DNP3

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DNP3 (Distributed Network Protocol) is a set of communications protocols used between components in process automation systems. Its main use is in utilities such as electric and water companies. Usage in other industries is not common. It was developed for communications between various types of data acquisition and control equipment. It plays a crucial role in SCADA systems, where it is used by SCADA Master Stations (a.k.a. Control Centers), Remote Terminal Units (RTUs), and Intelligent Electronic Devices (IEDs). It is primarily used for communications between a master station and RTUs or IEDs. ICCP, the Inter-Control Center Communications Protocol (a part of IEC 60870-6), is used for inter-master station communications. Competing standards include the older Modbus protocol and the newer IEC 61850 protocol.

Data acquisition all methods of simultaneous or sequential time measurement and counting for measurable or quantifiable data and groups of contiguous data

Data acquisition is the process of sampling signals that measure real world physical conditions and converting the resulting samples into digital numeric values that can be manipulated by a computer. Data acquisition systems, abbreviated by the acronyms DAS or DAQ, typically convert analog waveforms into digital values for processing. The components of data acquisition systems include:

Supervisory Control and Data Acquisition (SCADA) is a control system architecture that uses computers, networked data communications and graphical user interfaces for high-level process supervisory management, but uses other peripheral devices such as programmable logic controller (PLC) and discrete PID controllers to interface with the process plant or machinery. The use of SCADA has been also considered for management and operations of project-driven-process in construction.

Intelligent electronic device

An Intelligent Electronic Device (IED) is a term used in the electric power industry to describe microprocessor-based controllers of power system equipment, such as circuit breakers, transformers and capacitor banks.

Contents

DNP overview diagram DNP-overview.png
DNP overview diagram

History

While IEC 60870-5 was still under development and had not been standardized, there was a need to create a standard that would allow interoperability between various vendors' SCADA components for the electrical grid. Thus, in 1993, GE-Harris Canada (formerly known as Westronic, Inc.) used the partially completed IEC 60870-5 protocol specifications as the basis for an open and immediately implementable protocol that specifically catered to North American requirements. The protocol is designed to allow reliable communications in the adverse environments that electric utility automation systems are subjected to, being specifically designed to overcome distortion induced by EMI, aging components (their expected lifetimes may stretch into decades), and poor transmission media.

IEC 60870 part 5 is one of the IEC 60870 set of standards which define systems used for telecontrol in electrical engineering and power system automation applications. Part 5 provides a communication profile for sending basic telecontrol messages between two systems, which uses permanent directly connected data circuits between the systems. The IEC Technical Committee 57 have developed a protocol standard for telecontrol, teleprotection, and associated telecommunications for electric power systems. The result of this work is IEC 60870-5. Five documents specify the base IEC 60870-5:

Electromagnetic interference

Electromagnetic interference (EMI), also called radio-frequency interference (RFI) when in the radio frequency spectrum, is a disturbance generated by an external source that affects an electrical circuit by electromagnetic induction, electrostatic coupling, or conduction. The disturbance may degrade the performance of the circuit or even stop it from functioning. In the case of a data path, these effects can range from an increase in error rate to a total loss of the data. Both man-made and natural sources generate changing electrical currents and voltages that can cause EMI: ignition systems, cellular network of mobile phones, lightning, solar flares, and auroras. EMI frequently affects AM radios. It can also affect mobile phones, FM radios, and televisions, as well as observations for radio astronomy.

Security

Although the protocol was designed to be very reliable, it was not designed to be secure from attacks by hackers and other malevolent forces that could potentially wish to disrupt control systems to disable critical infrastructure.[ citation needed ]

Because smart grid applications generally assume access by third parties to the same physical networks and underlying IP infrastructure of the grid, much work has been done to add Secure Authentication features to the DNP3 protocol. The DNP3 protocol is compliant with IEC 62351-5. Some vendors support encryption via bump-in-the-wire for serial communications or virtual private networks for Internet Protocol-based communications. One of the most popular bump-in-the-wire methods began originally as AGA-12 (American Gas Association) in 2003, later becoming IEEE Std. 1711-2010. This standard was subsequently withdrawn March 27, 2014.

Smart grid Electrical grid operated by automatic control devices or controled and maintained from distance.

A smart grid is an electrical grid which includes a variety of operation and energy measures including smart meters, smart appliances, renewable energy resources, and energy efficient resources. Electronic power conditioning and control of the production and distribution of electricity are important aspects of the smart grid.

