Practical_modern_SCADA_protocols_-_dnp3,_60870-5_and_Related_Systems

366 Practical Modern SCADA Protocols: DNP3, 60870.5 and Related Systems
The UCA provides for two options at the T-Profile level. Utilities can use ISO network
and transport standards or IETF (Internet) network and transport standards.
At the application level (A-Profile), the UCA uses the ISO session and presentation
layers. It uses the association control service element (ACSE) to establish and release a
communications connection between applications.
Finally, UCA uses the manufacturing message specification (MMS) standard. MMS
provides the message structure, message syntax, and message dialog procedures for
monitoring and control information communications.
There has been interest expressed within the DNP3 technical standards committee to
extending DNP3 to include additional capability to allow DNP3 to be used instead of
MMS. This is likely to be a long term objective (2008?), rather than something that will
be available in the near future. The idea that UCA could be used in conjunction with
communications protocols other than MMS is an integral part of the philosophy of UCA,
although the protocol must have sufficient functionality to support the requirements of
UCA. For this reason IEC 870 is a potential candidate to support UCA, whereas Modbus
probably isn’t.
The UCA A-Profiles also allow ISO ‘Fastbyte’ session and presentation layers as an
option to the complete ISO session and presentation layers. This is intended to reduce
protocol overheads.
14.3.3 Recommended water/gas industry UCA profiles
The gas industry, through the Gas Research Institute (GRI), has explored the UCA in
several projects. One project, UCA/RAPS, has already evaluated UCA Version 2 and
recommended specific UCA profiles for gas applications. Figure 14.4 shows the recommended
water and gas industry profiles.
Figure 14.4
Water/gas UCA profile recommendations (Figure courtesy American Water Works Association)