intro to cdpd for scada & telemetry

7/28/2019 Intro to CDPD for SCADA & Telemetry

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February 2002

AirLink Communications, Inc.


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Intro to CDPD for SCADA & Telemetry

AirLink Communications Page 2 www.airlink.com

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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.

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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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