![Schweitzer Engineering Laboratories SEL-421-4 Instruction Manual Download Page 1322](http://html.mh-extra.com/html/schweitzer-engineering-laboratories/sel-421-4/sel-421-4_instruction-manual_12167261322.webp)
16.6
SEL-400 Series Relays
Instruction Manual
Date Code 20171006
DNP3 Communication
Introduction to DNP3
A feature called data link confirmation is a mechanism that provides positive
confirmation of message receipt by the receiving DNP3 device. While this fea-
ture helps you recognize a failed device or failed communications link quickly, it
also adds significant overhead to the DNP3 conversation. Consider for your indi-
vidual application whether you require this link integrity function at the expense
of overall system speed and performance.
The DNP3 specification 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 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 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 col-
lision 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 trans-
mits if there is no carrier or waits for a random time before rechecking for a car-
rier signal. 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 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 as a result of data collisions.
DNP3 LAN/WAN Considerations
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 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’s Group Technical Committee has recommended the
following guidelines for carrying DNP3 over a network:
➤
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
➤
TCP is strongly recommended for LANs
➤
UDP may be used for highly reliable single segment LANs
➤
UDP is necessary if broadcast messages are required
➤
The DNP3 protocol stack shall be retained in full
➤
Link layer confirmations shall be disabled
The Technical Committee has registered a standard port number, 20000, for
DNP3 with the Internet Assigned Numbers Authority (IANA). This port is used
for either TCP or UDP.
The Committee recommends the selection of TCP or UDP protocol as per the
guidelines in
.
Summary of Contents for SEL-421-4
Page 6: ...This page intentionally left blank ...
Page 14: ...This page intentionally left blank ...
Page 30: ...This page intentionally left blank ...
Page 104: ...This page intentionally left blank ...
Page 128: ...This page intentionally left blank ...
Page 536: ...This page intentionally left blank ...
Page 584: ...This page intentionally left blank ...
Page 616: ...This page intentionally left blank ...
Page 696: ...This page intentionally left blank ...
Page 750: ...This page intentionally left blank ...
Page 755: ...Instruction Manual PM400 01 NB SEL 400 Series Relays Instruction Manual 20171006 ...
Page 776: ...This page intentionally left blank ...
Page 932: ...This page intentionally left blank ...
Page 976: ...This page intentionally left blank ...
Page 1024: ...This page intentionally left blank ...
Page 1038: ...This page intentionally left blank ...
Page 1064: ...This page intentionally left blank ...
Page 1128: ...This page intentionally left blank ...
Page 1206: ...This page intentionally left blank ...
Page 1316: ...This page intentionally left blank ...
Page 1350: ...This page intentionally left blank ...
Page 1464: ...This page intentionally left blank ...
Page 1468: ...This page intentionally left blank ...
Page 1492: ...This page intentionally left blank ...
Page 1518: ...This page intentionally left blank ...