Explain in detail about the sequence involved in DNP3 message build up? [7 Marks]
(Homework Solution): Question: Explain in detail about the sequence involved in DNP3 message build up? [7 Marks]
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answers
DNP3 model is designed on the basis of OSI 7-layer model. Two are hardware layers and the topmost is the software layer.
The three layers are:
1. Application layer
2. DataLink layer
3. Physical layer
DNP3 uses 3 layers but adds some transport function using an intermediate layer called pseudo-transport layer which is somehow analogous to Transport and Network layer of OSI model.
DNP3 message buildup AT THE SENDER END takes place in a series of steps. Each layer receives the information passes from a higher layer and adds some extra information as Header on the front of the message, This also increases the size of the message as we go down the layers. AT THE RECEIVER END, the process is just the reverse.
The following steps show how the message is build up in each layer AT SENDER END:
1. Application Layer: We assume there is an application running on the system and wants to send some data to the destination. This application runs above the application layer and there is no restriction on the size of data to be sent. Application layer divides the data into a manageably sized segment called Application Service Data Unit (ASDU). This ASDU is combined with a header to create Application Protocol Data Unit (APDU). The header is referred to as Application Protocol Control Information (APCI) having size 2 bytes or 4 bytes depending on whether the message is request or response. The max size of each segment (APDU) is 2048 bytes.
2. Pseudo-Transport Layer: The APDU from application layer is referred to as Transport Service Data Unit (TSDU) in this layer. Now, this is the pure data to be transported, as considered by this layer and thus adds its own header. TSDU is broken down to data chunk each of max size 249 bytes. 1 byte header is added. Thus a fragment of max size 250 bytes is created called Transport Protocol Data Unit (TPDU).
3. Data Link Layer: Each TPDU is added with a 10-byte header. Along with the header, this layer also provides provision for error detection and correction functions and codes. A 16-bit CRC is used. Each TPDU is converted to a frame called LPDU. The max size of an LPDU is 292 bytes. Here, the frame format is known as FT3 frame format.
4. Physical Layer: Each frame or LPDU is converted into a stream of bits over the physical layer.
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