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7/28/2019 Intro to CDPD for SCADA & Telemetry
1/9
February 2002
AirLink Communications, Inc.
7/28/2019 Intro to CDPD for SCADA & Telemetry
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WHAT IS CDPD?
Cellular Digital Packet Data (CDPD) is an open IP-based standard for transmission of data
over cellular communications. It is an efficient, packet-switched digital wireless technology
that sends data over dedicated cellular network channels. Charges are based upon theamount of data sent to and from the cell tower and there is a small monthly fee to maintain
an active IP address. There is no charge to create the connection or to keep it open, only for
the data that moves across the connection.
CDPD carriers provide three alternatives for connecting a SCADA System to remote devices:
1. Using a Frame Relay connection into the CDPD Network
2. Using the public Internet
3. Using another CDPD Modem connected to the Host System;
WHAT IS TELEMETRY?
Telemetry is an automated communications process by which data is collected from
instruments located at remote or inaccessible points and transmitted to receiving equipment
for measurement, monitoring, display, and recording. Transmission of the information may
be over physical pairs of wires, telecommunication circuits, wireless radios or satellite.
WHAT IS SCADA?
SCADA (Supervisory Control And Data Acquisition) is an industrial measurement and control
system consisting of a central host (or Master) and one or more remote stations controlled by
standard and/or custom software. SCADA systems are similar to Distributed Control Systems
(DCS), which are LAN based control systems routinely seen in factories, treatment plantsetc.
SCADA systems often cover larger geographic areas, and rely on a variety of communications
systems. SCADA systems use a sophisticated database, provide graphing and reporting
functions, offer an interface to operate equipment and have software initiated alarms to alert
control engineers to specific conditions.
Some examples of Telemetry and SCADA applications include Water Supply Systems, Waste
Water Treatment Systems, Sewer and Drainage Runoff Systems, Natural Gas Transmission
and Supply, Electrical Power Transmission and Distribution, Weather Monitoring, Pipeline
Monitoring, Structural Integrity Monitors, and Transportation System Management.
RReemmoottee UUnniittssSSCCAADDAA HHoosstt
Frame
Relay
3
1 2
Internet
LocalCDPD
Network
Other
CDPDNetworks
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WHAT IS A PROTOCOL?
A protocol is an agreed-upon format for transmitting data between devices. Protocols used in
SCADA or Telemetry networks determine the essential communication elements, such as:
Network node address formats
How the sending device will indicate that it has started or finished sending a message
How the receiving device will indicate that it has received a message The data compression method used (if any) The type of error checking the devices will use
Protocols can be implemented in software and/or hardware. There are a variety of proprietary
and open-standard protocols to choose from, each having their relative advantages and
disadvantages. In the world of wireless communications (like CDPD), the protocols are
usually based on serial communications. Examples of such protocols include the following:
WHAT IS THE DIFFERENCE BETWEEN AN RTU, PLC AND DATA LOGGER?
Short for Remote Terminal Unit, an RTU is a device installed at a remote location that
collects data, codes it into a specified format and transmits that data back to a central station
(or Master). RTUs also collect information and initiate processes directed by the master. RTUs
are equipped with input channels for sensing or metering, output channels for control,
indication or alarms, and a serial communications port.
A PLC (programmable logic
controller) is a small industrial
computer originally designed toreplace relay logic. It has inputs
and outputs similar to an RTU,
but also contains a program
which will scan its inputs, and
takes actions based on certain
values. PLCs originally had no
communications capabilities, but
as they began to be used in
situations where communications
was necessary, modules for serial
and IP communications started to
emerge.
A DATA LOGGER is a device
designed for reliable, long-term,
unattended monitoring in typically harsh environments. It uses minimal power to measure
temperature, pressure, force or position using commonly available sensors, and stores the
data into memory or on a PCMCIA Card for retrieval via a PC or handheld terminal. Data
loggers are very easy to connect, and often use very simple protocols to communicate.
