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Section 2
DNP3 overview
12
support advanced RTU functions and messages larger than the maximum frame length
as defined by the IEC document 60870-5-1, the DNP3 data link is intended to be used
with the mentioned transport pseudo-layer. As a minimum, this transport layer
implements message assembly and disassembly services.
Physical layer
Even though the standard does not specify the physical layer, it does however specify
how to operate in a networked environment and also suggests how to avoid collisions
between simultaneously sending devices.
Many implementations use serial communication based on RS-232, RS-485 or even
fibre optics.
DNP3 can also be used over packet-oriented networks such as TCP/IP and UDP in
which, for example, Ethernet may be used. In this case DNP3 can be said to be
tunneled over TCP/IP or UDP.
Additional information on the DNP3 physical layer is available at the
DNP Users Group at www.dnp.org.
Data link layer
The DNP3 data link layer is designed to operate with asynchronous or synchronous bit
serial physical layers. Fully balanced transmission procedures were adopted to support
spontaneous transmissions from remotes.
Data link functions include:
•
Performing message data link retransmissions.
•
Packing user data into the defined frame format includes CRC and transmitting the
data to the physical layer.
•
Unpacking the data link frame received from the physical layer into user data,
checking and removing CRC.
•
Controlling the physical layer.
•
In unsolicited reporting mode, performing collision avoidance/detection
procedures to ensure reliable transfer of data across the physical link.
•
Responding to all valid frames received from the physical layer.
Data link responsibilities:
•
Exchange of Service data units (SDUs) between peer DNP3 data links
•
Error notification to data link user
•
Sequencing of SDUs
•
SDU delivery quality.
1MRK 511 391-UUS A
670 series 2.2 ANSI
Communication protocol manual
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