Practical_modern_SCADA_protocols_-_dnp3,_60870-5_and_Related_Systems

Fundamentals of distributed network protocol 75
Before moving to the EPA model, we will use the general OSI reference model to illustrate
message buildup. This will demonstrate the general principle and introduce the idea
of message headers. These concepts will assist us in understanding the details of message
construction under DNP3.
Figure 5.2
Generic message buildup using the OSI 7 layer model
Some of the features and terminology illustrated by the message buildup are now
discussed.
Data flow during message construction is from the application level down to the physical
level, then across the physical medium, then at the receiving device up through the model
layers until the original message is regenerated.
The application data is given an application header by the application layer. This forms
the application protocol data unit or APDU. The APDU is the data unit used by the next
layer, the presentation layer. This adds its own header, and so on.
The data may be split into smaller size units during the process of message construction.
In this example, the session protocol data unit (SPDU) is broken into multiple data units
at the transport level.
The data link layer adds both a header, and a trailer section containing an error
detection code, forming the link protocol data unit (LPDU). At the physical layer this is
broken down into 8-bit blocks, each with start and stop bits appended for asynchronous
transmission.