Cellular SCADA Approach

One of the major costs associated with SCADA is implementing data communications. This process can entail dial-up telephone service, leased telephone lines, private radio systems or satellite services.  A new, more affordable water/wastewater industry approach is to use cellular service providers.

Affordable

Traditional communication system charges can vary significantly, both initially and long term.  Gannett Fleming has found that CDPD service is not only cost effective for numerous applications, but meets common design criteria.  Although required hardware (CDPD modems, antennas, and appurtenances) costs slightly more than leased telephone line modems, the monthly service fee is typically much less (at $40/month).

Technology

CDPD uses existing cellular channels (824-894 MHz).  It sends data in packets (such as the Internet), rather than establishing a circuit (such as telephone lines).  This is called a “packet-switched” network, whereby telephone lines are typically “circuit-switched."  Devices which communicate on a packet-switched network send small bursts of data, which may be mixed with packets from other devices using the same communications media.  Packets are sorted based on the destination address attached to each packet.

Modems

CDPD modems are devices that interface serial communications from programmable logic controllers or computers to the CDPD network.  This is accomplished by encapsulating serial data in packets.  The modems are designed to read a serial data protocol and decipher the destination address.  This destination address is then associated with an internet protocal address of another CDPD modem.  Next, the data is unpacked and transmitted to the serial device.  To the host and remote device, this appears to be a direct serial cable connection.

Coverage

CDPD service is only an option in certain geographic areas (providers include AT&T Wireless and Verizon Wireless.)  If CDPD coverage is available, each site should be tested to verify that signal strength is adequate for reliable communications.

Assistance

To set up on-site testing, to see if CDPD coverage is available in your area or to learn more about this unique technology, contact SCADA expert Joe Bower.