BSAP (Bristol Babcock)
DF1 (Allen Bradley)
DNP
Fisher (ROC & FloBoss)
Mercury (Mercury)
Modbus (ASCII or RTU) Opto-22
Sixnet
SNP (GE Fanuc)
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The most basic Telemetry applications involve acquiring data from a remote device that is
designed to log a small number of analog or digital inputs for retrieval by a PC or handheld.
These devices are typically very simple in nature, and dont have significant processing
capabilities. Two means of communicating to these devices over CDPD are detailed here.
UNSOLICITED REPORTING
This application involves a REMOTE device that is set up to send data to a HOST server on a
continuing basis. This is often the case with remote devices that dont have a communications
protocol, and can only stream data out of a serial interface. By setting the CDPD Modem with
a pre-configured destination HOST, streaming data is sent without intervention or polling.
This is easy to configure, but requires that the HOST have the ability to manage the incoming
data. Data will look the same as it would if the HOST were connected via a local serial cable.
SOLICITED REPORTING (TERMINAL/TELNET AND SERIAL POLLING BASED CONNECTIONS)
Many devices have an internal menu driven data collection mechanism accessible using
terminal emulation programs such as HyperTerminal. Others have vendor provided softwarethat communicates via a direct serial interface. For Terminal or Telnet applications, the Raven
CDPD Modem is setup as a remote Telnet Server. Opening a Telnet session over an IP
network directly to the CDPD modems IP will provide the Host with the same character
interface seen over a direct serial link. Other serial applications will require a Host Raven or
COM Redirector. These are covered in Section 3 underDIRECT SERIAL POLLING TO CDPD MODEMS.
RReemmoottee CCoonnnneeccttiioonnStreaming data is sent from a remote CDPD modem direct to a Host CDPD Modem, which passes serial data to PC/Server.
RReemmoottee DDeevviiccee
The device is connected serially to the CDPD Modem.The CDPD Modem will appear to be a PC on the RS232 port.
HHoosstt AApppplliiccaatt iioonnssTerminal, Telnet, or Server Scripts open a terminal session directly to the CDPD Modem.
#105000000x
CDPDNetwork
#105000000x
F-Relayor
Internet
CDPD
Network RS-232
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CCDDPPDD DDiiaalliinngg uussiinngg aa HHOOSSTT RRaavveennSCADA system Dials remote devices using a Raven as a replacement for a landline modem.
33.. AAddvvaanncceedd SSCCAADDAA &&TTeelleemmeettrryy AApppplliiccaattiioonnss
Advanced applications involve a central facility communicating to remote devices using
disparate communications, including circuit-switched (Dial-up) connections, leased-lines,
and/or TCP/IP networks. This section will demonstrate how CDPD can fit into these unique
environments. We will break this down into (2) main areas (with specific subsets): CIRCUIT-SWITCHED EMULATION:
Dialing IP Addresses Using a Host CDPD Modem or COM Redirectors
DIRECT-SERIAL POLLING :
Using a Host CDPD Modem, SCADA IP Driver, COM Redirectors and Gateways
CIRCUIT-SWITCHED EMULATION: DIALING USING A HOST CDPD MODEM
Many SCADA networks dial out to RTUs that are connected to modems and telephone lines.
CDPD can be used instead of telephone lines by placing an AirLink Raven modem at the host
and one at each RTU. The AirLink Raven is configured so that remote devices can be called
by the HOST software. By utilizing AT Dial Commands and the remote IP address, (ie:
ATDT123.123.123.123), the HOST CDPD modem creates a pinned-up data connection.
Standard asynchronous serial protocols are used just as if the system were dialing units
connected to regular dial-up phone lines.
CIRCUIT-SWITCHED EMULATION: Dialing USING A COM REDIRECTOR
COM Redirectors enable applications that expect to communicate with a modem on a COM
port to transport their data over TCP/IP networks instead. Redirectors can dial remote
TCP/IP addresses as well as answer incoming connections. COM/IP is one example of a
COM Redirector. During installation, COM/IP creates virtual COM ports along with virtual
modems. These modems appear as Hayes-compatible modems to Windows software
programs. Instead of dialing via phone lines, the virtual modems dial the CDPD IP
addresses. Using a COM redirector allows you to continue using applications that require
modems, while gaining the performance and cost benefits of an IP based network connection.
