rating
IP65
IP65
Antenna Profile
420 mm length
270 x 205 x 15 mm
Weight
430 g
990 g
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Understanding Industrial WLAN – IEEE 802.11
Antenna Patterns
32
Understanding Industrial WLAN – IEEE 802.11
Cellular Antennas
GSM/GPRS Cellular Antennas
Product
Name
ANT-CQBASM-01
ANT-CQBAHSM-00-3m
ANT-CQBAHSM-03-3m
ANT-CQBAHSM-05-3m
Frequency
Range
850/900/
1800/1900 MHz
850/900/
1800/1900 MHz
850/900/
1800/1900 MHz
850/900/
1800/1900 MHz
850/900/1800/
1900/2100 MHz
850/900/1800/
1900/2100 MHz
Cable Type
---
RG174/U
RG174/U
RG174/U
---
RG174/U
Typical
Antenna
Gain
max. 1 dBi
0 dBi
3 dBi
5 dBi
1.5 dBi
4 dBi
Impedance
50 ohms
50 ohms
50 ohms
50 ohms
50 ohms
50 ohms
Polarization
Type
Linear
Linear
Linear
Linear
Vertical
Vertical
V.S.W.R.
---
<2
<2
<2
1 : 6.4
<2
Connector(s)
SMA(M)
SMA(M)
SMA(M)
SMA(M)
SMA(M)
SMA(M)
3.3 mm length
100 mm length
250 mm length
370 mm length
104 mm length
110 mm length
---
3m
3m
3m
---
2.5 m
Antenna
Profile
Cable
Length
33
UMTS/HSDPA/WCDMA Cellular
Antennas
ANT-WCDMAANT-WCDMAASM-1.5
AHSM-04-2.5m
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Moxa Performance Test Report
ANT-WDB-O-2(Omni dipole dual band 2dBi antenna)
Distance(Km)
2.4G-AP
Station (Mbps)
2.4G-Station
5G-AP
0.1
AP (Mbps)
Station (Mbps)
5G-Station
AP (Mbps)
0.2
0.5
1
18.38
16.62
7.869
4.471
20.13
17.471
6.82
4.008
20.371
11.725
12.959
N/A
19.068
13.637
13.123
N/A
N/A: Not Available
ANT-WSB-ANM-05(Omni dipole 2.4G 5dBi antenna)
Distance(Km)
2.4G-AP
Station (Mbps)
2.4G-Station
AP (Mbps)
0.1
0.2
0.5
1
24.257
17.597
10.781
4.965
20.67
20.315
10.246
5.922
N/A: Not Available
2
ANT-WSB-ANF-09(Omni directional 2.4G 9dBi antenna)
2.4G-AP
Station (Mbps)
2.4G-Station
AP (Mbps)
0.1
0.2
0.5
1
2.3
18.725
17.557
12.678
7.649
9.259
20.452
19.953
15.259
6.637
9.855
N/A: Not Available
ANT-WSB5-ANF-12(Omni directional 5G 12dBi antenna)
Distance(Km)
5G-AP
Station (Mbps)
5G-Station
AP (Mbps)
1
2.3
5.3
10
11.92
N/A
N/A
N/A
9.259
N/A
N/A
N/A
N/A: Not Available
ANT-WSB-PNF-12(Uni-directional 2.4G 12dBi antenna)
Distance(Km)
2.4G-AP
Station (Mbps)
2.4G-Station
AP (Mbps)
1
2.3
5.3
10
11.92
15.579
8.02
N/A
11.905
16.078
8.022
N/A
N/A: Not Available
ANT-WDB-PNF-1518(Uni-directional dual band 15/18 dBi antenna)
Distance(Km)
2.4G-AP
Station (Mbps)
2.4G-Station
5G-AP
AP (Mbps)
Station (Mbps)
5G-Station
AP (Mbps)
1
2.3
5.3
10
15.686
11.892
11.066
0.306
14.703
13.176
11.691
0.204
24.519
12.889
6.934
N/A
25.001
15.035
7.553
N/A
Understanding Industrial WLAN – IEEE 802.11
Distance(Km)
N/A: Not Available
ANT-WSB5-PNF-18(Uni-directional 5G 18dBi antenna)
Distance(Km)
5G-AP
Station (Mbps)
5G-Station
AP (Mbps)
1
2.3
5.3
10
25.022
16.75
9.662
N/A
26.456
18
9.508
N/A
N/A: Not Available
ANT-WSB-PNF-18(Uni-directional 2.4G 18dBi antenna)
Distance(Km)
2.4G-AP
Station (Mbps)
2.4G-Station
AP (Mbps)
1
2.3
5.3
10
19.627
23.552
14.168
9.715
18.051
21.22
15.564
8.559
N/A: Not Available
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Understanding Industrial WLAN – IEEE 802.11
2.5 Mobile Optimization
In mobile applications that involve multiple access points (APs), the speed and roaming (handover) mechanism can
be crucial to a project’s success. In the world of wireless, roaming refers to when a client moves between two or
more access points. As a result, seamless connection is required for the client to roam from one AP to another.
Simply put, as the client physically moves from one AP to another, the signal from the first AP will drop while the
signal strength from the other AP will increase. By the time the signals from the first AP drop below the signals from
the second AP, the client will have roamed to the second AP.
There are a number of factors that will affect the smoothness of roaming. These include the topology of the access
points, gain and coverage of the antennas, and the roaming threshold settings on the client. To ensure smooth
roaming, we first have to take into consideration the routes of the moving object and carefully plan the wireless AP
deployment configuration.
Different applications encounter different roaming conditions, but we will use linear roaming as an example to
illustrate how roaming is conducted and what factors we need to pay special attention to.
Roaming Under Linear Movement
A client is moving from left to right across three different APs. As the client moves, the signal from the first
AP drops and the signal strength from the second AP increases. Most of the commercial wireless clients only
monitor the communication quality as the basis for roaming decisions. That is to say, when the signal from
first AP drops and frames can not be transmitted, the client, in an IEEE 802.11b application, will first drop
the communication speed from 11 Mbps to 5.5 Mbps and then to 2 Mbps and 1 Mbps. If the communication
quality is still poor and frame transmission continues to fail, the client will decide to roam from the first AP to
the second AP.
A roaming mechanism of this sort might be able to satisfy many non-critical applications. However, this type
of mechanism severely impairs the smoothness of data transmission for video or audio applications, which
require higher quality data transmission.
Limitations of High Speed Roaming
The first limitation of the Threshold-based Handover Algorithm is wireless resource consumption because the
client is constantly scanning for APs that also need to respond in due time. To solve this problem, the network
topology and system configuration needs to be carried out in greater detail.
The second limitation is that there is currently no unified high speed roaming standard so roaming among
different APs from different manufacturers might not be possible.
35
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Roaming Speed Acceleration
To increase the roaming speed, a common method is to use the Threshold-based Handover Algorithm.
2
In short, the Threshold-based Handover Algorithm means the client will constantly scan for the best AP signal
quality and roam only when the threshold is reached. This can prevent the ping-pong effect, unnecessary
handovers that might take place when the client moves back and forth between two APs. Also, this can
increase the roaming speed to deliver a smoother data transmission.
The second way to increase the roaming speed is to unify AP channels to avoid wasting channel hopping time
during roaming. However, a unified channel selection will also cause interference. Users are advised to make
proper channel separation among roaming APs to reduce interference.
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Understanding Industrial WLAN – IEEE 802.11
The Threshold-based Handover Algorithm allows roaming only when the current AP’s signal drops below a
certain threshold so that roaming to another AP would improve the transmission quality and at the same time
provide a stronger signal.
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Understanding Industrial WLAN – IEEE 802.11
2.6 Advanced WLAN Technologies
Dual RF Redundancy
According a recent VDC report, more than 40% of wireless users are concerned about interference. In
industrial and critical applications, this issue is even more important. Normally, interference occurs in a
dedicated frequency. So, if we can use 2 or more different frequencies to communicate at the same time, then
data transmission will not be stopped, even if there is interference in one of the frequencies. The picture below
depicts the standard architecture for wireless infrastructure. As you can see, access points (AP) can connect
many Clients to an Ethernet network.
Figure: Traditional Wireless Architecture
For network redundancy, simply use APs and Clients with dual RF and keep the existing architecture (usually,
these 2 RFs are set to 2.4 GHz and 5 GHz to make sure prevent interference). To ensure that data can be
delivered between the AP and Client, even when there is interference in one of the frequencies, Moxa devices
are equipped with a special protocol with almost ‘0’ switching time for seamless redundancy. For reliability
beyond wireless redundancy, Ethernet redundancy is also required. Fast ring redundancy like RSTP or Turbo
Ring is important on the Ethernet side.
Figure: Redundant Wireless/Ethernet Typology
37
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Introduction to Moxa’s Dual RF Redundancy
Moxa’s advanced AP/Client AWK-5000/6000 series product line provides this kind of redundancy. The
configuration is very easy. All you need to do is select redundant AP on the AP side and redundant Client
on the client side. Then, set a different SSID for each RF. As shown in the following figure depicting the Web
console UI for Moxa’s AWK-5222, set SSID1 for WLAN1 and SSID2 for WLAN2.
