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Achieving secure, reliable communications with the DNP3 Protocol

In our previous article, Making the Case for DNP3 in Upstream Oil & Gas, we discussed the growing adoption of the DNP3 protocol in the oil and gas automation market and how this adoption will benefit end users of automation equipment, particularly RTUs and SCADA. The focus of the article was on the opportunity to reduce the costs and risks associated with integrating equipment and software from multiple vendors. We will now take a closer look at how this interoperability is achieved.

DNP3 is an open, public protocol and is fully documented in the latest revision of the standard (IEEE Std 1815™-2012). It was designed from the outset to meet the requirements of the automation industry where a ‘wide area’ communications protocol was needed that could efficiently transfer large amounts of data reliably over long distances and a poor quality communications infrastructure, primarily between ‘outstation’ devices (field equipment such as RTUs and PLCs) and ‘master’ stations (SCADA servers).

The DNP3 protocol implements three distinct layers of the standard OSI 7-layer Model:

  1. Data Link: Error checking, link control, prioritization, addressing, and fragmentation management
  1. Transport: Framing, sequencing management of connections, and reliability
  1. Application: Data formats, function codes, and the definition of supported operations.

Physically, the DNP3 protocol has been implemented over the most common media used within the automation industries; initially, RS232/485 over radio, copper or fiber, and more recently over Ethernet and the wide range of physical media that it represents. As such, the adoption of DNP3 will rarely require changes to physical infrastructure.

The DNP3 protocol supports a wide range of standard data formats and permits the ‘outstation’ and ‘master’ to select those that work best for the application at hand. For example, an analog value can be transferred in any one of multiple data formats, varying from a simple, unsigned integer to a 64-bit high-resolution, floating-point value. The different formats, and the way to request them, are fully defined in the standard, and outstation devices are self-describing (using an XML profile and ‘Class 0’ request) to determine the data and functionality that is available. This information can be used by the DNP3 master to automatically configure communications, with just the final selection of required values, formats and frequencies to be determined by the user. Compare this scenario to Modbus where configuration of a link is often a manual, parameter-by-parameter (or byte-by-byte) investigation of what is required and available.

Data can be transferred by traditional, scheduled ‘polling’ for current values but is normally transferred using ‘DNP3 events’ where the outstation is polled. Only the data that has changed is transferred together with a timestamp indicating (to the millisecond) when the data changed. During interruption of communications, these events can be buffered and then transmitted once the link is re-established, ensuring no data is lost.

This approach achieves the reliability, precision and accuracy of reporting that can only be dreamed of with legacy polling protocols over high latency networks. A similar mechanism can be used to timestamp and log the historical data that is especially critical to electronic flow measurement (EFM) in the oil and gas industry. This ability to associate RTU field measurements with millisecond resolution timestamps, even over relatively low bandwidth networks, opens up a wide range of applications that can benefit from highly accurate, timestamped data streams. For example, leak detection and cathodic protection are a few applications that typically require far higher performance networks.

For the most exacting requirements, DNP3 events can be transmitted from outstation to master by exception, reducing bandwidth requirements even further.

Extensibility is yet another advantage of the DNP3 protocol that benefits both users and vendors. As a simple example, Emerson’s FB1000 and FB2000 Series Flow Computers and FB3000 RTU include a range of standard files for the transfer of electronic flow measurement (EFM) data used in gas measurement reports. These files can be retrieved by any SCADA platform supporting standard DNP3 functions and are already supported by industry leading SCADA platforms, including AUTOSOL and Cygnet.

These and other features will only serve to make the DNP3 protocol more attractive to a wider range of customers and equipment vendors, helping to speed adoption throughout the oil and gas industry.

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