Practical_modern_SCADA_protocols_-_dnp3,_60870-5_and_Related_Systems

84 Practical Modern SCADA Protocols: DNP3, 60870.5 and Related Systems
5.4.3 Some terminology
Communications in a SCADA system will generally have a structure where some stations
may be identified as master stations, and others as slave stations, sub-master stations, or
outstations. In a hierarchical structure, there may be some devices that act both as slave
stations and master stations.
At the data link level, the terms balanced and unbalanced are used to describe whether
all stations may initiate communications or not. In ‘unbalanced’ systems, only master
stations will initiate communications. That is, only a master station will be a primary, or
originating, station, and slave stations will always be secondary, or responding, stations. In
these systems when a slave station has data that it needs to transmit to the master, it must
wait until the master station polls it.
The DNP3 protocol support balanced communications at the data link level. This provides
greater flexibility by allowing non-master stations to initiate communications.
In DNP3 any station can be an originator or primary station.
The terminology ‘balanced’ and ‘unbalanced’ can be confused with the terms ‘master’
and ‘slave’. This confusion can arise because it is easy to imagine that in a balanced system,
all stations must be equal. This is not the case, however. The reason is that the terms
balanced and unbalanced apply only to the lower data link level communications, whereas
the master/slave distinction applies at the application level. In DNP3 the terms master and
slave have real meaning, and a station is configured as one or the other. The key to understanding
is that only a master station can issue a request, and only a slave can provide a
response. These are each application level messages.
In the case of an outstation (a slave station) having unsolicited data to send to the master
station, it issues an unsolicited response to the master. At the application level this is a different
type of message to a request. At the data link level, however, the message frame
appears no different from any other message frame whether issued by a master or a slave.
These concepts are illustrated in the following communication sequence diagram.
The diagram below shows how these terms relate to the communication process. The
diagram illustrates a request for data from a master station to a non-master station. This
could be a poll for current data, a ‘static’ poll. The diagram illustrates the communication
sequence by showing the parties on each side, with message directions shown between
them. The time sequence is shown from top to bottom.
MASTER
STATION
REQUEST MESSAGE
(Primary)
CONFIRM - ACK
(Secondary)
SLAVE
STATION
CONFIRM - ACK
(Secondary)
RESPONSE
(Primary)
Time
Message Direction
Figure 5.7
Example of communication sequence diagram