Practical_modern_SCADA_protocols_-_dnp3,_60870-5_and_Related_Systems

162 Practical Modern SCADA Protocols: DNP3, 60870.5 and Related Systems
6.4.4 Certification authorities
The DNP3 Users Group has authorized three test houses to carry out conformance testing
and issue conformance certificates for DNP3. Third party conformance testing is not a
requirement to use DNP3, but if certification is required by an end user, this can only be
obtained from one of these authorities.
Authorized Testing Authorities:
• Advanced Control Systems (Georgia, USA)
• Reltronics (Canada)
• Subnet Solutions (Calgary, Canada)
6.5 DNP3 polling and communications options
The designer of a SCADA system utilizing DNP3 needs to select an operational mode for
the acquisition of data from the system. Whereas many older SCADA systems are based
on purely polled data acquisition, DNP3 offers the choice of report-by-exception
operation. The advantage that this operational mode gives is a substantial saving in bandwidth
usage.
Instead of slave devices reporting the status of all of their points in response to master
station polls, they report changes only. This method assumes, and requires, that the
SCADA master can reliably retain its record of the present status of the slave and that any
changes to that status will be reliably reported to the master. When the typical operation of
SCADA systems is considered it is apparent that such a system can make considerable
savings in the amount of data that is communicated. This reduction in communication
requirements can translate to allowing a much larger number of slaves to be connected to
a given communication path than might otherwise be the case.
In fact, the combination of polled versus non-polled operation, and report-by-exception
combine to make four variations. The DNP3 documentation identifies the following four
operating modes and identifies them in order of decreasing efficiency in use of communications
bandwidth.
A mode commonly used is unsolicited report-by-exception. This mode gives security
against failure of communications systems, while still retaining the benefit of limited
bandwidth usage. This addresses a problem that can occur with quiescent operation, which
is undetected communication failures. In a fully quiescent system, if a link should fail
there is no way to detect the failure because the master may not be attempting to initiate
any communications over that link. In this case the failure would only be apparent if
the master attempted to carry out a control action.
When implementing an unsolicited report-by-exception operating mode, the frequency
of background polling and the amount of information sought by this can be adjusted to
provide optimal system performance. For example, a periodic background poll might be