Practical_modern_SCADA_protocols_-_dnp3,_60870-5_and_Related_Systems

308 Practical Modern SCADA Protocols: DNP3, 60870.5 and Related Systems
10.1.2 Differences
In this section a number of areas are compared briefly to summarize the points of difference
between T101 and DNP3.
10.1.3 Addressing
• T101 uses both link addresses and application addresses
• This gives greater flexibility in routing messages
• T101 has larger point address range, up to 3 bytes gives 16 777 216 addresses
• DNP3 uses link addresses only, no application layer addresses
• DNP3 link carries both source and destination addresses
Overall, T101 has greater flexibility in its addressing system, both by including data link
and application level addresses, and through the use of variable address lengths. The
benefit of variable lengths is that they allow savings on communications bandwidth when
only small numbers of addresses are required.
Data link communications
• T101 uses unbalanced and balanced (limited to point–point only)
• T101 does not support unsolicited messages on multidrop communications
• DNP3 uses balanced communications only
Both DNP3 and IEC 60870-5-101 support balanced or peer–peer communications,
however, IEC’s balanced communications are limited to point-to-point configurations.
This can be a significant limitation if a situation requiring a multidrop configuration is
contemplated. An example would be where there are a large number of outstations
connected to a limited bandwidth channel such as a VHF radio link. In such a case polling
for data may require an unacceptable bandwidth, and DNP’s support for multidrop
balanced communications would be an advantage in implementing a reporting by exception
system.
Frame format
• T101 uses FT1.2 frame; 8 bit checksum, length up to 255 byte
• T101 frames are fixed and variable length
• DNP3 uses FT3 frames; 16 bit CRC, length up to 255 byte
• DNP3 uses variable length only
When the fixed length frame option is used under T101, a very short and simple
message is created in comparison to DNP’s message. This reduces communications
overheads substantially.
Application functions and data objects
Application functions:
• T101 allows only one control point per message
• T101 uses single character application acknowledgment
• DNP3 allows control over multiple points in one message