CELLULAR DIGITAL PACKET DATA TECHNOLOGY
FOR SCADA AND TELEMETRY APPLICATIONS
Twin Cities Technologies
P.O. Box 61927, Midland, TX 79711
BACKGROUND In 1992, eight cellular service providers came together
to develop specifications that would become the
The introduction of Cellular Digital Packet Data standard for the transmission of digital packet data over
technology (CDPD) came about after the build out of the the cellular network. These companies were: Bell Atlantic
cellular voice systems in the United States. The designers Mobile Systems Inc., GTE, Mobilnet Inc., Ameritech
of those systems realized a part of that technology was Mobile Communications Inc., Southwestern Bell Mobile
not being used that could send short burst packets of Systems, PacTel Cellular, NYNEX Mobile
data without interfering with the voice transmissions. The Communications, Contel Cellular Inc., and McCaw
definition of CDPD would be: “CDPD is a technology Cellular Communications Inc. By the end of 1996, the
intended to provide packet data networking services technology had been installed in more than 60% of the
to mobile hosts. The method used consists of radio Metropolitan Services Areas of the United States and
channels typically used for Advanced Mobile Phone covered almost one half of the population. CDPD is not
Systems (AMPS) cellular service.” built out in all areas of the United States and it was not
connected in a complete networking arrangement as of
Note the preceding paragraph where it defined the May 1999. The needs of the end user will dictate the
development of CDPD as intended to provide packet complete build out in the more rural areas of the country.
data for “mobile” hosts. Please do not be confused by CDPD is designed for bursts of data from 3K to 5K. It is
this as our discussion within this paper is mainly intended IP addressable thus making it easily transferable over
for fixed remote RTU communications. When CDPD was the Internet. The data sessions are very short, from 1 to
originally deployed, it was to offer the mobile cellular 2 seconds, compared to 20 to 30 seconds for a cellular
user the ability to send data via his or her laptop or other switched dial up connection.
data device over the cellular overhead more quickly, and
without interrupting the user’s use of his or her cellular Since CDPD is designed to support routing of unaltered
telephone. You might be needing a mobile data for your Internet Protocol (IP) packets, it allows the use of
needs, too. CDPD can do both. Transmission Control Protocol/Internet Protocol
(TCP/IP) as the method of forming and handling the
As CDPD become more built out, SCADA designers and packets across the network. The use of this standard
end users became interested to see if this technology protocol has greatly enhanced the usability of CDPD.
could be an option for the fixed remote terminal units Within TCP/IP, there are two protocols that can be used.
which had cellular communication capability or RS-232 They are the Transmission Control Protocol (TCP) and
ports to which a modem could be installed. Many had the User Datagram Protocol (UDP). UDP has been
been using switched cellular devices already. Switched found to be less reliable in delivering the packet to the
cellular data sending took more time and the cost was destination. However, as UDP does not generate the
higher. CDPD used short burst messaging. This short extra packets that help for a reliable packet delivery, as
burst messaging refers to the packet data method of does TCP, it is less costly, as many billing systems bill
sending information. Packet data contains not only the for every packet sent, including the extra packets
desired data that the user wants to send, but it also generated by the TCP method. To get around this, there
includes the information imbedded that is necessary to are middleware packages that will filter out packets
deliver the packet to a destination. The packets are generated by TCP that do not need to go out over the
routed according to the information in the packet to a airlink to help in controlling the billing cost when TCP is
given destination. CDPD allows for the packets to move the choice of protocols.
from one cell site to another in a given network.
Transmissions are completed during the idle time WHEN CONSIDERING THE DATA COMMUNICATION
between voice transmissions on a cellular network in METHODS AVAILABLE FOR SCADA APPLICATIONS,
place in the geographical area. Studies completed by WHEN WOULD YOU SELECT CDPD AS THAT
the system designers determined that during the heaviest METHOD? YOU MIGHT CONSIDER CDPD FOR A
voice uses, 30% or more of the air time of the frequency SCADA IF:
was idle. This would be more than enough time to (1)The end devices are scattered over a wide geographic
establish a link to modem, send the data, then switch to area.
another frequency. (2)You do not have a license for a licensed data radio
system and have little chance of getting one.