ATDT123.123.123.123
123.123.123.123
CDPD
Network
F-Relayor
Internet
CDPD
Network
Modem provides RING and CONNECTmessages as well as DCD, DSR, CTS & RTS
CCDDPPDD DDiiaalliinngg uussiinngg aa CCOOMM RReeddiirreeccttoorrSCADA system Dials remote devices through a virtual Com port and virtual modem.
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Screenshot of Wireless ACE modem configuration software
DIRECT SERIAL POLLING TO CDPD MODEMS
Some applications do not have dialing capabilities, but instead send a poll or message
directly over a dedicated communications link. For example, when a Master Polling Station
(PC) or Data Concentrator polls remotes connected to a multi-drop network, private radio
network, or Telco leased lines, the Master sends messages (Modbus, BSAP, DF1, etc.)
assuming that the device is listening on the network. Remotes look at each message to see
whether they are the intended recipient and reply as needed. No dialing ever takes place.
To enable these systems to work with CDPD, the SCADA protocol messages need to be
converted to TCP/IP packets. This p r o t o c o l c o n v e r s io n or IP encapsulation takes place at
the Host and Remote ends, and is transparent to the Master and remote devices. Each
message is wrapped in an IP packet, sent to a specific CDPD modem, unwrapped, and the
original message is passed to the remote over an RS232 connection. This encapsulation can
be done in one of (3) ways:
Using a HOST Raven CDPD Modem
Through a serial-to-IP driver supplied by the SCADA application vendor, or
Using a third-party COM redirector application such as COM/IP.
All three methods have their respective benefits. We will examine each in detail.
USING A HOST RAVEN CDPD MODEM
Since SCADA messages are broadcast to
groups of remotes, messages contain a
remote unit ID. The remote whose unit ID
matches the message will be the only one
to respond. When using a Raven CDPD
Modem at the Host, the remote unit ID is
matched against a lookup-table of
remote IP addresses maintained inside
the Host Raven. (Shown at right). Each
message is then assembled into a UDP
packet and sent to that IP Address.
Remote modems are set to listen for
these packets, unwrap them, and pass
the data over the serial link to the remote
device. Return messages are sent to the
same Host IP.
RTU-1 = IP x.x.x.1RTU-2 = IP x.x.x.2RTU-3 = IP x.x.x.3RTU-4 = IP x.x.x.4 IP x.x.x.2
IP x.x.x.1
TThhee HHOOSSTT RRaavveennA Raven uses an internal Lookup Table to decide where to route messages.
Software sends messages as if remotes were on a serial or radio network.
The remote device receives only theSCADA messages meant for that ID.
CDPDNetwork
Lookup-table looks at eachmessage to find Unit ID.
Easy to maintain lookup-tableholds up to 60 modem entries.
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Screenshot of COM/IP showing serial activity & IP connection
SCADA SYSTEM SERIAL-TO-IP ENCAPSULATION DRIVERS
Serial-to-IP Encapsulation drivers are sometimes available from SCADA System vendors.
They may include an I/O driver as part of their application software. A good example is
Bristol Babcocks Open BSI, which includes the ability to assign CDPD Modem IP addresses to
remote unit profiles. While the unit may already have a BSAP Unit ID, (BSAP is Bristol
Babcocks serial-based messaging protocol), it also has an accompanying IP Address.
Another option is to acquire a driver or OPC server from a third-party developer, such as
Software Toolbox. These third party tools perform exactly the same as SCADA I/O drivers,
with all messages being sent to remote units via an IP based LAN connection to the CDPD
network. CDPD modems attached to the remotes work the same as in the previous scenario.
SENDING SCADA PROTOCOL MESSAGES USING A COM REDIRECTOR AT THE HOST
As previously described, COM
Redirectors send serial data on TCP/IP
networks. With COM/IP, we described
the softwares ability to create virtual
COM ports and modems. COM/IP can
actually create up to 256 virtual COM
ports on one single Windows machine!
