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You may have heard it that some people say this or that protocol is or isn’t good for wireless environment. Here is what is different in wireless environment (yes, I admit this is a big generalization) compared to a dedicated line is:

·         Higher level of noise and bit error rate

·         Single channel is shared by many remote devices

·         Higher propagation delay

·         Hidden nodes, meaning that there may be nodes that cannot see each other and as a result step on each other’s toe

·         Wireless signal issues such as fading and bouncing

So a communication protocol needs to have enough ammunition to combat against these issues. I like to see these features in the protocol to overcome the above problems:

·         Moderately small size of data packets so that the probability of packet corruption goes down

·         Ability to fragment the frames so that the longer frames can be sent in smaller independent packets

·         Enough number of CRCs inside and at the end of the packet to prevent accepting the corrupt packets

·         Link acknowledgment to signal the other side that the packet is received

·         Sequence numbers so the consequent packets can be differentiated and also a single packet cannot be accepted more than once

·         Being able to send unsolicited messages to prevent wasting the bandwidth through repeated polls

DNP3 seems to have all of these features. So why do people complain so much? Because:

·         The RTU is not intelligent enough when to send the response, meaning it may send it when the channel is already seized by someone else. The remote device needs to be monitoring its own radio signals before putting data on the line

·         The radio communication is such that the nodes cannot see each other. If we suffice to packet-acks, the throughput would not go beyond 20% of the channel capacity. There are better ways to combat the hidden node issue.

·         Most often DNP3 master drivers are not designed to consider the limitations of the wireless environment. One of the most common mistakes is overloading the channel with many requests at the same time.

·         Timing, Timing, Timing: It is very important to tune your timing parameters at the master so that for example a delayed packet is not rejected due to link timeout setting.

I am planning to put together a DNP crash course in near future for those who like to know more about this popular protocol, stay tuned.

Take care for now, Rod