Practical_modern_SCADA_protocols_-_dnp3,_60870-5_and_Related_Systems

70 Practical Modern SCADA Protocols: DNP3, 60870.5 and Related Systems
4.5 System topology
System topologies include:
• Master–slave
• Multidrop from one master
• Hierarchical with intermediate data concentrators
• Multiple master
These are illustrated in the following drawing.
Figure 4.1
DNP3 network topologies
DNP3 supports multiple-slave, peer-to-peer and multiple-master communications.
It supports the operational modes of polled, and quiescent operation. The latter is also
referred to as reporting by exception. Quiescent operation is so called because polls to
check for changes are not required. This is because the master station can rely on the
outstation to send an ‘unsolicited response’ when it has a change that needs to be reported.
Thus, in the absence of change the system remains quiescent, or in a quiet state, with
neither polls from the master station, nor responses from the outstations. This mode of
operation provides for better use of the communications system capacity.
In a quiescent system, generally a periodic background poll is still used, perhaps at
hourly intervals, to guard against undetected communications failure. If this was not done,
the master station would have no way of detecting the failure of communications with the
outstation should it occur. It would just assume that nothing had changed.