Practical_modern_SCADA_protocols_-_dnp3,_60870-5_and_Related_Systems

300 Practical Modern SCADA Protocols: DNP3, 60870.5 and Related Systems
9.4 Network operation
9.4.1 General introduction
This sub-section covers the operation of the IEC 60870-5-101 (or T101) protocol over
networks using standard transport profiles. This, in other words, is the operation over
networks using the TCP/IP protocols. Since the early use of the T101 protocol there has
been an increasing level of interest in being able to use standard network profiles to carry
protocol messages. This has occurred no doubt with the ever increasing penetration and
usage of local and wide area networks, which may offer a ready-made communications
infrastructure that is attractive for SCADA due to savings that can be achieved over
installing separate communications infrastructure.
The interest in doing this led to some attempts to encapsulate the T101 messages,
including the link layer components, for transmission over networks. This is referred
to briefly in the introduction to IEC 60870-5-104 which mentions the use of packet
assembler disassembler (PAD) type stations to provide access for balanced communications.
However, such an approach is not based on standardization and so could only be
applied in limited circumstances.
The need for a standard for carrying the communications protocol over data networks
was addressed with the release of IEC 60870-5-104 in 2000. This standardized the use of
the application functions and data objects of the protocol with a different message transport
mechanism, which completely replaced the link and physical layers specified by the
T101 profile. The structure of this in terms of standards is shown in Table 9.2.
Layer Source Selections
User Process
Application
Transport
IEC 60870-5-101
IEC 60870-5-101
Application Functions
ASDUs and Application Inform ation
Elements
Network
TCP/IP Transport and Network Protocol Suite
Link
Physical
Table 9.2
Standards selection for IEC 60870-5-104
Table 9.2 shows that the user process functions and application level definitions of T101
have been retained, but that the levels below these have been replaced by the transport and
network profiles of TCP/IP. The lower data link and physical layers are not specified at all
as these are not implemented directly by users of T104. This is because the boundary of
the protocol is at the network interface level.