Practical_modern_SCADA_protocols_-_dnp3,_60870-5_and_Related_Systems

182 Practical Modern SCADA Protocols: DNP3, 60870.5 and Related Systems
8.2 Protocol architecture
8.2.1 EPA and OSI reference models
As for DNP3, IEC 60870-5 is based on the three-layer enhanced performance architecture
or EPA model for data communications. These models are described in detail later in this
text, which shows how the EPA model is a simplified form of the OSI seven layer reference
model to provide optimum performance for telecontrol applications.
In the Figure 8.1, the relationship between the OSI model and the EPA model is
represented. This shows that the EPA model basically omits the presentation, session and
transport layers of the OSI model.
Figure 8.1
Relationship of EPA model to OSI 7 layer model
The structure of the EPA model is appropriate for a continuously operating system that
operates over a single network. One layer is normally added to the top of the basic EPA
model representation is identified as the user layer. This is included to represent the
various functions or processes that must be defined to provide telecontrol system
operations. These are required to be defined to provide for the interoperability between
equipment that will result in a fully operable telecontrol system, rather than merely a data
communication system.
For the first defined companion standard IEC 60870-5-101 or T101 profile, a four
layer model as illustrated at the right-hand side of Figure 8.1 provides an accurate
representation of the architecture of the protocol. In the case of the networked version IEC
60870-5-104, or the T104 profile, additional layers of the OSI model must be included to