Figure: Dual RF—Wireless Redundancy Mode
2
Figure: Single RF Connection
Understanding Industrial WLAN – IEEE 802.11
If both the existing Clients and dual RF clients support redundancy in the same network, Moxa’s AWK5000/6000 Access Point can connect both types of clients to an Ethernet network. As shown in the figure
below, enter SSID (Moxa_1_1) in the 2nd column for the AP to connect the traditional wireless clients with
this SSID to the AP.
In addition to wireless redundancy mode, Moxa’s AWK-5000/6000 advanced AP/Client devices offer
another dual RF feature called “Wireless Bridge” mode. This is designed to optimize WDS mode because of
the throughput problem for WDS. The normal eruption for throughput is throughput = 25Mbps/(n-1), where
n is the nodes number for WDS. With Wireless Bridge mode, we can keep the throughput at 10 to 15 Mbps.
Configuration is simple; simply link the Wireless Bridge master to the Wireless Bridge slave, as shown
below.
25 Mbps
(n-1)
Ex. Around 8Mbps with 4 mesh nodes
Poor Performance
Throughput =
Single RF—Mesh Network
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Understanding Industrial WLAN – IEEE 802.11
Figure: Wireless
Bridge Mode
Wireless Bridge mode can also connect wireless clients to another SSID, as shown below, so it can be used
in environments where APs cannot be wired.
Figure: Bridge Mode for Extra APs
Mesh Technologies
Mesh technologies are generally considered to be wireless communication systems that are interconnected to
each other. However, there are two distinctive ways to build up a so called mesh network: wireless distribution
systems (WDS) and mesh routing. Both of these methods create Layer 2 connections to one or more bridges /
mesh routers to allow data to be passed between them.
WDS differs from mesh routing in many ways. Generally WDS has the nature of a more static network
configuration without significant demand for redundancy. That is, a wireless bridge is configured to point to the
adjacent bridge with a predefined MAC address. So when a bridge fails and while there is no adjacent bridge is
configure to serve as a backup path, the link will be lost.
A wireless mesh routing link, on the other hand, can provide greater redundancy because it can create a
redundant path in the event of node failure. In other words, the mesh router automatically detects a new node
when the original node fails and dynamically determines the best path.
While a WDS is more of a standard and a mesh routing link is more of a proprietary standard, they are being
adopted in accordance with users’ needs. A WDS is often employed in a hierarchical network topology
for bridges that can not prevent broadcast storms. As a result, a WDS is often configured in spanning tree
topologies. A bridge loop is often avoided to prevent a broadcast storm. However, there are software solutions
which utilize Spanning Tree Algorithms (STA) to compute the best path between two nodes while putting all
other paths in blocking mode. This realizes communication redundancy in a WDS but it can be time consuming
to create a workable bridge loop. So a WDS is often adopted in a small network that requires manual
configuration for each node. Once the connection is established, it is not easily interrupted.
Mesh routing on the other hand is often adopted in systems that require higher redundancy. It often needs few
manual configurations for each node and provides greater expandability when more nodes are to be added in
the future. In summary, redundancy is the primary concern when choosing mesh routing links. It is also more
of a suitable choice when the connections are subjected to constant disruptions, for example, by passing by
buses.
39
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Wireless VLAN
A Virtual LAN (VLAN), as defined in IEEE, is a group of hosts grouped together as if they were attached to the
Broadcast domains in a Layer 2 network. Traditional networks use routers to define broadcast domain, but it
is now possible to set the broadcast domain boundaries with Layer 2 switches. That is to say, a VLAN can add
two or more hosts that were in different subnets to be grouped into the same LAN segment disregard of their
geographical locations. VLAN provides the leeway to the network administrators to address network security,
management and scalability issues.
2
By borrowing the same concepts, it is now also possible to apply VLAN to an IEEE802.11 wireless
network. Many wireless access points (AP) are now equipped with VLAN capability. A single AP can now
be configured to assign a different service set identifier (SSID) to different VLANs. Also the authentication
settings like MAC, EAP, and VLAN ID are required to configure a wireless VLAN.
Understanding Industrial WLAN – IEEE 802.11
How to Set Up VLAN with IEEE 802.11
Every time a packet is sent from one switch to another over a VLAN, VLAN tagging is required. VLAN
tagging is the practice of inserting a VLAN ID in the packet header so the packet can be identified and
forwarded to the right port or interface. The IEEE 802.1q standard is the most commonly seen VLAN tagging
protocol created by IEEE group. The tagging protocol supports a maximum of 4096 VLANs.
Wireless VLAN Limitations
Wireless VLANs bring many benefits to WLAN applications, but there are some potential limitations when a
wireless VLAN reaches a certain scale. The first limitation arises from its 12 bit VLAN identifier (VID). The size
of the VID limits the number of wireless VLAN to 4094. The number might look big enough to accommodate
most WLAN applications. However, as wireless applications grow at a tremendous pace, it will soon be not
enough for some large scale WLAN applications.
Large scale Wireless VLAN also causes the second and third limitations. That is, when the Wireless VLAN
grows too large, traffic flowing through the routers also increases. This large volume of traffic makes routers
another potential bottleneck for the Wireless VLAN.
The third constraint is the potential security loopholes. As the VLAN grows, there is a possibility that the
wireless VLAN will stretch over large geographical areas that require the VLAN to pass through a third party
network. This creates security loopholes as there is almost nothing to stop the virus from spreading inside
the VLAN.
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Understanding Industrial WLAN – IEEE 802.11
QoS for Video/Audio and Control
The dramatic growth of WLAN has also led to demands for video and audio transmission over WLAN. Quality
of Service (QoS) is therefore becoming an important topic for wireless communications. QoS is a network
term for controlling and measuring data transmission rates, throughputs and error rates. QoS is not of a big
concern for simple data transmission. However, it becomes more of an issue when it comes to audio and
especially video. Video requires high quality of flow and throughput control and lower error rates. All these are
In multimedia data transmission, it is important for delays arising from network latencies to be undetectable to
users.
IEEE 802.11e was set up to answer this call for QoS in WLAN. IEEE 802.11e is the amendment that defines
wireless LAN QoS enhancement in IEEE 802.11. WLAN QoS is achieved through modifications to the Media
Access Control (MAC) layer, solving the latency delay problem that is sensitive to multimedia and voice data
transmission.
Wireless Management
QoS is essential for wireless communication. It is an important element for wireless applications when it comes
to management. There are three layers of management, namely device management, network management,
and centralized management.
Device Management
When it comes to network management, device management is always the most basic task for all network
administrators. Often, wireless APs/Clients come with a management utility or web console that allows
network managers to locate and remotely configure the wireless APs/Clients.
Network Management
Above device management is the network
management layer. This layer requires a higher
level of software utilities to manage all wireless
nodes. The network management utility should
be able to perform multiplatform monitoring,
event management, alerting, real-time
performance monitoring, network discovery,
and topology mapping.
Centralized Management
More advanced network management tools
provide a complete solution for network
administrators including VPN, firewall, and
UTM. It also allows centralized management for
device maintenance.
41
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2.7 Industrial Certification
EN50155 Certification
EN50155 are the standards that define requirements for railway cars, or rolling stock. It clearly outlines the
requirements for power input voltage fluctuation, transient, ambient temperature ranges, shock and vibration
and as well as fog and salt spray. Electronics that are used aboard railway rolling stock are required to meet the
standards of EN50155. The target of EN50155 certification for all electronics is for the devices to be operable
24/7 for as long as 20 years. To meet this standard, an electronic device needs to meet the requirements as
summarized below:
Power Input Voltage Fluctuation
A nominal 110 VDC power input system needs have at least 77 to 137.5 VDC voltage fluctuation range with no
time limits. The fluctuation may sometimes become too extreme so electronic devices should also be able to
withstand 66 to 154 VDC power input for at least 100 ms.
Transient and Ambient Temperature
Shock and Vibration
EN50255 follows the test standards of EN67373. Electronic devices must withstand at least 1 G vibration
when mounted on a DIN-Rail. Chassis mounted devices are required to withstand a shock rating as high as
50 G.
Atmospheric Pollutants
Combustible dust accompanied by oil, sulphur dioxide, and salt spray in the air create a hazardous
environment for rolling stock applications. As a result, an EN50155 compliant device must have a high IP
rating.
Air Cooling
Force air cooling systems are not allowed. EN50155 electronic devices must have conductive-only
mechanism designs to eliminate potential maintenance problems that arise from fan cooling systems.
Moxa AWK Series Meet EN50155 and EN50121-3-2/50121-4 Standards for Rail Traffic
Certified to Meet Industry Standards Rail vehicles require the highest standards of stability due to random
vibrations that occur during normal operation. The EN 50155 standard covers electronic equipment used on
rolling stock, and EN 50121-4 defines the emission and immunity of
signaling and telecommunication apparatus. They outline the issues
that need to be addressed to ensure that railway electrical systems
are integrated successfully.
