Practical_modern_SCADA_protocols_-_dnp3,_60870-5_and_Related_Systems

98 Practical Modern SCADA Protocols: DNP3, 60870.5 and Related Systems
Processing on receipt of message:
• Receive T′ = M followed by R′
• Invert R′ and form T = M appended with R
• Divide T by P (using modulo-2 division)
• Discard quotient QC, keep calculated remainder RC
• If RC = 0, result is GOOD, copy M to data output
• If RC not = 0, result is BAD, reject message
It is noted that this CRC has a ‘Hamming distance’ of 6. This means that at least 6 bits
must have been received in error to obtain a false GOOD DATA result.
Summary of data link operation
Summary of data link layer:
• Receives link service data units from higher layer
• Forms the LPDU by including addresses, the control byte, and CRC codes
• Uses procedures to deliver the frame across the physical layer
• Function codes provide for link initialization and error recovery
• Reassembles LSDU at secondary station
• Provides status indications to higher levels
5.5 Transport layer (pseudo-transport)
5.5.1 Description of transport layer
The primary function of the transport layer in DNP3 is to implement message disassembly
and reassembly. This allows for the transmission of larger blocks of data than can be
handled by the data link layer. Because this functionality is fairly limited compared to the
wider OSI definition of the transport layer, the prefix ‘pseudo’ is often used. For simplicity
the layer is referred to in this text as the transport layer.
5.5.2 The transport protocol data unit
The transport layer takes the user data, the transport service data unit (TSDU) and breaks
it down into one or more transport protocol data units (TPDUs) and sends each to the data
link layer for sending. The TPDUs become the user data within the data link layer, that is
the LSDUs.
Recalling that the LSDUs can be a maximum of 250 bytes of user data, this defines the
maximum size of the LPDU, including any transport layer overheads. In fact the transport
functions are accomplished with a single header byte, leaving 249 bytes for carrying data.
This is pictured in the diagram following.