Practical_modern_SCADA_protocols_-_dnp3,_60870-5_and_Related_Systems

8.4.5 Unbalanced and balanced transmission
Fundamentals of IEC 60870-5 191
The terms ‘unbalanced transmission’ and ‘balanced transmission’ are related to the terms
‘primary’ and ‘secondary’. Unbalanced transmission refers to the configuration where the
controlling station acts as a primary on the link, and one or more controlled stations act as
secondary stations. The stations are not peer-to-peer at the link level, and so are unbalanced
in their functionality.
This is the situation in Figure 8.8 for each of the two communications links. In this
configuration the controlling station must acquire data from the controlled stations by
polling each in turn for data. This is because they cannot initiate transmissions on their
own. The advantage of unbalanced communication is that there is no possibility of
collisions between controlled stations attempting to transmit information at the same time.
Balanced transmission refers to the configuration where any station on a link may act as
a primary, which means it can initiate communications. This configuration is also known
as peer-to-peer communications.
Under IEC 60870-5-101, only point-to-point (that is two station) links can be balanced.
Multi-point links must be unbalanced. This is in contrast to DNP3, which uses balanced
transmission only, and therefore has to have procedures to overcome collisions which can
occur when more than one outstation commences communications simultaneously.
A balanced communications link is shown in Figure 8.9. In this case a master station is
connected via a point-to-point link to a sub-master station. Note that each station can act
as a primary and a secondary at the ports for this link. These may in fact be thought of as
two separate processes within each station, which in fact is how they are logically within
the stations. Station A has a primary process and a secondary process operating simultaneously
for that link, and Station B has the same.
Figure 8.9
Balanced communications
Figure 8.10 shows the primary and secondary processes for unbalanced transmission.
The processes are represented by circles identified as primary (P) or secondary (S). The
primary station implements the primary process only, and the secondary stations each
implement a secondary process only. Note that in effect there is a separate logical link for
each secondary station and it is necessary for the primary to keep a record of the state of
each link.