Understanding Industrial WLAN – IEEE 802.11
Railway rolling stock require an operating temperature range of -25 to 40°C, or -40 to 85°C, for electronic
devices. When the trains enter and leave a tunnel, the temperature change can be very drastic. As a result,
electronic devices must also be able to withstand a 3°C/sec temperature change and thermal shock that
leads to condensation on the PCB.
2
The AWK series is engineered to resist extreme vibrations and
shocks based on the railway standards (EN50155/EN50121-32/50121-4).
Rail Traffic
- EN 50155 (Environmental)
- EN 50121-3-2 (EMC)
- EN 50121-4 (EMC)
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Understanding Industrial WLAN – IEEE 802.11
ATEX/Class I Division 2
ATEX is the term used when referring to the European Unions (EU) Directive 94/9/EC. ATEX governs
the regulations on the equipment used in potentially explosive atmospheres. All equipment meeting the
requirements are free to circulate within EU boarders. The directive applies to all equipment or protective
systems used in areas subject to explosion risks, gas vapors, mist, or dust. The directive also sets the
standards for safety devices, control equipment, and calibration equipment.
About ATEX/Class I Division 2
Class I Division 2/Zone 2
Class I Locations:
These locations are defined as places where the air may contain flammable gases or vapors in sufficient
quantities to cause explosive or ignitable mixtures.
Division 2:
The division defines the conditions under which the hazard exists. Division 2 refers to the following
conditions:
• Liquids and gases in closed containers or systems are handled, processed, or used.
• Concentrations are normally prevented by positive mechanical ventilation.
• The specified area is adjacent to a Class I, Division 1 location.
Zone 2:
This is defined as an environment where an explosive gas atmosphere is not likely to occur during normal
operation, but could occasionally arise for brief periods of time.
ATEX/Class I Division 2 in Oil & Gas Industry
Safety is of the utmost importance and cannot be negotiated in dangerous industrial environments. In
particular, oil and gas exploration sites, as well as energy field development, production, and transportation
facilities face a real risk of explosions. Therefore, engineers must ensure prior to installation that all the
equipment and protection systems have to meet or exceed regulatory standards used under hazardous
conditions.
Moxa Offers Specifically Designed for Hazardous Sites
Designed with the highest safety standards in mind, Moxa’s AWK series of WLAN solutions are constructed
in strict accordance with global acceptance for explosion protection in hazardous locations. Product
offerings include rugged AWK-3121 and AWK-4121. With ATEX/Class 1 Division 2, and UL C1D2
certifications, these stringently proven WLAN products help build safe and reliable networks in the oil and
gas industry.
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Chapter 3
Cellular Networks
3.1 Cellular Basics
What is Cellular?
Cellular is a radio based communications system that enables customers to call and be reached over a
wide area, supporting both hand-over and roaming. Cellular networks are connected to the PSTN to give
transparent incoming and outgoing access to fixed network subscribers.
3
The following diagram shows a typical GSM cellular network architecture:
Cellular Networks
Supports the switching
functions, subscriber profiles,
and mobility management
Moxa OnCell G2100/3100
series, PDA or PC
connected to the ME
• Radio Interface Connects
to MS
• A Interface Connects to
MSC
Data Service of GSM
Here we show how to implement these three data services. We hope this will help you understand how these
technologies work so you can select the most suitable application.
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Cellular Networks
Short Message Services (SMS)
SMS messages, as specified by the ETSI organization (documents GSM 03.40 and GSM 03.38) and can be
up to 160 characters long, where each character is 7 bits according to the 7-bit default alphabet. Eightbit messages (max. 140 characters) are usually not viewable on phones as text messages. Instead, they
are used for data in 16-bit messages (max. 70 characters) and used for Unicode (UCS2) text messages
viewable on most phones.
SMS is a point-to-point store and forward technology with 2 basic functions:
• Transmit a message from the short message service center to the mobile station. SMS-DELIVER PDU
(Protocol Data Unit)
• Transmit a message from the mobile station to the service center. SMS-SUBMIT PDU (Protocol Data Unit)
SMS messages contain up to 140 octets which is equivalent to
• 160 Latin characters (7 Bit Coding) in Text Mode, or
• 70 Unicode characters (double byte), such as Arabic or Chinese characters.
SMS Summary
• Up to 140 octets or 160 characters for every message
• No IP-based communication, only suitable for sending/receiving serial data
Circuit Switch Data (CSD)
Circuit Switched Data (CSD) is the original form of data transmission developed for Time Division Multiple
Access (TDMA) based mobile phone systems like Global System for Mobile Communications (GSM). GSM
CSD bearer service is the most widely used data service providing a “non-transparent” data rate of 9.6
Kbps. “Non-transparent” in this context means error correction and flow control.
Usually, GSM network operators support the non-transparent CSD bearer service through a modem
interworking function. This means that a mobile station (mobile phone) initiates a data call and the network
routes the call to the modem interworking function, which is located at the Mobile Switching Center (MSC)
of the GSM network, which then dials the number supplied by the mobile station.
Confirm that your cell phone service provider allows fax/data type connections for your cellular service plan.
Please note that we are not referring to high-speed wireless data services or SMS. We are referring to your
cellular service plan’s ability to transmit a “fax call” connection, which has been widely included in cellular
service plans for years. This feature is typically referred to as CSD (circuit switched data).
• CSD Summary
- Up to 9.6 Kbps
- Circuit-switched connection
- IP-based communication possible with dedicated link, but speed is slow and billed by connection setup,
most operators remove CSD service
- Most GSM operators provide the service
- In North America, CSD has been completely phased out by the end of 2007
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Packet Switching Data Solution: GPRS
General Packet Radio Service (GPRS) provides Packet Switching service to GSM systems. A GSM system
is traditionally a Circuit Switching network that provides optimized voice transmission service. For instance,
a call between Party A and Party B will exclude other parties. Even if Party A and Party B temporarily fall
into silence during their conversation, the call (network resource) won’t be released until either end hangs
up. As a result, the network resource is wasted when a data call is utilized in a GSM network. That is
because data transmission doesn’t necessarily need real-time transmission, which is primarily required for
voice or video communications. A few seconds delay won’t change the correctness and consequence in
data transmission. Take email uploads and downloads for example. The network resource is occupied only
when data is transmitted through the packet switching system. As a result, other users can freely send their
data when the system is “inactive.” Accordingly, consumers benefit from the low cost and real-time data
transmission of GSM networks.
General packet radio service (GPRS) is a packet-oriented mobile data service available to 2G and 3G GSM
users.
• Serving GPRS Support Node (SGSN)
A Serving GPRS Support Node (SGSN) is responsible for the delivery of data packets from and to the
mobile stations within its geographical service area. Its tasks include packet routing and transfer, mobility
management (attach/detach and location management), logical link management, and authentication and
charging functions. The location register of the SGSN stores location information (e.g., current cell, current
VLR) and user profiles (e.g., IMSI (ID code of the SIM card) addresses used in the packet data network) of
all GPRS users registered with a particular SGSN.
• Gateway GPRS Support Node (GGSN)
The Gateway GPRS Support Node (GGSN) is a main component of GPRS networks. The GGSN is
responsible for connecting the GPRS network to external packet switched networks such as the Internet
and X.25 networks.
The GGSN stores the current SGSN address of the user and his or her profile in its location register. The
GGSN is responsible for IP address assignment and is the default router for the connected user equipment
(UE). The GGSN also performs authentication and charging functions.
Other functions include subscriber screening, IP Pool management and address mapping, QoS and PDP
context enforcement.
3
Cellular Networks
A GSN (GPRS Support Node) is a network node that supports the use of GPRS in the GSM core network.
All GSNs should have a Gn interface and support the GPRS tunneling protocol. There are two key variants
of the GSN, namely Gateway (GGSN) and Serving GPRS Support Node (SGSN).
• Summary
- General Packet Radio Service (GPRS)
- Connectionless
- Bill by packets
- IP-based communication, Internet access and increasing speed with 3G, HSDPA, HSUPA, etc.
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Cellular Networks
APN in Packet Switch
Each external network is given a unique Access Point Name (APN) that is used by the mobile user to establish
the connection to the required destination network.
PDP context activation procedures are as follows:
1. Mobile phone sends out PDP context activation request and other relative parameters (e.g.,APN, QoS)
2. SGSN begins verification based on previously stored GPRS Attach information
3. DNS mechanism in SGSN analyzes the APN and returns a GGSN address
4. SGSN and GGSN build logic links
5. GGSN will instruct an IP address for the mobile phone and send it to the MS via SGSN. The external
network can then start a session with the MS.
• APN (Access Point Name)
- Access Point Name is a label according to DNS naming conventions describing the access point to the
external packet data network (PDN).
- An APN is a logical way to name a GPRS service.
- The Domain Name Service (DNS) server translates the APN into the GGSN IP address.
- APN string naming comes from the mobile operator. There is no common rule so customers need to
request (1) GPRS service and (2) the APN string from their operator.
- Some operators offer different APNs according to the GPRS service level, such as public fixed IP
addresses, non-port blocking, or VPN.
