When you have finished this study on the difference between DNP3 and IEC 60870, you should be able to distinguish and identify the similarities between the two communication protocols.
Table of Contents
Comparison between DNP3 and IEC 60870-5-101
DNP3 (Distributed Network Protocol 3) and IEC 60870-5-101 are two communication protocols widely used in the field of industrial control systems (ICS) automation and supervisory control and data acquisition (SCADA) systems. Although they have similar objectives, they differ in their design and usage. It is therefore probably useful to start by identifying their common characteristics.
Common Characteristics of DNP3 and IEC 60870-101
- High-security data transmission (higher for DNP3)
- Operation by polling and by exception
- Unsolicited messages (limited in IEC 60870-5-101)
- Data definitions based on objects adapted to SCADA systems
- Time synchronization
- Time-stamped events
- Freeze and reset of counters
- Select before operate for open/close commands
- Groups or classes of data
- File upload and download
In functional terms, it is clear that the two protocols have many similarities.
Addressing
- IEC 60870-5-101 uses both link addresses and application addresses (ASDU address). This provides greater flexibility in message routing.
- IEC 101 has a wider point address range, up to 3 bytes.
- DNP3.0 uses only link addresses, without application layer addresses.
- The DNP3.0 frame carries both source and destination addresses.
Overall, IEC 101 offers greater flexibility in its addressing system, both by including data link and application level addresses, and through the use of variable address lengths, allowing bandwidth savings by using small numbers as addresses.
Data Link Communications
- IEC 60870-101 uses asymmetric and symmetric communications (limited only to point-to-point).
- IEC 101 does not support unsolicited messages on multidrop communications.
- DNP3.0 uses only symmetric communications.
DNP3 and IEC 60870-5-101 support symmetric or peer-to-peer communications. However, IEC 101’s symmetric communications are limited to point-to-point configurations. This can be a significant limitation if a multidrop configuration is considered, for example, a large number of remote stations connected to a single IEC 101 link, in which case polling data might require high-bandwidth communication support. For this, the DNP3 protocol for multidrop symmetric communications would be an advantage for setting up a system based on unsolicited messages.
Frame Format
- IEC 101 uses the FT1.2 frame, length up to 255 bytes.
- IEC 101 frames are of fixed and variable length.
- DNP3 uses FT3 frames, length up to 255 bytes.
- DNP3 uses only variable lengths.
When the fixed-length frame option is used under IEC 60870-101, a very short and simple frame is created compared to the DNP3 frame. This significantly reduces communication overhead.
Application Functions and Data Objects
Application Functions
- T101 allows only one control point per message.
- T101 uses a single-character application acknowledgment.
Single-character frame
- DNP3 allows control of multiple points in a single message.
Data Objects
- T101 allows one type per message.
- T101 combines function and data types in the type code.
- T101 data objects are oriented toward substation communications.
- The DNP3 protocol allows multiple data objects in a single message.
- DNP 3.0 uses distinct function codes.
- DNP 3.0 has one function code per message, which applies to all data objects in the message.
There are considerable differences between IEC 60870-101 and DNP3 regarding supported application functions and data objects. The separation of functions and data objects in DNP3 perhaps offers greater flexibility, but also implies greater complexity.
Security
- T101 relies on data link confirmation before resetting events.
- DNP3 requires application confirmations before resetting events.
- Error checking is more robust in DNP3.
- Both have select before operate.
Although DNP’s error detection capability is stronger than for IEC 60870-101, the importance of this difference would depend on the error rate on the communication lines as well as the message lengths. Since T101 messages tend to be shorter than DNP3 messages, the overall effect might not be substantially different.
Interoperability
- T101 has no official certification procedures or authorities.
- There are companies that provide testing for T101.
- The DNP3 protocol has defined subset levels for IEDs.
- DNP3 has defined conformance testing procedures.
- DNP3 has certification authorities in North America.
