wilamowski-b-m-irwin-j-d-industrial-communication-systems-2011

58-6 Industrial Communication Systems
(low priority, usually cyclic data) unless the previous response indicates that there is high-priority data
available (by setting the access demand flag), in which case the controlling station requests Class 1 data. In
this manner, the controlled station is fundamentally in control of the correct sequence of reporting of data
to the controlled station. In DNP3, the outstation always replies with exactly the data that the master has
requested: The master has full control over what will be sent. In order for the correct data reporting to be
preserved in DNP3, the master must only request static data (the current value of inputs) if it is also requesting
all buffered events for those same inputs in the same request. This is critical for correct data reporting.
Note: In all the protocols, an understanding of the underlying philosophy is required when implementing
the protocol. It is possible to poorly implement T101 unbalanced in a controlled station so that the
critical requirement to report all measurements in the order that they are measured is violated by arcane
configuration settings. Similarly, it is possible to violate this same requirement in a DNP3 master by not
requesting buffered events when static data is requested.
Differentiators between IEC 60870-5-101/-104 and DNP3 are
• The T101 profile is strictly for use over serial links, T104 is strictly for use over TCP/IP transports.
The data link frame (IEC 60870-5-1 FT1.2) defined for T101 is replaced by a cut-down frame for
T104 together with different message confirmation rules. T101 is permitted to use unbalanced
data link services (required for use on multi-drop channels) or balanced data link services (requiring
a dedicated full-duplex point-to-point link between pairs of devices). The operation of T104 is
basically the same as the operation of T101 over balanced data links with the differentiation that a
configurable number of T104 information messages (I-frames) can be sent before a confirmation
is received, whereas the T101 rules require a confirmation of each individual frame. DNP3 always
uses the serial data link frame (based on IEC 60870-5-1 FT3) and “balanced transmission” rules
modified to allow multi-drop operation on half-duplex channels. When DNP3 is used with TCP/
IP or UDP/IP, the serial data link frame is encapsulated in a TCP or UDP packet. This process
simplifies the software for frame handling (same for serial or IP) and also simplifies the use of
DNP3 with terminal server type interfaces that simply wrap or unwrap the serial message within
the TCP/IP frames and do not need to apply a new serial data link frame.
• T101/T104 messages can only contain a single type of data (e.g., single-bit binary inputs or integer
analog measurands). DNP3 messages can contain multiple types of data in one message, as long
as the same “function” (e.g., read, control command, report response, etc.) applies to all data in
the message. Because of this, the response to a command such as an integrity poll can consist of
several smaller messages in T101 or T104 and a small number of large messages in DNP3. This
can affect the efficiency of reporting the different protocols, especially over data links with special
properties such as data radios (turn-around time may be long compared to transmit time) and
satellite links (pay per byte: More packets = more overhead = more cost).
• T101/T104 messages are limited to the maximum size of a single data link frame. This typically
allows about 250 bytes of data per message. DNP3 messages have no logical size limit: DNP3 uses
a transport function and application layer fragmentation to build messages of any length and
transmit them as a series of application fragments (typically 2048 bytes in size) split into many
transport segments in order to fit the data link frames.
• DNP3 uses an explicit application confirmation mechanism to verify reporting of application
data. T101 deduces correct application reporting by reliance on data link confirmation of each
link frame or toggling of a single sequence number bit (known as the Frame Count Bit or FCB) in
a subsequent request. T104 relies on the confirmation of a number of transmitted messages. The
maximum window size in T104 is configurable (parameter k) but is typically set to 12. DNP3 also
permits the use of the IEC data link confirm (except on TCP/IP links), however, the use of this
provides no functional benefit and is not recommended.
• Within a device, T101 and T104 model every kind of data as being a member of a single set of
objects, identified by the information object address (IOA) of the data element. The IOA values
© 2011 by Taylor and Francis Group, LLC