• IP Address Allocation in GPRS
- Fixed addressing
IP address is stored in HLR
HLR sends IP address to SGSN, then SGSN sends IP address to the MS
IP address is sent to the MS when the MS wants to send data
- Dynamic addressing
GGSN receives the IP address (DHCP/local address pool/RADIUS)
When the MS opens PDP context, GGSN assigns an address to the MS
• Obtaining an IP Address
- From a local address pool on the GGSN
- Via DHCP
- Via RADIUS from an external RADIUS server
- From the customer’s network via an L2TP tunnel from the GGSN
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3.2 Private IP Solution
There are two limiting factors you are almost assured to encounter when setting up a wired LAN with private IP
addresses in an office: (1) short transmit latency and (2) limited IP addresses to connect to the Internet for the
cellular WAN interface. As a result, delay times for the WAN interface are very different from the local area network’s
delay times. WAN port IP addresses are also very different from those in office LANs.
Private IP vs. Public IP
From the LAN point of view:
According to RFC 1918 standards, office networks are allocated private IP addresses according to class
A, B, and C. The Internet Assigned Numbers Authority (IANA) has reserved the following three blocks of IP
address space for private networks:
10.0.0.0
10.255.255.255 (10/8 prefix)
172.16.0.0
172.31.255.255 (172.16/12 prefix)
192.168.0.0 192.168.255.255 (192.168/16 prefix)
3
From the cellular WAN port point of view:
Type of Address
Description
Role(s) of Configured Devices
Floating private IP
address
Mobile operator keeps a pool of
private IP addresses and assigns one
to the GPRS subscriber
Always Client role to access server
Class A 10.xx.xx.xx
Floating public IP
address
Mobile operator keeps a pool of public
IP addresses and assigns one to the
GPRS subscriber
Can be Client or Server role, IP
address is always changeable, needs a
notification mechanism to update public
IP address
Fixed public IP
address
Mobile operator keeps a dedicated IP
address for each SIM card based on
the SIM card’s (IMSI) ID code and user
service level
Can be Client or Server role,
IP address is fixed, needs a special bill
rate from operator
Cellular Networks
Normally, IP addresses for cellular WAN ports will have:
As you can see from the table above, the kind of WAN IP address obtained from your cellular operator will
affect network planning and determine the role of the devices configured with the IP address.
Private IP addresses are suitable for Client role.
Public IP addresses are suitable for Client role and Server role.
Delay Time
Latency in a packet-switched network is measured either one-way (the time from the source sending a packet
to the destination receiving it) or round-trip (the one-way latency from source to destination plus the one-way
latency from the destination back to the source). Round-trip latency is more often quoted, because it can be
measured from a single point. Note that round trip latency excludes the amount of time that a destination
system spends processing the packet. Many software platforms provide a service called ping that can be used
to measure round-trip latency. Ping performs no packet processing; it merely sends a response back when it
receives a packet (i.e., performs a no-op) so it is a relatively accurate way of measuring latency.
Where precision is important, one-way latency for a link can be more strictly defined as the time from the start
of packet transmission to the start of packet reception. The time from the start of packet reception to the
end of packet reception is measured separately and called “Serialization Delay.” This definition of latency is
independent of the link’s throughput and the size of the packet, and is the absolute minimum delay possible
with the link latency of the LAN you can measure by specified device with input to output delay time in
serialization.
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Cellular Networks
As a result of WAN latency from cellular networks, you cannot count the number of nodes in your link as the
timing is different for each link. Therefore, delay time in cellular networking is immeasurable and not suitable for
real-time systems.
Solution for Private IP
A common problem in M2M network planning is that cellular operators and service providers usually only
provide private and dynamic IP addresses. The reasons for doing so are threefold:
1. In the world of Internet communication, one of the biggest problems is that the number of public IP
addresses is running out quickly. It is therefore reasonable that cellular network operators issue private and
dynamic IP addresses to conserve valuable public IP resources.
2. The bandwidth of a cellular network is so narrow that it is very vulnerable to cyber attack. Private IP
effectively prevents cyber attacks from paralyzing the networks.
3. Require only uplink from cellular device to WLAN. However, in most M2M applications, it is a requirement
that data exchange is a two-way communication either from server to client or client to server.
To solve this private IP issue for M2M applications, three major solutions are available:
1. Users can pay extra money to get a public and fixed IP SIM card. This way, cellular M2M system
configuration will be very similar to LAN architecture.
2. Users can get cellular VPN services from their ISP (Internet Service Provider) or a second tier operator
known as an MVNO (Mobile Virtual Network Operator). They offer services to allow cellular links between
nodes in a VPN that uses open connections or virtual circuits in a larger network, such as the Internet.
With the help of VPNs, cellular devices act as a VPN client that can initiate a connection with a VPN server
building a two-way communication environment for M2M applications.
3. Despite VPN being the most commonly seen solution to the private IP problem in M2M applications,
the data exchange inside a VPN can take up too much of the network resources for it requires heavy
duty encryption and decryption of data. To save valuable cellular network resources, some M2M
solution providers offer a software solution to help customers cope with the private IP problem. That is,
manufacturers provide middleware that works as a communication gateway.
Moxa OnCell Central Manager
The OnCell G3100 series is Moxa’s cellular IP gateway that connects serial or Ethernet devices to cellular
networks. As a leading industrial networking solution provider, Moxa is aware of the private IP issue troubling
many of our M2M customers. To solve this problem, Moxa has introduced OnCell Central Manager. Simply
install OnCell Central Manager onto a Server PC with a public IP address and register the IP addresses of
OnCell devices in your private network on the Client OnCell IP gateway. As soon as the OnCell IP gateway is
powered on, it will initiate the connection to OnCell Central Manager. Thanks to OnCell Central Manager, your
access points can now communicate with field devices via cellular networks so you don’t need to worry about
paying extra VPN service fees or wasting valuable cellular network resources.
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3.3 Security
One of the major concerns faced by system integrators when adopting an Ethernet solution is the security and
confidentiality of data transmissions over the network. Wireless networks are especially vulnerable because
they need to transmit data through open air and are vulnerable to sniffing. To protect the security of wireless
connections, one of the most commonly seen solutions is the VPN.
The Virtual Private Network (VPN)
A VPN is a computer network that links up two or more networks or nodes by using open connections or
virtual circuits. Many people believe that a VPN offers sufficient data transmission security. However, a VPN, by
itself, does not guarantee information security. In response to the lack of security when tunneling through the
network, L2TP and IPSec are often used to enhance network security.
Layer 2 Tunneling Protocol (L2TP) is a tunneling protocol used in a VPN. L2TP is sent in a UDP datagram. It
contains no security feature on its own so it is often implemented along with IPSec.
IPSec is an open communication standard created to ensure data transmission security over public networks.
IPSec is also a Layer 4 security protocol, which is the most widely used way to ensure security for it is a more
balanced solution than Layer 1 and Layer 7 security control.
IPSec also contains the Internet Key Exchange protocol that is used to negotiate IPSec Connection Settings,
authentication endpoints, and secret keys, as well as to define the security parameters, manage updates, and
more.
Cellular Networks
IPSec uses either Authentication Header (AH) or Encapsulating Security Payload (ESP). AH can protect
packet headers and data integrity but provides no encryption functionality. On the other hand, ESP provides
encryption and conserves the integrity of the packet, but cannot protect the outermost IP header as AH can.
ESP is the most commonly used protocol in a VPN because encryption is more of an important requirement in
a VPN while header protection is not.
3
As far as the data compression technologies go, IPSec uses IP Payload Compression Protocol (IPComp) to
compress data before encryption; this also allows communication to be carried out in a more efficient way.
Firewall
Except for data encryption, using a firewall is the most common method to protect both wired and wireless
connections from outside attacks. There are multiple ways in which the firewall acts to deny cyber attacks
including inspecting data packets for suspicious contents or filtering IP addresses.
The most protection a firewall can offer is to set up a list of accessible IP addresses that limits access from
WANs. In most M2M applications, this is the most effective and direct way to protect a LAN from WAN attacks.
Moxa’s OnCell IP router offers two kinds of firewall protection for users to choose from. One way is to filter
WAN IP addresses to accept or deny WAN connectivity requests. Another way is to set up a virtual server that
allows remote users to access the Host or FTP services via a public IP address, and automatically redirects
them to local servers in the LAN. This firewall feature will filter out any unrecognized packet to protect your
LAN.
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Cellular Networks
3.4 How to Connect Serial Devices to Cellular Networks
Traditional Modems
Serial port connections are very popular in traditional industrial applications but their transmission distance
is limited. The Hayes command modem (AT command) offers a good solution for enlarging the transmission
distance. It connects two serial devices through PSTN via an AT modem. This traditional modem always
occupies the line as a voice call so even when you are not talking (transmitting data), you will still be charged
by the minute for staying on the line.
In addition, the serial device (such as a PLC) link to the modem needs dial-up capability for call controls, such
as dialing a number, checking if the called side is busy, retrying the call, and hanging up. If the connected link
is an IP domain, then the serial device needs built-in PPP (Point to Point Protocol) capability to access the IP
domain, whether it is an Internet or VPN. Serial devices also require many call control capabilities in order to
link to traditional modems, resulting in heavy loading.
