DNP3.0 - Distributed Network Protocol 3 

What is the DNP3.0 protocol?

DNP3 — Distributed Network Protocol 3

Dis­trib­uted Net­work Pro­to­col 3 (DNP3) is a set of com­mu­ni­ca­tions pro­to­cols used between com­po­nents for automa­tion sys­tems in elec­tric, indus­tri­al and water sectors.

It is a key pro­to­col in SCADA sys­tems, where it is pri­mar­i­ly used for com­mu­ni­ca­tions between a mas­ter sta­tion and RTUs or IEDs.

  • While IEC 60870–5 was still under devel­op­ment and was wait­ing to be stan­dard­ized, there was a need for an inter­op­er­a­ble com­mu­ni­ca­tions stan­dard to inter­con­nect var­i­ous mul­ti-ven­dor com­po­nents in the network. 
  • Thus, in 1993, the par­tial­ly com­plet­ed IEC 60870–5 pro­to­col spec­i­fi­ca­tions were used as a base for an open and imme­di­ate­ly imple­mentable pro­to­col that specif­i­cal­ly catered to North Amer­i­can requirements.

  • DNP3 is a lay­er 2 pro­to­col on the OSI model.

  • The pro­to­col pro­vides data frag­men­ta­tion, error detec­tion, mul­ti­plex­ing, link con­trol, pri­or­i­ti­za­tion, and lay­er 2 address­ing ser­vices.

  • For error detec­tion, it relies on cyclic redun­dan­cy check codes.

  • Fur­ther, it defines a trans­port func­tion (ana­log to lay­er 4) and an appli­ca­tion lay­er (lay­er 7) that defines func­tions and gener­ic data types for com­mon SCADA appli­ca­tions. 

  • Designed for harsh envi­ron­ments, DNP3 is more robust, effi­cient, and inter­op­er­a­ble than old­er pro­to­cols like Mod­bus, which is still some­times pre­ferred for its sim­plic­i­ty. 

Modbus vs DNP3

Mod­bus is an appli­ca­tion lay­er pro­to­col, where­as DNP3 con­sists of both an appli­ca­tion and data link lay­er. Anoth­er dif­fer­ence is that DNP3 also sup­ports unso­licit­ed mes­sag­ing. This means that RTUs can send updates when a change of sta­tus hap­pens, with­out wait­ing to be polled by the master.

Both pro­to­cols can be used over var­i­ous media, such as RS-232, RS-485, and TCP/IP.
While Mod­bus has a spe­cif­ic vari­ant for TCP/Ip com­munca­tions, DNP3 needs to be wrapped with­in TCP/IP.

Since you can send more data in small­er pack­ets and unlike Mod­bus, it is an event-dri­ven pro­to­col, mean­ing that net­work devices are able to trans­mit unso­licit­ed respons­es and con­ti­nu­ity is not required, using DNP3 can save lots of band­width. Fur­ther­more, DNP3 is high­ly stan­dard­ized and pro­vides high com­pat­i­bil­i­ty and inter­op­er­abil­i­ty between devices from many dif­fer­ent vendors.

Still, some pre­fer Mod­bus for its sim­plic­i­ty and the high num­ber of devices that sup­port the pro­to­col. So, both Mod­bus and DNP3 can be imple­ment­ed in func­tion­al and effi­cient SCADA sys­tems, mak­ing it strong­ly depen­dent on the project network.

IEC 104 vs DNP3

DNP3 and IEC-104 have both been specif­i­cal­ly devel­oped for SCADA com­mu­ni­ca­tion beyond the sub­sta­tion lev­el, for instance in order to con­trol a cir­cuit break­er from a Con­trol center.

How­ev­er, there are a few advan­tages of DNP3 that make it supe­ri­or in cer­tain applications.

  • For the trans­mis­sion of larg­er data vol­umes over longer dis­tances, DNP3 is pre­ferred as it sends larg­er but few­er data pack­ages than IEC-104, which sends a high num­ber of small packages.
  • DNP3 also enables faster data trans­mis­sion by oper­at­ing in high­er baud rates (num­ber of data bits trans­mit­ted in one sec­ond) than the IEC standards.
  • At last, if the band­width is lim­it­ed, DNP3’s data pri­or­i­ty class­es (Class 1,2,3) allow the con­troller to poll in var­i­ous fre­quen­cies, as opposed to IEC-104, which only pro­vides a con­stant polling frequency.

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