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(3) The messages are fairly short compared to a the slaves? Considerations would be to find out if the
continuous polling SCADA system with long messages. protocol is proprietary, Modbus, Opto22, BSAP, or
(4) You are not required to have as a continuous link with others?
all the remote sites 100% of the duty time. C. Are there timing considerations built into the host
master site? Is the communication device on the
CONSIDERATIONS remote RTU supposed to reply in a given time frame?
If so, what are those timing parameters and are they
Many end users find it an invaluable decision to seek out configurable?
a communication specialist from the very beginning of
their SCADA plan when CDPD is being considered. While (3) HOST HARDWARE AND SOFTWARE QUESTIONS
getting a modem installed in your home PC and then
getting it on the Internet or for e-mail, may not be a task, A. What type of machine and operating system is the
getting a remote CDPD modem can be. Unless you, or host software running on?
others in your company, have a fairly detailed working B. Can the host software communicate with the remote
knowledge of your master SCADA software, the hardware devices using TCP/IP?
com port configuration, and the remote communication C. Can the host software communicate using a modem
capability of the field devices, it might serve your long connected to the host hardware serial port, and, if so,
term frustration to call in outside consultants. Call area can it use standard AT commands?
radio shops to see if they are doing telemetry integration. D. Can the host software AT command strings be
If they are, they will know exactly what you need. If they altered? Can the phone number in the ATD command
do not support all area of telemetry communication, keep be replaced by an IP Address?
searching. Now then, on to some specific considerations. E. Does the host polling application have its own protocol
to ensure that all the data is received in the right order?
There are several areas that need to be reviewed when F. What is the number of typical bytes received per
considering CDPD. These would include: response and what would be the maximum amount
that might be needed for the SCADA information
(1) AVAILABILITY OF CDPD IN YOUR AREA OF needed?
CONCERN G. How often does the host poll each remote?
H. Does the polling application use or require hardware
Do not assume that just because your cellular phone flow control?
works in area in which you want to install a SCADA I. Does the polling application use DTR to control each
communication system, that CDPD is installed. There poll?
are two types of carrier modes as well. There are A and J. Does the polling application use RTS to control radio
B carriers and the two cannot talk directly to each other transmission?
to create a continuous network. Therefore, once you
believe you meet the requirements of short data (4) REMOTE SCADA HARDWARE AND SOFTWARE
messages, etc., contact your area cellular provider and QUESTIONS
ask if they have CDPD available in the areas you need
SCADA communication. A. Does the manufacturer of the RTU being considered
supply a communication port RS-232 connectivity?
Does that carrier provide either the A or B carrier If so, do you have the pin layout?
mode in all the area you need CDPD and are they B. Does the RTU use flow control? (Usually not needed
networked back to where you need a master with very small responses)
receiving CDPD modem installed? C. Does the RTU require DCD or DSR to signal an
For more information you might want to contact the D. Does the RTU originate messages? For instance, can
Wireless Data Forum at http: //www.wirelessdata.org. the RTU report exceptions, alarms, etc?
E. Does the RTU run off AC power? Solar panel? Other?
Now that you have been told that CDPD is available, or F. Does the RTU issue any specific modem setup string?
that the provider will make it available for your needs, If so, what are the AT commands?
what is next? G. Will the CDPD modem have room for internal mounting
in the RTU enclosure?
(2) COMMUNICATION QUESTIONS THAT NEED TO H. Is Class 1 Div 2 required?
GETTING STARTED WITH THE PROVIDER
A. How can the host software communicate with the
slave RTU’s in the field? Can the host software The first step you must take in getting set up for operation
is to contact your CDPD Service Provider in your area.
communicate via dial up modem, leased lines, private
As noted, it is normally your cellular phone provider. From
radio (data radios), etc? Is a 232 port available?