IP modems, equipped with call setup and PPP capability to reach IP Internet domains, offer a viable solution
that reduces loading for serial devices so they can focus on transmitting and receiving serial data. Cellular
networks are everywhere so you can make calls without a wired telephone connection, providing industrial
automation machine-to-machine applications with additional benefits.
IP Gateways
IP gateways are not only call setup intelligent but also come with built-in TCP/IP capability. Due to the
popularity of cellular networks around the world, you will be able to use them to communicate from just about
anywhere. Moreover, IP gateways can help your serial devices transfer and receive data conveniently.
Moxa‘s cellular IP gateways solution offers flexible communication for serial devices.
How OnCell Cellular IP gateways can help your serial device
access an IP domain:
• Keep existing software (Real COM / Reverse Real COM)
• Standard TCP IP connection (TCP server / Client)
• Real-time serial data send/receive solution
• Short message connection (SMS Tunnel)
Network capabilities for data bearer:
• IP gateways over CSD
• IP gateways over GPRS
Keep Existing Software (Real COM / Reverse Real COM)
Moxa’s Real COM and Reverse Real COM operation modes are used to link remote serial devices to the
control center. By simply installing OnCell Driver Manager onto a host computer at the control center, your
applications will be able to keep existing serial interface software to transfer and receive data from the serial
devices.
Here’s how it works:
• Installing OnCell Driver Manager onto the host PC allows the host PC to connect directly to remote serial
device.
• The serial device is connected to the OnCell G3100’s serial port.
• The connection is maintained automatically between OnCell Driver Manager and the OnCell G3100 device
to access the serial device over a cellular network.
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• Depending on whether the OnCell G3100 device is acting in a Client role or Server role, the user can select
Real COM mode or Reverse Real COM mode. Normally, the role of the OnCell G3100 device depends on
the IP address obtained from your cellular service provider. If your OnCell G3100 device’s SIM card is able
to obtain a public IP address, then the OnCell device can act as a Server and you can select Real COM
mode to connect the host PC (Client role).
OnCell Device’s IP Address
Suitable Role
Operation Mode Selection
Server role,
Real COM mode,
Client role
Reverse Real COM mode
Client role
Reverse Real COM mode
Public address
Private IP address
Like 10.x.y.x or 172.xx……
3
Real COM mode diagram
Cellular Networks
If both the host PC (at the control center) and the OnCell G3100 device have private IP addresses, you can
use Real COM mode on the OnCell G3100 to resolve the private IP to private IP problem.
Reverse Real COM mode diagram
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Cellular Networks
Socket Mode, Standard TCP IP Connection (TCP Server/Client)
If your application involves a socket-based TCP server or TCP client, you can set your OnCell G3100 device
to Socket Mode Socket operation mode by simply using the OnCell web console. You do not need to
install any additional utilities onto the host PC but the socket software on the host PC will need to set up a
socket connection with the OnCell G3100. The IP address of the SIM card on the OnCell G3100 device will
determine the role of socket connection.
OnCell Device’s IP Address
Suitable Role
Socket Mode Selection
Public address
Server role, Client role
TCP Server, TCP Client
Private IP address
Like 10.x.y.x or 172.xx……
Client role
TCP Client
If both the host PC (at the control center) and the OnCell G3100 device have private IP addresses, you can
use OnCell Central Manager on the OnCell G3100 to resolve the private IP to private IP problem and select
TCP Server for the OnCell G3100’s socket mode.
IP gateway play TCP Server role
IP gateway play TCP Client role
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Short Message Connection (SMS Tunnel)
Short Message Service (SMS) is a very popular service offered by cellular providers. You can send and
receive serial data via SMS by setting up the OnCell G3100 device’s SMS Tunnel Mode.
The OnCell G3100’s SMS Tunnel Mode uses a serial port to serial port communication tunnel to send SMS
messages. There are 3 key inputs involved in transmitting SMS serial data.
1.Target phone number of the mobile device that will receive transmitted data
2.Pre-approved recipient phone numbers; unapproved numbers will be filtered out
3.Data format (i.e., ASCII, binary, UCS2)
3
Cellular Networks
Unlike GPRS and CSD, SMS employs a “store and forward” mechanism so messages are not transmitted in
real time.
IP Gateways and CSD
Some cellular providers offer Circuit Switch Data (CSD) service, which transmits data over voice channels
that are always connected, such as fax and modem service. However, CSD calls always occupy the phone
which means you will still be charged for service even when you are not sending data. In additional, data
throughput is limited and takes up too many resources. As a result, cellular providers usually do not offer
CSD and the service is no longer offered in some countries. At the same time, 2G service is also being
phased out in favor of 3G technologies. Check with your cellular provider to see if they offer these services.
The OnCell G3100 IP gateway can send IP stacks using PPP (Point to Point Protocol) capability over CSD
service. There are 2 mechanisms for setting up your PPP connection:
1. The originating OnCell device (PPP) dials the terminating OnCell device (PPPD).
PPP(Dial out) to PPPD(Dial-In)
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Cellular Networks
2.The originating OnCell device dials the phone number of the ISP (Internet Service Provider) just like an
analog modem. Normally, ISPs offer free accounts and passwords for you to access the Internet.
PPP to Internet via ISP
IP Gateways and GPRS
GPRS is designed for packet data communication and provides GSM users with access to the Internet from
a cellular network. Before activating the GPRS function on your OnCell G3100, make sure that the SIM card
is GPRS-enabled with the right APN string.
In order to activate GPRS service on your OnCell device, your SIM card must first be connected to a GSM
network. That is why the GSM LED on your OnCell device will be lit for a while before the GPRS LED turns
on. You will also need the APN (Access Point Name) and account password (not required by some cellular
providers). The APN is a string that determines which Internet gateway (GGSN) from the cellular provider’s
network you can access. This may affect the IP address and services, such as port blocking and VPN, you
are able to obtain from your provider. Cellular providers also maintain a database (HLR) to record and bill
your service level.
GPRS mode to Internet
If your SIM card is already GPRS-enabled and has the right APN, then your OnCell G3100 will automatically
set up the IP link for your application whether you’re using socket mode or operation mode.
Modem Extension Mode (Virtual Modem)
Modem Extension Mode is a solution for
• Extending the distance from the control center to a pure AT modem, and
• Sending pure AT commands to control your OnCell G3100 device via Ethernet (longer distance).
If you need to control your modem from a longer distance, consider using an Ethernet link. This solution
allows you to keep the serial control interface at the control center and send AT commands from the control
center to your OnCell G3100 device by Ethernet.
You will first need to install OnCell Driver Manager on the controller side to create a virtual COM port, which
will allow you to control the serial port as if it were a traditional modem and send AT commands to set up
CSD calls, send SMS messages, or link to GPRS. You also can install the OnCell modem driver so you can
set up dial up networking for your laptop.
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3.5 How to Connect Ethernet Devices to Cellular Networks
Traditionally, Ethernet-based devices can only establish TCP/IP connections through wired LAN lines. At best, you
may be able to deploy a WLAN environment to communicate with Ethernet devices in the field. A WLAN system can
eliminate wires and cabling problems when installing and operating the devices. It also provides greater mobility,
especially when the Ethernet devices are moving. When it comes to range, however, WLAN systems are still limited
to local area networks and places with hardwire Internet connections.
What if Ethernet devices could be accessed through a cellular network? This would allow Ethernet devices to
be accessible almost anywhere as long as cellular coverage is available. Ethernet to cellular technology can also
provide primary and backup network connectivity. Moxa offers an easy and cost-effective means of connecting
virtually any remote location or device to a corporate IP network. It is ideal for applications where wired networks
(e.g., lease line / frame relay, CSU/DSU, fractional T1) are not feasible or where alternative network connections are
required.
3
From WAN to LAN (TCP Server)
Cellular Networks
In this case, the Ethernet device, when acting as a server, is to be reached from the public domain. The TCP Server
may be an industrial PC server, an I/O device with LAN interface, or any Ethernet routing device. As long as the
device uses a LAN interface running on TCP protocols (even MODBUS TCP), the device can be reached. There may
even be multiple Ethernet devices with different IP addresses connected to the IP gateway.
When a TCP Client device attempts to connect to the TCP Server, it will first need to make a TCP connection
with the IP modem (OnCell), and then have the OnCell port forward the synchronization request to the TCP
Server connected to it. Basically, the OnCell plays the role of a virtual server to allow clients to make a direct TCP
connection to it before forwarding traffic to the actual server. Much like a WLAN router, the traffic from the WAN port
is directed to the devices connected to the LAN port of the router.
It is important to note that your OnCell device will need to obtain a public WAN IP address from your cellular
provider in order for it to be visible to the public domain. Private IP addresses are hidden from the public Internet so
TCP Clients will not be able to find it on a public network. The WAN IP address of your OnCell device may be static
or dynamic, but it must be a public IP address. If the public WAN IP address is a dynamic IP address (changes
every time the OnCell reconnects to the cellular network), a useful function is to enable to DDNS (Dynamic DNS).
