Practical_modern_SCADA_protocols_-_dnp3,_60870-5_and_Related_Systems

5.3.6 Full-duplex procedures
Fundamentals of distributed network protocol 83
Direct link
No collision can occur as both master and slave have their own channel to each other.
Multidrop link
Time delays are used as for half-duplex multidrop, except that the fixed time delay may be
reduced, as it is not necessary to provide this for the master. In the full-duplex multidrop
environment, the master has its own channel to all receivers.
5.4 Data link layer
5.4.1 Description of data link layer
The purpose of the data link layer is to establish and maintain reliable communication of
data over the physical link.
Link establishment involves setting up the logical communications link between sender
and receiver. DNP3 is capable of supporting either connection oriented or connection-less
operation. Thus, if a channel operates over a PSTN line and requires connection by dialling
before communication can commence, the data link layer manages this without any
direction from higher levels.
As seen previously, the data unit at the link layer level may be called the frame.
The frame has a maximum size of 292 bytes including CRC codes, and carries a total of
250 bytes of information from the higher levels. The frame includes 16-bit source
and destination addresses in its header. These provide for 65 536 different addresses. The
address range FFF0–FFFF is reserved for broadcast messages, which are intended to be
processed by all receivers. Note that addresses are logical in the sense that it is possible
for one physical device to have more than one logical address. In such a case the different
addresses would appear as separate devices to the master station.
The frame header also contains a function code. The functions supported by this are
those required to initialize and test the operation of each logical link between sender and
receiver. As an additional security feature every frame transmitted can request a confirmation
of receipt. This is termed link layer confirmation.
These services and functions are expanded upon in the following sections.
5.4.2 Services provided
The following services are all provided by the data link layer. Understanding how these
are provided begins with examining the frame format. The frame format includes necessary
control bits for controlling message flow, functions, error detection and correction.
Services provided by the data link layer:
• Establish and maintain the communication channel
• Report on link status to higher layers
• Detect and correct errors that may occur during transmission
• Conversion of data between LSDUs and LPDUs
• Error notification to higher levels
• Sequencing and prioritized delivery of LSDUs