B. What are the communication protocols available within them you will obtain your IP Address for that specific
modem. Every remote and master modem must have a
the host software that will poll or receive data from
specific IP Address. To get an IP Address you must
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provide them the modem’s ID (EID) which is on the label were a hardwire. The call out is discontinued and the
of the modem you have purchased. This IP Address is a action is over. A few things come into play here.
valid Internet address and is matched up with your unique
EID. It would be good to request an IP Address that can RSSI
be used as a target for PINGS. You will use PINGS to
test to see if you are communicating with your modems. When you install the CDPD modem the Received Signal
Strength is vitally important. It ensures a good handshake
SETTING UP YOUR MODEM or communication path between the modem and the
CDPD network. It is given in minus terms with a (-113) to
Now you must configure your modem to the IP Address (- 100) being considered as Off and a RSSI of a (-69) as
and side preferences (A side carrier or B side carrier) being Solid On. Testing the RSSI when a modem is
provided by the Carrier. To do this you must connect installed, or better yet, testing the RSSI in the
your modem to a PC, normally with a DB-9 male geographical area you are considering, is vital. Most
connector for the modem end and a DB-9 or DB-25 for CDPD modems pull about 600 mW when transmitting,
the PC. Use a straight through wiring connection, not a are full duplex with integrated TCP/IP, use serial protocols
null modem. From this point you will follow the (AT Commands, SLIP and PPP), work from 9 VDC to 30
configuration methods provided by the modem VDC, use an Input Current from .10 to .54 amps and
manufacturer you have selected. transmit at .34 amp and receive at .14 amps. They
typically use serial interfaces of either a RS-232 or a DB-
COSTS 9F connections. They transmit and receive from 824-
849 MHz and from 869-894 MHz. They can operate
HARDWARE Windows 95/98/NT Dial-Up Networking.
CDPD modems have a retail cost from $400.00 to DATA RATES
$700.00. Add to that a power supply (when not
connected to the power supply of the RTU), an antenna CDPD can easily operate at 9600 baud and may transmit
and some cable. A user can normally install a CDPD at even higher speeds. Some manufacturers advertise
modem for a total cost of from $550.00 to $850.00 per rates from 300 - 19,200 bps.
site plus a couple hours of labor if a licensed radio shop
handles the installation and start up. SYSTEM DESIGN FROM THE CELLULAR CARRIER
TO THE END USER
Different cellular carriers or resellers have their own
developed use fees that they offer. This could be from Cellular carriers, who are offering CDPD in their working
one low flat fee per site (From $15.00 to $20.00 per areas, are adhering to some agreed upon concepts
month), up to a fixed fee per the first 100k used (Example: drawn up by the originators of the CDPD technology.
$12.00 first 100k) and then a fee (Example: $3.00 to $5.00 The key area of the CDPD specifications include:
per every 100k after that). There is not a national standard • Compatibility: CDPD will be integrated into
of charge methods being used. I would suggest current networks, minimizing the amount of
negotiating the use fee for your area based on the number modifications to the system.
of sites and amount of data you will be sending. • Deployment: CDPD networks will be completed
in phases to include areas in connectivity,
We have seen cases where a cellular carrier in a specific security and accounting, network management,
area might try to charge a set up or IP fee, but this is not and application services.
the norm and should be discussed if the setup person • Open architecture: CDPD will be designed so
you call to get an IP address mentions it. that any vendor can develop CDPD, and it will
not be dependent upon proprietary
OPERATION developments. Vendors can stay loyal to the
uniform design, however they still can offer
You have now installed a CDPD modem at the master different features or options to sell their product.
PC in which your polling software has been installed. • Security: The CDPD network is designed so as
You have a method to watch the polls as they occur. to protect the identity of the subscriber and to
When the request for data is transmitted by the master protect that subscriber’s data from casual
software you will see an initiation string and the IP eavesdropping.
address come up. A call up link from the CDPD modem • Mobile users: CDPD will provide the services and
is begun and a handshake into the cellular network facilities necessary to support mobile use. As
begins. If all is working right, within a few seconds, a noted, we now know that CDPD is a workable
handshake occurs with the polled remote and the data option for fixed remote site data communications
is seamlessly transferred over the modem lines as if it as well.