DDNS allows the TCP clients to access the OnCell device by domain name. So even as the OnCell device’s WAN IP
address changes, the changed IP addresses continue to map to the same domain name through DDNS updates.
In cases where only private IP addresses are available from the cellular provider, the OnCell can still play the server
role by enabling the OnCell Central Manager (see section 3.2 for details) function proprietary to Moxa’s IP gateways.
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Cellular Networks
How to configure the OnCell device as a virtual server
In the OnCell device’s web console, you will find a virtual server settings page where you can set up multiple
Ethernet devices connected to the OnCell device:
As you can see, virtual server setting is basically setting the forwarding ports. For example, you select an
available public port that the OnCell’s WAN IP will be listening on. A TCP client device will connect directly
to the OnCell’s WAN IP/Public Port when making a TCP connection with the server. Next, enter the actual
server’s IP address (Internal IP) to allow the OnCell to locate the server in the local network. An internal port
(listening port on the actual server) is then specified so that the traffic coming through from the public port
will be forwarded to the internal port. Lastly, you’ll notice that both TCP and UDP traffic can be forwarded by
the virtual server.
The previously mentioned DDNS function can be enabled on the OnCell device to compensate for dynamic
WAN IP addresses:
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From LAN to WAN
3
We now switch the role of the Ethernet device from TCP Server to TCP Client:
When the OnCell is acting as a client, its WAN IP address will not be limited to public WAN IP addresses. The
WAN IP address of the OnCell IP modem can be public or private, static or dynamic without any extra settings.
How to configure the OnCell as a gateway to the public domain
For the OnCell G3100 series, you do not need to perform extra settings to use the OnCell as a gateway for
connected Ethernet devices to be able to access the public internet. The built-in NAT function is enabled by
default, so by simply setting the default gateway IP address of the Ethernet device to the OnCell’s LAN IP
address, the Ethernet device will be able to connect to the Internet and initiate TCP connection requests to the
TCP server.
Cellular Networks
Now the OnCell device is a gateway for the TCP Client to route its traffic to the public domain through the
cellular network. The NAT function built into the OnCell device allows the WAN and LAN interfaces to direct
traffic to each other. The Ethernet device can now locate the server on the public domain to establish a remote
connection. For example, multiple Ethernet devices at a remote site can act as TCP clients and all connect to
the same server in the control center for central management.
Please note that if you are going to be accessing a domain name on the Internet, not only do you need to set
the OnCell as your default gateway, but a public DNS server is also required for domain name access.
The OnCell can be both TCP Server and TCP Client
In some applications, Ethernet devices act in both the server and the client role. In these cases, the OnCell
can play both roles as well. The virtual server settings and gateway settings can be used simultaneously in the
same way a traditional router can forward traffic in both directions. As shown in the network diagram below,
you only need two OnCell devices for two Ethernet devices to communicate with each other. The Ethernet
devices do not need to be located in a hardwire Internet-ready area as long as there is a cellular signal present
and the TCP Server/Client pair of the OnCell device can bridge a LAN connection.
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Cellular Networks
3.6 How to Connect I/O Devices to Cellular Networks
SCADA Meets Ethernet
SCADA (Supervisory Control and Data Acquisition) is a computer-based industrial control system that plays an
important role in the field of automation today. In particular, SCADA systems are used to monitor and control
a process, including manufacturing, production, power generation, fabrication, refining, water treatment,
distribution, wastewater collection and treatment, oil and gas pipelines, electrical power transmission and
distribution, facility processes, and more. Some of the benefits provided by SCADA include:
• Setting up Communications for Data Acquisitions
• Graphics HMI
• Alarms
• Trends and Process Analyst
• Commands and Controls
SCADA usually refers to centralized systems that monitor and control entire sites, or a network of systems
spread out over large areas. Most control actions are performed automatically by intelligent I/O devices (i.e.,
Moxa’s Active Ethernet I/O), remote terminal units (RTUs), or programmable logic controllers (PLCs). Host
control functions are usually restricted to basic overriding or supervisory level intervention. For example, a PLC
may control the flow of cooling water through part of an industrial process, but the SCADA system may allow
operators to change the setpoints for the flow, and enable alarm conditions, such as loss of flow and high
temperature, to be displayed and recorded. The feedback control loop passes through the Active Ethernet I/O
or RTU or PLC, while the SCADA system monitors the overall performance of the loop.
Modem Extension Mode (Virtual Modem)
Data acquisition begins at the RTU, PLC, or I/O device level, and includes meter readings and equipment
status reports that are communicated to the SCADA system as required. Data is then compiled and
formatted in such a way that a control room operator using the HMI can make supervisory decisions to
adjust or override normal I/O controls. An HMI is usually linked to the SCADA system’s database and
software programs, to provide trending, diagnostic data, and management information such as scheduled
maintenance procedures, logistic information, detailed schematics for a particular sensor or machine, and
expert-system troubleshooting guides.
Data may also be fed to a commodity database to allow trending and other analytical auditing. SCADA
systems typically implement a distributed database, commonly referred to as a tag database, which
contains data elements called tags or points. A point represents a single input or output value monitored
or controlled by the system. Points can be either “hard” or “soft”. A hard point represents an actual input
or output within the system, while a soft point results from logic and math operations applied to other
points. (Most implementations conceptually remove the distinction by making every property a “soft”
point expression, which may, in the simplest case, equal a single hard point.) Points are normally stored as
value-timestamp pairs (a value and the timestamp when it was recorded or calculated). A series of valuetimestamp pairs gives the history of that point. It’s also common to store additional metadata with tags,
such as the path to a field device or PLC register, design time comments, and alarm information.
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The SCADA system reads the measured
flow and level, and sends the setpoints to the PLCs
PLC2 compares the measured level to
the setpoint, controls the flow through
the valve to match level to setpoint
‘‘Mix-and-Match” SCADA
SCADA/HMI systems evolved in three stages, including Monolithic, Distributed, and Network SCADA
systems. Monolithic SCADA involves an independent system for single station and uses a vendor’s
proprietary communication protocols. As the number of monitoring sites increased, multiple stations were
required for monitoring and control in a Distributed SCADA system. The introduction of LAN technology
in the late 1990s provided SCADA systems with real-time monitoring capabilities. At the time, most
communication protocols were proprietary.
Cellular Networks
PLC1 compares the measured flow to
the setpoint, controls the speed pump as
required to match flow to setpoint
3
Due to a limited choice of equipment when requirements changed, open communication protocols became
increasingly popular, such as Modbus RTU and Modbus ASCII (originally both developed by Modicon) over
RS-485. By 2000, most I/O makers offered completely open interfacing such as Modbus TCP over Ethernet
and IP.
Today, Network SCADA systems, which use open system architecture, standards, and protocols, distribute
functionality across a WAN rather than a LAN. It is now easier to connect third party peripheral devices
because of the adoption of information technology. IT field protocols, such as Internet Protocol (IP), are
used for communication between the master station and communications equipment. Due to the use of
standard protocols, many Network SCADA systems are accessible from the Internet.
SCADA systems are coming in line with standard networking technologies. Ethernet and TCP/IP based
protocols are replacing the older proprietary standards. A key protocol is OPC Client/Server protocol.
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Cellular Networks
Although it allows different equipment from different vendors to communicate with each other, it does
not utilize the bi-directional and “push” technology advantages of Ethernet networks. For example, if an
intelligent device wanted to send alarms and execute front-end logic, it could take advantage of Ethernet
network communication technology used in IT. The vast majority of markets have accepted Ethernet
networks for their HMI/SCADA systems.
What does the future have in store? Experts foresee the next generation of SCADA to be a mix-and-match
system that takes advantage of XML, web service, push, and other modern web technologies.
New Push Technology from Moxa Active OPC Server
Active OPC Server Lite is a software package provided by Moxa that operates as an OPC driver for an HMI
or SCADA system. It offers seamless connection from Moxa’s ioLogik series products to SCADA systems,
including Wonderware, Citect, and iFix. Active OPC Server Lite meets the latest standard of OPC DA 3.0,
which allows connections to various kinds of devices and host OPC machines.
Communication from I/O to SCADA
There are many kinds of communication methods between SCADA/HMI software and remote devices, such
as I/O devices, RTUs, or PLCs. In the past, proprietary protocols were provided by device manufacturers who
always supplied drivers to make their devices compatible with SCADA/HMI software or provided their own
SCADA software. Today, Modbus protocol is quickly becoming the standard among hardware manufacturers
with SCADA/HMI software vendors also jumping on the wagon. Although Modbus offers compatibility
between most devices and SCADA software, not all devices support Modbus protocol. Achieving a standard
mechanism for communicating with numerous data sources, such as devices on the factory floor or a database
in a control room, is the motivation for the development of OPC. SNMP protocol is used in IT to manage
various Ethernet devices, and OPC Client/Server architecture supports SNMP. Moxa’s innovative Active
Ethernet I/O products support Modbus/TCP, OPC Client/Server, and SNMP for greater flexibility compared to
passive remote I/O.
