D.5
Date Code 20100129
Instruction Manual
SEL-751A Relay
DNP3 Communications
Introduction to DNP3
DNP3 Serial
Network Issues
Data Link Layer Operation
DNP3 employs a three-layer version of the seven-layer OSI (Open Systems
Interconnect) model called the enhanced performance architecture. The layer
definition helps to categorize functions and duties of various software
components that make up the protocol. The middle layer, the Data Link Layer,
includes several functions for error checking and media access control.
A feature called data link confirmation is a mechanism that provides positive
confirmation of message receipt by the receiving DNP3 device. While this
feature helps you recognize a failed device or failed communications link
quickly, it also adds significant overhead to the DNP3 conversation. You
should consider whether you require this link integrity function in your
application at the expense of overall system speed and performance.
The DNP3 technical bulletin (
DNP Confirmation and Retry Guidelines 9804-
002
) on confirmation processes recommends against using data link
confirmations because these processes can add to traffic in situations where
communications are marginal. The increased traffic will reduce connection
throughput further, possibly preventing the system from operating properly.
Network Medium Contention
When more than one device requires access to a single (serial) network
medium, you must provide a mechanism to resolve the resulting network
medium contention. For example, unsolicited reporting results in network
medium contention if you do not design your serial network as a star topology
of point-to point connections or use carrier detection on a multidrop network.
To avoid collisions among devices trying to send messages, DNP3 includes a
collision avoidance feature. Before sending a message, a DNP3 device listens
for a carrier signal to verify that no other node is transmitting data. The device
transmits if there is no carrier or waits for a random time before transmitting.
However, if two nodes both detect a lack of carrier at the same instant, these
two nodes could begin simultaneous transmission of data and cause a data
collision. If your serial network allows for spontaneous data transmission
including unsolicited event data transmissions, you also must use application
confirmation to provide a retry mechanism for messages lost due to data
collisions.
DNP3 LAN/WAN
Overview
The main process for carrying DNP3 over an Ethernet Network (LAN/WAN)
involves encapsulating the DNP3 data link layer data frames within the
transport layer frames of the Internet Protocol (IP) suite. This allows the IP
stack to deliver the DNP3 data link layer frames to the destination in place of
the original DNP3 physical layer.
The DNP User Group Technical Committee has recommended the following
guidelines for carrying DNP3 over a network:
NOTE:
Link layer confirmations are
explicitly disabled for DNP3 LAN/
WAN. The IP suite provides a reliable
delivery mechanism, which is backed
up at the application layer by
confirmations when required.
➤
DNP3 shall use the IP suite to transport messages over a LAN/
WAN
➤
Ethernet is the recommended physical link, though others may
be used
➤
TCP must be used for WANs
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