That means that if your total numberof remote devices is 256 or less, you
can assign each remote device its own
separate COM port in your SCADA
system. By configuring each COM port
with its own unique destination CDPD
modem, all messages are separated
from each other and you actually have
256 dedicated packet-data circuits.
SSCCAADDAA SSyysstteemm DDrriivveerrThe SCADA system driver encapsulates the protocol into IP packets for transport
over TCP/IP networks right inside the software. No external devices are used.
F-Relay
orInternet
CDPD
Network
CDPD
Network
F-Relay
orInternet
IP Address on right means that a
TCP Connection has been made
LEDs on left show activity on COM2
NOTE:
Much like the Host Raven, COM/IP will wrap each serial message into an IP packet. However, COM/IP doesnot support UDP, only TCP. While TCP is a robust protocol, it may be unnecessary given that most serial
protocols are already very robust. TCP also has packet-size and session overhead you may want to avoid.The use of UDP vs. TCP for data transfer is something you should always consider when designing a solution.
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Gateways
Gateways facilitate data acquisition by acting as the communications front-end, accepting
multiple data streams to and from remote devices. Gateways accept message packets from
multiple serial interfaces and pass the data into a PAD (IP Packet-Assembler/Dis-assembler)
for transport onto a TCP/IP network. Gateways are often sophisticated systems that can be
tuned to interface with multiple protocols, manage large amounts of traffic, provide
assistance with message timing issues, and keep detailed communication logs.
THE AIRLINK TELEMETRY GATEWAY
The AirLink Telemetry Gateway
(ATG) provides a gateway andinterface between legacy SCADA
systems and Frame Relay or Internet
connections to Wireless IP networks,
such as CDPD. The AirLink Telemetry
Gateway allows customers to
seamlessly collect data from
hundreds of remote devices through
a single IP interface, reducing
network communications
complexities and enhancing SCADA
system performance.
ATG is a complete hardware andsoftware solution that interfaces
directly to the serial interface on the
legacy SCADA system while also
residing as a node on the IP based
LAN. ATG can accept up to 18 RS-
232 connections from a single
SCADA Master (or concentrator),
and manage those connections while
simultaneously providing the SCADA
protocol conversion and data
encapsulation for transport onto the
IP network.
Some of the features of ATG include:
Intuitive User Interface
Easy to Configure and Manage
Support for any serial protocol
Support for timing variances
System Performance Analyzer
Detailed Communications Log
F-Relayor
Internet
CDPDNetwork
AirLink Telemetry Gateway
COM Port configuration screen
Protocol Configuration Screen
(Showing Modbus RTU Config)
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EXPERIENCE
Since 1993, AirLink has been a recognized leader in the wireless data industry. Our founders
participated in the development of CDPD, and have focused on developing data collection and
control solutions for carriers, public utilities and transportation agencies. Our customer list isa whos-who of industry leaders, and theyre using our products for everything from
monitoring oil wells in Oklahoma to tracking tour bus fleets in Hawaii. You can find our
products attached to valves, pumps, switches, flow computers, data loggers, traffic counters,
message signs or tracking ambulances and tow-trucks. Anywhere theres a need for instant
wireless information.
PROVEN SOLUTIONS
From RTUs and PLCs to Data Loggers and SCADA software, if you use it, weve probably
integrated with it. To give you an idea of what that means, we will let the logos speak forthemselves. Here are a few you may recognize
PRE AND POST SALES ENGINEERING SUPPORT
AirLink always provides you with the exact information you need to integrate CDPD into your
solution. Our Product Application Notes show you step-by-step how to set up your hardware,
cabling, and CDPD modem configurations, with pictures and diagrams whenever possible.
AirLinks personalized pre-sales engineering and post sales technical support is beyond
compare. Our Field Engineers understand wireless communications, and know how to make
things work. They come from all aspects of the automation, utility, and communication
industries, and have walked in your shoes.
So, if after reading this, you think youre ready to try CDPD, then were ready to help.
Visit us on the web at www.airlink.com or call us at 510-226-4200.
AirLink Communications
We Make Wireless Data Work
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