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Passive I/O
Active Ethernet I/O
RTU
PLC
Proprietary Protocol
No
No
No
Yes
Modbus/TCP
Yes
Yes
Yes
Yes
SNMP
No
Yes
No
No
OPC Client/Server (Polling)
No
Yes
No
Needs driver
OPC Client/Server (Push)
No
Yes
No
No
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General OPC servers typically use the “poll/response,” or so-called “pull” architecture, to connect to Ethernet
I/O devices, which involves an HMI/SCADA system continuously sending out commands to collect relevant
data. Moxa’s Active OPC Server, with its non-polling architecture, supports the standard OPC protocol, but
also offers active (or “push”) communication with Moxa’s ioLogik series of Active Ethernet I/O products to HMI/
SCADA systems, providing instant I/O status reports.
Pull-based OPC Server
General OPC Server
Polls continuously
Local Network and Fixed IP
Connection only
Remote I/O
3
Active OPC Server
No polling required
Router
Internet and Dynamic IP
Connection
ioLogic
I/O Response that’s 7 Times Faster and Provides 80% off Bandwidth Usage with Event-driven Tag
Updates
Cellular Networks
Push-based Active
OPC Server
Adding additional I/O channels will tend to bog
down an HMI/SCADA system’s operation, resulting
in a longer response time , and high network
bandwidth occupation, all because of the traditional
“pull” architecture. Active tags created by Active
OPC Server Lite and ioLogik series products report
the I/O status only when it changes.This type of
event-driven tag status update results in an I/O
response time that is 7 times faster than other OPC
Server packages (using a testing environment with
2,560 I/O channels). In a different test of network
bandwidth usage, Active OPC Server Lite and
the ioLogik caused an apparent 80% reduction in
network traffic. The end result is that I/O access is
more precise, and the cost of communicating with
remote I/O devices is substantially lower, especially
when the remote site has limited bandwidth (e.g.,
satellite, microwave, and cellular communication).
At the same time, the CPU usage of the SCADA/
HMI system is also reduced by 35% with this
innovative push-based architecture, so that less
maintenance effort and lower level hardware
devices can be implemented.
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Cellular Networks
Automatic Tag Generation
Active OPC Server Lite and ioLogik series products support “Auto Tag Generation,” which eliminates the
headache of specifying target IP addresses, I/O channels, and data formats one by one, or editing and
importing configuration text files, since Active OPC Server Lite creates the tags for the target ioLogik
automatically. Simply select the channels that you need to update, and the tags are generated and
configured automatically. Generally speaking, tag generation is 50 times faster with Active OPC Server Lite
than with traditional OPC server packages. One of the biggest payoffs is that users will no longer need to be
trained to install and configure your OPC.
5 steps for channel, interface, and protocol definition: take 20 seconds.
13 steps for device, IP address, and other communication parameters: take 30 seconds.
1 step to look up address table: takes 100 seconds
General OPC Server Many queries
5 seconds to select channels and update configuration by clicking the
It takes 2.5
minutes to
create only 1 tag
button
Just 5 seconds to create 20 or more tags
Active OPC Server No queries
ioLogic
Dynamic IP/WAN Connection
Unlike the fixed IP requirements of Ethernet I/O with a traditional OPC server, Active OPC Server Lite and
ioLogik products provide the flexibility of configuring the ioLogik to use dynamic IP addresses. The ioLogik
connects directly to the Active OPC Server Lite instead of being polled, which makes dynamic IP addressing
and WAN Access to the Ethernet I/O device possible, and adds even greater flexibility by allowing
connections across firewalls. I/O devices for traditional data acquisition applications are not capable of
using this approach.
OPC Fundamentals
OPC (OLE for Process Control) is an industry standard created by the collaboration of a number of leading
worldwide automation hardware and software suppliers, working in cooperation with Microsoft. The standard
defines methods for exchanging real-time automation data between PC-based clients using Microsoft
operating systems. “The OPC Specification is a non-proprietary technical specification that defines a set of
standard interfaces based upon Microsoft’s OLE/COM/DCOM platform and .NET technology. The application
of the OPC standard interface makes possible interoperability between automation/control applications, field
systems/devices and business/office applications. Traditionally, each software or application developer was
required to write a custom interface, or server/driver, to exchange data with hardware field devices. OPC
eliminates this requirement by defining a common, high performance interface that permits this work to be
done once, and then easily reused by HMI/SCADA, control and custom applications.
OPC simplifies system integration in a heterogeneous computing environment. However, functions such as
security, batch and historical alarm, and event data access belong to the features that are addressed. OPC
interfaces can be used in many places within an application. At the lowest level they can get raw data from the
physical devices in a SCADA/HMI system, or from the SCADA/HMI system in the application. The architecture
and design makes it possible to construct an OPC Server that allows a client application to access data from
many OPC Servers provided by many different OPC vendors running on different nodes via a single object.
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General OPC Architecture and Components
The architecture of OPC leverages the advantages of the COM interface, which provides a convenient
mechanism to extend the functionality of OPC. OPC specifications always contain two sets of interfaces;
Custom Interfaces and Automation interfaces.
3
Cellular Networks
The OPC Specification specifies the COM interfaces but not the implementation. It specifies the behavior
that the interfaces are expected to provide to the client applications that use them. Like all COM
implementations, the architecture of OPC is a client-server model where the OPC Server component
provides an interface to the OPC objects and manages them. There are several unique considerations
in implementing an OPC Server. The main issue is the frequency of data transfer over non-sharable
communications paths to physical devices or other databases. Thus, we expect that OPC Servers will either
be a local or remote EXE which includes code that is responsible for efficient data collection from a physical
device or a database. An OPC client application communicates to an OPC server through the specified
custom and automation interfaces. OPC servers must implement the custom interface, and optionally may
implement the automation interface. In some cases the OPC Foundation provides a standard automation
interface wrapper. This “wrapperDLL” can be used for any vendor-specific custom-server.
OPC Servers now register with the system via Component Categories. This allows the Microsoft
ICatInformation (IID_ICatInformation) Interface on the StdComponentCatagoriesMgr (CLSID_
StdComponentCategoriesMgr) to be used to determine which OPC servers are installed on the local
machine. The problem is that this does not work for remote machines because the Component Categories
Manager is a DLL and the ICatInformation interface only works in-process. As a result, there is no easy
way for a Client (including the Foundation supplied Automation Wrappers) to obtain a list of OPC Servers
installed on a remote machine. The OPC Foundation supplied Server Browser OPCENUM.EXE can reside
on any machine, will access the local Component Categories Manager, and provide a new interface
IOPCServerList that can be marshaled and used by remote clients. This server has a published classid
(see below) and can be installed once on any machine which hosts OPC servers. The client still needs to
know the nodename of the target machine however he can now create this object remotely and use its
IOPCServerList interface to determine what types and brands of servers are available on that machine.
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Cellular Networks
OPC and DCOM: 5 Things You Need to Know
OPC technology relies on Microsoft’s COM and DCOM to exchange data between automation hardware
and software; however it can be frustrating for new users to configure DCOM properly. If you have ever
been unable to establish an OPC connection or transfer OPC data successfully, the underlying issue is likely
DCOM-related. In the following, we will discuss the steps necessary to get DCOM working properly and
securely. A simple and effective strategy to establish reliable DCOM communication involves the following
steps:
Remove Windows Security
The first step to establish DCOM communication is to disable the Windows Firewall, which is turned on by
default in Windows XP Service Pack 2 and later. The Firewall helps protect computers from unauthorized
access (usually from viruses, worms, and people with malicious or negligent intents). If the computer resides
on a safe network, there is usually little potential for damage as long as the Firewall is turned off for a short
period of time. Check with the Network Administrator to ensure it is safe to turn off the Firewall temporarily.
Set Up Mutual User Account Recognition
To enable both computers to properly recognize User Accounts, it is necessary to ensure that User
Accounts are recognized on both the OPC Client and Server computers. This includes all the User Accounts
that will require OPC access. If there are no User Accounts or Passwords already on the computers, please
add them to both computers.
Configure System-wide DCOM Settings
OPC specifications depend on Microsoft’s DCOM for the data transportation. Consequently, you must
configure DCOM settings properly. It is possible to configure the default system-wide DCOM settings, as
well for a specific OPC server. The system-wide changes affect all Windows applications that use DCOM,
including OPC application. In addition, since OPC Client applications do not have their own DCOM settings,
they are affected by changes to the default DCOM configuration. OPC communication only requires
“Connection-Oriented TCP/IP”, so add “Anonymous Logon” (required for OPCEnum) and “Everyone” to the
list of “Group or user names” in each tab.
Configure Server Specific DCOM Settings
Once the system-wide DCOM settings are properly configured, turn attention to the server-specific DCOM
settings. In the OPC-Server specific settings, only the Identity tab needs to change from the default settings.
After opening the DCOM setting windows, find the OPC Server to configure and right-click on it. Select
the Properties option in the list of objects in the right window pane. Choice the The system account
(services only). The OPC Server will take the identity of the Operating System (or System for short). This
is typically the desired setting for the OPC Server as the System Account is recognized by all computers
on the Workgroup or Domain. In addition, no one needs to be logged on the computer, so the OPC Server
can execute in an unattended environment. Disable this option if the OPC Server is not setup to execute
as a Windows Service. If this is the case, simply configure the OPC Server to execute as a service before
configuring this setting.
