Practical_modern_SCADA_protocols_-_dnp3,_60870-5_and_Related_Systems

80 Practical Modern SCADA Protocols: DNP3, 60870.5 and Related Systems
5.2.3 The pseudo-transport layer
The APDU from the application layer may be referred to as the transport service data unit
within the pseudo-transport layer. It is interpreted purely as data to be transported by the
transport layer. The transport layer breaks the TSDU down into smaller units termed transport
protocol data units or TPDUs. These are made up of a one byte header, followed by a
maximum of 249 bytes of data.
The overall size of the TPDUs, which is 250 bytes, was determined so that each TPDU
will fit within one ‘frame’ or LPDU at the data link layer.
5.2.4 The data link layer
This layer takes the TPDUs from the pseudo-transport layer and adds a 10 byte header to
each. As the data link layer is also responsible for providing error detection and correction
functions, error checking codes are introduced here. A 16-bit cyclic redundancy code
(CRC) is used. Each TPDU is converted to a frame of up to 292 bytes in length.
It is worth noting at this point that the frame format is known as the FT3 frame format,
which was originally described by IEC 870-5-1. This frame format represents a historical
point of common ground between DNP3 and IEC 870-5. This frame format will be
returned to in the next section.
5.2.5 The physical layer
The physical layer converts each frame into a bit stream over the physical media. In the
original DNP3 documentation, a bit-serial asynchronous physical layer is specified. It calls
for 8-bit data, 1 start bit, 1 stop bit, no parity, and RS-232C voltage levels and control
signals.
5.2.6 Summary of message buildup
The buildup of DNP3 messages has been shown from a top-down perspective to have the
following features:
• Application functions may or may not require the passage of data
• Commands will often require no data
• The application layer parses the data into APDUs
• The APDU maximum size is 2048 bytes
• The pseudo transport layer parses the APDU into smaller TPDUs
• TPDU maximum size is 250 bytes
• The data link layer adds headers and CRCs to form the LPDU
• LPDU maximum size is 292 bytes, of which 250 bytes are data
In the following sections the detailed functioning of each of the model layers will be
examined. Looking at the functionality of each layer it will be necessary to examine and
understand the message header sections applied at each layer, and to see how the functions
of the layer are performed using them.
5.3 Physical layer
The physical layer may be described in terms of the network topology as well as the
characteristics of the communication paths.