The existence of testing procedures and authorities, combined with defined minimum implementation levels, is recognized as a strength of DNP3. However, although DNP3 took an early lead in this area, future developments could reduce this difference as the use of the IEC 60870-101 standard evolves.
Complexity
In many ways, IEC 60870-101 is a simpler protocol and can operate in a simpler manner. Here are some examples:
- No separate application function codes.
- Data objects are simpler, without variations as in DNP3.
- The point addressing scheme is simpler than in DNP3.
- Can be configured to have fixed-length frames.
- Can be configured to use unbalanced data link transmissions.
- This simplifies communications as collisions are avoided.
- Uses single-byte ACK transmissions on the data link layer.
- The FT1.2 format is simpler (but offers less error protection).
- The absence of a transport layer and a single data type per message simplifies parsing.
On the other hand, it has been noted that IEC 101 may have more low-level aspects requiring configuration, which can increase difficulties during system integration. Additionally, although DNP3 is more complex in many ways, not all features need to be implemented. DNP3’s minimum implementation subsets are limited to a small number of functions and addressing modes.
Efficiency
Some of T101’s characteristics, such as the fixed frame length option, single-character application acknowledgments, and less rigorous error checking, can result in shorter messages. However, T101 may require a large number of messages to send information, so some of the benefit of reducing message headers is lost.
Without knowledge and analysis of the specific data to be primarily transported on a system, it is probably difficult to conclude whether one or the other protocol will perform more efficiently. When either protocol needs to be transported over networks, message length will impact efficiency, as the message will be encapsulated in additional data. In this case, T101’s shorter messages could lead to a loss of efficiency.
Protocol Support
IEC 60870-5-101 is dominant in Europe compared to DNP3, but is limited to the electrical industry. DNP3 is becoming increasingly recognized in Europe in the utilities industry. An example of this is the use of IEEE Std 1379 “Recommended Practice for Data Communications Between Intelligent Electronic Devices and Remote Terminal Units in a Substation.”
This recommended practice recognizes both DNP3 and IEC 101 for use in this application. IEC 60870-101 and DNP3 are used to similar degrees in Asia, and DNP3 is dominant in Australia. T101 is supported by several major manufacturers, just like DNP3. In summary, it is clear that both protocols enjoy significant support, but this varies by industry and geographic location.
Which One Will Prevail?
The choice between IEC 60870-101 and DNP3 for an organization will depend on many factors. In functional terms, both achieve similar results. For the product developer, the supported protocol will depend on customer needs, which will stem from their industry and location.
For many products, support for both protocols will be necessary. From the user’s perspective, this choice will be dictated by the already installed system or, in the case of an entirely new system, by comparing the characteristics of available equipment using one or the other protocol.
The fact is that both protocols are used worldwide and are here to stay in the foreseeable medium-term future.
In the longer term, industry trends will have a significant influence. The Utilities Communication Architecture (UCA) project has the potential to emerge as a major force for achieving large-scale interoperability and plug-and-play. In this case, the extent to which the protocols adapt and integrate into this system may influence their ultimate longevity.
Conclusion
IEC 60870-5-101 and DNP3 are open protocols specifically designed for telecontrol applications. They were developed to meet common needs, have a common origin, and emerged over a similar period. Both have also been developed to meet the needs of the electrical industry, although DNP3 has more data types oriented toward general SCADA use than for electrical use in particular.
The comparison of each’s characteristics has shown that each has differences that can translate into advantages in some situations and disadvantages in others. In several ways, IEC 101 seems to be a simpler protocol, but this does not necessarily mean a simpler implementation.
DNP3’s implementation subset levels, defined testing procedures, and certification authorities provide a solid foundation for ensuring interoperability of products from different manufacturers. The choice between IEC 60870-5-101 or DNP3 for an organization will depend on many factors.
In functional terms, both achieve similar results. For the product developer, the supported protocol depends on customer needs, derived from their industry and location. For many products, support for both protocols will be essential.