IEC 62351 is a standard developed by WG15 of IEC TC57. This is developed for handling the security of TC 57 series of protocols including IEC 60870-5 series, IEC 60870-6 series, IEC 61850 series, IEC 61970 series & IEC 61968 series. The different security objectives include authentication of data transfer through digital signatures, ensuring only authenticated access, prevention of eavesdropping, prevention of playback and spoofing, and intrusion detection.

The term bump-in-the-wire (BITW) refers to a communications device which can be inserted into existing (legacy) systems to enhance the integrity, confidentiality, or reliability of communications across an existing logical link without altering the communications endpoints. The term was originally used to indicate that the device should introduce only a relatively small increased latency in communications compared to the original, unsecured, approach.

The DNP3 protocol is also referenced in IEEE Std. IEEE 1379-2000, which recommends a set of best practices for implementing modern SCADA Master-RTU/IED communication links. These include not just encryption but other practices that enhance security against well known intrusion methods.

Technical details

The DNP3 protocol has significant features that make it more robust, efficient, and interoperable than older protocols such as Modbus, at the cost of higher complexity.

Modbus is a serial communications protocol originally published by Modicon in 1979 for use with its programmable logic controllers (PLCs). Modbus has become a de facto standard communication protocol and is now a commonly available means of connecting industrial electronic devices. The main reasons for the use of Modbus in the industrial environment are:

In terms of the OSI model for networks, DNP3 specifies a layer 2 protocol. It provides multiplexing, data fragmentation, error checking, link control, prioritization, and layer 2 addressing services for user data. It also defines a Transport function (somewhat similar to the function of layer 4) and an Application Layer (layer 7) that defines functions and generic data types suitable for common SCADA applications. The DNP3 frame strongly resembles, but is not identical to the IEC 60870-5 FT3 frame. It makes heavy use of cyclic redundancy check codes to detect errors.

The improved bandwidth efficiency is accomplished through event oriented data reporting. The Remote Terminal Unit monitors data points and generates events when it determines that the data should be reported (for example, when it changes value). These events are each placed in one of three buffers, associated with "Classes" 1, 2 and 3. In addition to these, Class 0 is defined as the "static" or current status of the monitored data.

The Remote Terminal Unit is initially interrogated with what DNP3 terms an "Integrity Poll" (a combined Read of Class 1, 2, 3 and 0 data). This causes the Remote Terminal Unit to send all buffered events and also all static point data to the Master station. Following this, the Master polls for the event data by reading Class 1, Class 2 or Class 3. The reading of the classes can all be performed together or each class can be read at a different rate, providing a mechanism to create different reporting priorities for the different classes. After an Integrity Poll, only significant data changes are sent. This can result in significantly more responsive data retrieval than polling everything, all the time, irrespective of whether it has changed significantly.

The Remote Terminal Unit can also be configured to spontaneously report Class 1, 2, or 3 data, when it becomes available.

The DNP3 protocol supports time synchronization with an RTU. The DNP Protocol has time stamped variants of all point data objects so that even with infrequent RTU polling, it is still possible to receive enough data to reconstruct a sequence of events of what happened in between the polls.

The DNP3 protocol has a substantial library of common point-oriented objects. The focus of this extensive library was to eliminate the need for bit-mapping data over other objects, as is often done in many Modbus installations. For example, floating point number variants are available, so there is no need to map the number on to a pair of 16 bit registers. This improves compatibility and eliminates problems such as endianness.

A Remote Terminal Unit for the DNP3 protocol can be a small, simple embedded device, or it can be a large, complex rack filled with equipment. The DNP User Group has established four levels of subsets of the protocol for RTU compliance. The DNP Users Group has published test procedures for Levels 1 and 2, the simplest implementations.

The protocol is robust, efficient, and compatible with a wide range of equipment, but has become more complex and subtle over time. Increasingly demanding industrial applications are part of the challenge. Also, SCADA concepts are technically simple but field applications that integrate several types of equipment can become complex to set up or troubleshoot due to variances in vendor implementations.

IEEE Standardization

The IEEE adopted DNP3 as IEEE Std 1815-2010 on July 23, 2010. [1] IEEE Std 1815 was co-sponsored by the Transmission and Distribution Committee and Substations Committee of the IEEE Power & Energy Society, with additional input from the DNP Users Group.

In April 2012, the IEEE approved Std 1815-2012 for publication. IEEE Std 1815-2010 has been deprecated. The 2012 version of the standard includes features for Secure Authentication Version 5. The previous version of secure authentication in IEEE 1815-2010 used pre-shared keys only. The new version is capable of using Public Key Infrastructure, and it facilitates remote key changes.

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