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HARDWARE SYSTEM DESIGN transmit the polled data back to a Master PC many miles
from the SCADA system being polled. This is especially
The end user is defined as the SUBSCRIBER. The useful when the radio SCADA system does not have a
subscriber provides two hardware items: (1) A Fixed path to the Master PC software at an office or the miles
Master Receiving site, called the F-ES, which stand for are just too great for a radio system. Western Gas
FIXED END SYSTEM, where one CDPD modem is Resources Inc.’s, Phil Rutledge, in the Midkiff area of
installed as a master, usually to a PC where the host West Texas, has tested the use of this Master Spread
SCADA software is installed, and (2) A remote CDPD Spectrum to CDPD modem installation with the
modem either fixed at an RTU site for SCADA TOTALFLOW EFM’S and found that 35 days of data
applications, or one of the mobile modems used for those could be easily polled within 40 seconds. Real time data
applications. This piece of hardware, or remote CDPD was polled within just a few seconds. Mr. Rutledge, as
modem, is called the MOBILE END SYSTEM, OR M-ES. well as Enron’s Jerry Ball, are among those pioneering
this technology in order to gather data from SCADA
The cellular carrier for any geographical area is referred systems many counties away from the Master PC while
to as the SERVICE PROVIDER. As a service provider, seeking means to control costs at the same time. Many
the cellular carrier is responsible for the system hardware of the major and independent oil and gas operators have
to send and receive the packets of data and for the had successful data gathering systems utilizing CDPD,
hardware that will keep track of the location of the modem but remember this all came from many hours of planning
sending the data. The Service Provider provides three and working on understanding what could and could
basic components of the system: (1) A MOBILE DATA not be accomplished and many more hours in the field
BASE STATION (MDBS). This is the part of the system getting the systems set up correctly.
that sends and receives the packets of data over the
cellular frequencies. (2) MOBILE DATA-INTERMEDIATE SUMMARY
SYSTEM (MD-IS). This part of the system does the
tracking of the remote modem in use. When a mobile The use of Cellular Digital Packet Data will continue to
CDPD is in use, this part of the system is responsible for expand with the needs and the willingness of the
handing the modem off to another MD-IS where the providers to complete the build out and keep the use
mobile CDPD modem is now in use. (3) INTERMEDIATE cost down. More work needs to be completed to widen
SYSTEM (IS). This part of the system is supposed to the network operability and to work on a more unified
work primarily as a network relay, a function usually means of transmitting data from the A to the B carrier
performed by network routers. It is the CDPD modem’s side at a cost that is also affordable before wide area
interface to the outside world. Review (Figure 1.) CDPD SCADA systems will be the norm. The data
carriers will have to place the same emphasis on the
CDPD ARCHITECTURE quality of the CDPD network as they do the voice network
and still make the cost at a level that the user can afford
M-ES REMOTE OR MOBILE for a large RTU system. All of this might not happen
CDPD MODEM before other technologies (such as the LEO, or Low Earth
Orbit satellites) catch up in build out operation and lower
the use cost for the data being polled. Users are looking
MDBS MD-IS Tracks for the most efficient and cost effective means for data
Sends locations of transmission. Our goal, as SCADA integrators, is to
Receives remote modems continue to look for means to provide that.
Finally, even though the setup of a CDPD seems fairly
straight forward, we have found that the actual setting
F-ES MASTER up working of the modem with a given RTU back to a
CDPD MODEM IS Router
Master still requires a person with a RF background. It
might be useful in the beginning of your proposed SCADA
FIGURE 1. system integration to plan on the use of a qualified
communications shop indicating specific telemetry
experience that will follow the project from beginning to
OTHER CDPD APPLICATIONS end and offer field service support for the long term
efficiency of the system.
Recently we have successfully tested the use of a CDPD
modem connected by a null modem cable to a Master Bob Halford
Spread Spectrum 900 MHz data radio. We have found Twin Cities Technologies
that we can design a spread spectrum radio system to PO 61927
gather data from a multiple RTU system at one Master Midland, TX 79711
Site and then use the CDPD modem connected to the (915) 561-8449
spread spectrum master data radio to seamlessly email@example.com
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AirLink Communications CDPD Users Manual December
Uniden DATA Modems Users Manual 1997
Salamone, Salavtore, December 1995 Untangling
Wireless, BYTE Magazine Volume 20, Number 12
Using Cellular Digitized Packet Data (CDPD) In a SCADA
System by Dean Gordonwood Kimray, Inc.
Gallant, John, October 13, 1994 The CDPD Network
Phil Rutledge Western Gas Resources Midkiff, Texas
(915) 535-2485 Ext. 3001
Jerry Ball Enron O&G Midland, Texas (915) 686-3715
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