Practical_modern_SCADA_protocols_-_dnp3,_60870-5_and_Related_Systems

Fundamentals of IEC 60870-5 183
provide for transport of messages over networks using standard network protocols. These
are the transport and network layers corresponding to the use of the TCP and IP protocols.
The two architectures are shown in Figure 8.2 below.
IEC 60870-5-101
Non-Networked
IEC 60870-5-104
Networked
Figure 8.2
Architectures for T101 and T104
As is clear from Figure 8.2, the operation of the lower layers of the networked version,
IEC 60870-5-104 is completely different from that of the non-networked version,
IEC 60870-5-101. These layers correspond to all the layers below the application layer,
which for these architectures are the layers concerned with message transport.
The remainder of this chapter will focus on the non-networked version only. The
networked version will be returned to in Chapter 9.
8.2.2 Selections from standards
The benefit of the OSI reference model, and the EPA models derived from it and shown in
Figure 8.2, is that they provide a framework for description of protocol operation.
Describing the operation of the protocol is a matter of specifying the functions of each
layer, and specifying the structure of information passing between the layers.
Under the IEC standard, it is the companion standard IEC 60870-5-101 that completely
specifies the protocol. It does this by referring to the main sections of the IEC 60870-5
standard, and by making particular selections from options that may be available within
those sections.
Table 8.3 shows how the different sections of the IEC 60870-5 set of standards
correspond with the layers of the model.