Restore Windows Security
Once you establish the OPC Client/Server communication, it is important to secure the computers again.
This includes (but is not limited to):
a. Turn on the Windows Firewall again. This will block all unauthorized network traffic. You will also need to
provide exceptions on two main levels:
• Application level: specify which applications are able to respond to unsolicited requests.
• Port-and-protocol level: specify that the firewall should allow or deny traffic on a specific port for either
TCP or UDP traffic.
b. Modify the Access Control Lists (ACLs) to allow and deny the required User Accounts. This can be
accomplished either through the system-wide settings of DCOMCNFG, or in the server-specific settings.
Remember that OPCEnum requires the “Anonymous Logon” access. You may wish to remove this
access. The consequence of this action will simply be that OPC Users will be unable to browse for OPC
Servers on the specific computer where Anonymous Logon access is not available. However, users will
indeed be able to properly connect to and exchange data with the OPC Server.
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We encourage you to complete your DCOM setup with this step. Integrators frequently establish OPC
communication and don’t spend the necessary time to secure the computers again. This can lead to
catastrophic results if network security is compromised due to a virus, worm, malicious intent, or simply
unauthorized “experimentation” by well-meaning coworkers.
For more detail information, please refer to the OPC Training Institute: http://www.opcti.com
Enhance OPC Capability for Cellular Communications
GPRS is a communication technology that allows data acquisition systems to overcome the difficulty of
cabling for wide area remote sites. GPRS applications are becoming more and more prevalent due to the ease
with which they can be implemented, but the dynamic IP address issues associated with GPRS networking
continue to frustrate system integrators. The fact that most GPRS devices use dynamic IP addresses can be
somewhat frustrating. What this means is that telecom service providers (commonly referred to as carriers)
often assign temporary IP addresses to their clients to access the Internet. Compared with static IP addresses,
using dynamic IP addresses make it difficult for the control centers to keep in constant contact with remote
devices.
3
The Traditional Polling Architecture of GPRS Networks
Solution 1: Public Static IP Address
The first choice is to get a public static IP address; some carriers can assign a static IP address to a specific
SIM card. This way, all the I/O devices will have their own static IP address and the entire system will
operate in the same manner as a traditional monitoring system that uses physical wiring. Perhaps the main
benefit of this solution is that it behaves the same as a wired solution. However, not all carriers offer this kind
of service, and when they do the cost is relatively high.
Cellular Networks
Traditional monitoring and alarm systems use a polling architecture that will only work properly if the host
knows the IP addresses of the I/O devices used by the system. The trouble with I/O devices with GPRS
capability is that the devices receive a different IP address every time they connect to the GPRS network.
Three distinct solutions have been developed to tackle this obstacle:
Solution 2: VPN Service Provided by Carrier/MVNO
A VPN (Virtual Private Network) is a secure LAN solution that groups specific devices together. VPN has
two major functions—security and grouping—and for the GPRS world the VPN grouping concept solves
the dynamic IP address issues. The grouping of the devices into one private network prevents unauthorized
persons from accessing the data. For this VPN solution, customers are required to buy a number of different
GPRS on-line services, and to apply for access to a Virtual Private Network (VPN). When the GPRS device
dials up, the carrier will assign a private IP address to it and because the private IP address is on the same
network segment as the host, the host and devices can maintain bi-directional communication using a
polling architecture.
Solution 3: DDNS
Using dynamic IP addresses is often necessary since many ISPs do not provide static IP addresses, or
because the cost of obtaining a static IP address is too expensive. The Dynamic Domain Name System
(DDNS) is used to convert a device’s name into a dynamic IP address so that remote devices can
communicate with the control center using a fixed domain name. When GPRS devices get an IP from the
carrier, they will automatically connect to the GPRS network. Each time a GPRS device’s built-in DDNS
client gets a new IP address, it will send the IP address to the DDNS sever. The mapping table in the DDNS
server is refreshed each time the DDNS receives a new IP address from the devices.
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Cellular Networks
New Push Architecture for GPRS Networks
Push Architecture is a mobile centric solution. Service providers such as web portals and e-mail servers
use a fixed domain name. Clients such as mobile phones get information from these service providers by
“pushing” the connection request to the Web and e-mail servers, and when a connection is established,
the communication is bi-directional. Unlike the so-called polling architecture, push technology makes
bi-directional communication possible for GPRS networks that are using either a dynamic or a static IP
address. A remote device with front-end intelligence can report its I/O status to the host and connect to the
GPRS network when it needs to. Since Moxa’s Active OPC Server supports push technology, our GPRS I/O
family of products creates a software-based gateway that makes communications easier. By using a static
IP address on the Active OPC Server, the GPRS I/O device can connect to the GPRS network and Active
OPC Server without needing to worry about the IP address issues. The topology is described below:
Compared with polling architecture, push technology not only solves the IP address issues but also reduces
network loading as well as bandwidth consumption.
Moxa’s ioLogik W5340 Active GPRS I/Os takes full advantage of all the benefits of push technology and
Active OPC Server. What Active GPRS I/O and Active OPC Server provide are:
1. SCADA Data Acquisition by OPC protocol.
2. SCADA Data Acquisition by Modbus/TCP protocol.
3. ioAdmin.exe: active GPRS I/O’s configuration software.
Alarm messages, such as e-mail and SNMP trap or user definable TCP/UDP raw packets, can all be actively
pushed to e-mail servers, SNMP trap servers, or TCP/UDP servers. SMS can be pushed from the Active
GPRS I/O to an engineer’s cellular phone.
Active OPC server is an exceptionally powerful gateway for Active GPRS I/O and plays the role of managing
IP addresses, GPRS I/O device names, data acquisition gateways, and configuration gateways. This is truly
the easiest solution for the GPRS industry to eliminate IP address and communication problems.
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Benefits of Using Active GPRS I/O
Moxa’s W5340 Active GPRS I/O devices come equipped with 4 analog inputs, 8 software configurable DI/
Os, and 2 relay outputs. In addition, the built-in GPRS communication, front-end intelligence, and data
logging function give users the advantage of a highly integrated solution. The W5340 also features a 3-in-1
serial port (RS-232/422/485) for connecting field serial devices such as instruments or meters. The benefits
of using Active GPRS I/O include:
A cost-effective solution for GPRS telemetry applications.
The best choice for solving the dynamic IP issue: whether the IP is public or private, dynamic or static.
Easy installation: with Active OPC’s support, the W5340 can push IP addresses and I/O status to
Active OPC server.
Flexible Event Handling thanks to the Click&Go logic inside.
Rich Alarm functions: SMS, SNMP trap, and e-mail.
Lower bandwidth consumption: push architecture reduces bandwidth by 80% compared to the polling
architecture.
Data Logging: Local data logged to SD card and pushed to host by TFTP.
Conclusion
Remote monitoring and alarm systems used in water distribution, pipeline management, and environmental
monitoring applications must be capable of covering a wide area and function reliably. Most importantly, the
cost must be affordable. A remote monitoring and alarm solution with Moxa’s Active GPRS I/O devices and
Active OPC Server help users overcome the frustrations associated with using dynamic IP addresses, and
makes it extremely easy to connect to SCADA systems.
Cellular Networks
Faster response time because of push technology and event handling
3
Reference:
[1] http://en.wikipedia.org/wiki/SCADA
[2] http://www.citect.com/index.php?option=com_content&view=article&id=1502&Itemid=1366
[3] How to put SCADA on the Internet, http://www.controleng.com/article/CA321065.html
[4] OPC & DCOM: 5 Things You Need to Know, www.opcti.com
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Creating Excellence Since 1987
About Moxa
For more than twenty years, industrial systems integrators
have relied on Moxa products in major device networking
installations around the world. Moxa offers industrial-grade
solutions backed by an excellent
warranty and highly-specialized technical support for a
diverse range of applications, including connecting PLCs to
a wireless control network, transmitting
temperature signals over long distances,
and automating device control monitoring
at remote locations.
Moxa Inc.
Moxa Americas
Moxa China
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info@moxa.com
Toll-free: 1-888-MOXA-USA
(1-888-669-2872)
Te l: +1-714-528-6777
Fax: +1-714-528-6778
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usa@moxa.com
Shanghai Office
Moxa Europe
Te l: +86-10-6872-3959/60/61
Fax: +86-10-6872-3958
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Te l: +49-89-3 70 03 99-0
Fax: +49-89-3 70 03 99-99
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Te l: +886-2-8919-1230
Fax: +886-2-8919-1231
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asia@moxa.com
©2009 Moxa Inc. All rights reserved.
The MOXA logo is a registered trademark of Moxa Inc. All other
logos appearing in this document are the intellectual property of
the respective company, product, or organization associated with
the logo.
P/N: 1900040901050
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Fax: +86-755-8368-4148
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Trademark Credits
The Moxa Inc. logo is a registered trademark of Moxa Inc. All
other trademarks mentioned in this document are the property of
their respective owners.