Practical_modern_SCADA_protocols_-_dnp3,_60870-5_and_Related_Systems

Advanced considerations of distributed network protocol 165
As DNP3 was originally conceived, its physical layer was specified as using RS-232C
bit-serial asynchronous communications. This is consistent with the historically typical
communications system described above and was a natural choice.
However, DNP3 was developed in the electrical industry, which is characterized by
short-range as well as long-range communications requirements. For example, in a substation
environment, a large number of devices such as meters, relays and other IEDs may
be utilized.
When DNP3 is used in a short-distance situation such as this, a multidrop configuration
can be achieved by using RS-485. However, this is limited to half-duplex operation as only
one device can drive the line at one time, and there is a limit of 32 devices on an RS-485
circuit. Regardless of the technical merit of this solution though, there are wider trends
operating that are taking communications in a different direction.
Where the end devices are intelligent, that is they have on-board processing, they may be
connected to the rest of the system via a data concentrator or control device such as an
RTU. It is the fact that these are now intelligent devices that have changed the nature of
communications requirements. Where in older systems, the SCADA system would stop at
the RTU and then be hard-wired to the end devices, this is no longer true. Today, the need
for communication of the protocol spans from the end device upwards.
At the other end of the scale, the requirements for communications have also become
more extensive. Where once individual operating plants were largely islands, little connected
with other locations, this is also no longer true. Today’s operating environments
have changed so that organizations can operate over larger geographic areas, and the
communications systems have changed in parallel with this. A major component of this is
the widespread adoption of local and wide area networks (LANs and WANs) to provide
data connectivity across organizations. These technologies are being increasingly
utilized in the electrical utility industry as in many other industries for the many benefits
they bring.
Thus, coincidental with the growth of the open SCADA protocols of DNP3 and
IEC60870-101 there has been a revolution to organization-wide connectivity, and this is
based on the use of LANs and WANs. Within a substation or operating plant a LAN may
be used to provide high-speed reliable communications between local equipment as well
as via routers to nearby or remote LANs, or to an enterprise level WAN. Because of the
benefits these technologies provide they have become ubiquitous, and are now found in
control rooms, the plant floor, and the substation, all areas where not too many years ago
they were rarely found.
It is hardly surprising then that pressure has emerged to carry DNP3 over a network
environment, and this has resulted in extension of the DNP3 specification to include this.
The DNP3 User Group Technical Committee has defined a method for carrying DNP3
involving the use of the Internet protocol suite for the transport and network layers, and
the Ethernet physical layer. The technicalities of doing this are discussed in the following
sections.
The following diagram shows the topology of a networked system using DNP3 and
gives an idea how the control system may fit into the overall networked system.