A protocol defines the rules by which devices talk to each other. DNP3 is a protocol for transmitting data from Point A to Point B via serial and IP communications. Although DNP is most commonly used by electric, gas, water and wastewater utilities, it can be used anywhere a SCADA system is exists.

Why Do Devices Need to Talk to Each Other?
Utility providers commonly have lots of operations they need to monitor. Typically, there is a central operations center plus remote equipment in the field.  The central operations center houses their main computer.  Installations/substations house remote equipment in the field.  DNP is used to facilitate communication between the main computer and remote equipment, enabling the main computer to remotely open/close circuit breakers, measure line voltages, start/stop motors, open/close valves, check for errors, etc.

View Previous Blog Series on DNP3:

DNP3 Part 1 – Date/Time Stamping

DNP3 Part 2 – Communications Options

DNP3 Part 3 – Security

DNP3 Part 4 – Reliability

DNP3 Part 5 – Compliance

The first area that cellular comms started getting attention was for water supply and wastewater collection systems – as a backup to radio.

The requirements would state that radio was the primary communications and cellular GPRS was the backup on more important stations.

We also saw a few utilities who requested cellular as their primary communications method, either because they had had a lot of problems with radio in the past, or because their geography meant that building a radio network was clearly a lot more expensive than using cellular.

Let’s look at the thinking behind the backup first of all.

Historically, in water and wastewater, radio has the been the main method of RTU communications, with PSTN (phone lines) coming a long way behind in second place. One of the main benefits of radio was the fact that the utility owned the infrastructure and therefore “felt” some level of control over it.

In theory, if you own the infrastructure then you are able to run an operation which ensures an uptime that you are happy with, i.e., that meets the organizational requirements. If communications to one site goes down then you have a problem – but one that you can theoretically fix. If a repeater goes down, it’s the same situation.

Contrast that with cellular comms where if a cell tower goes down, or for some reason there are other comms problems in the backbone or to one area, you have to wait for the cellular operator to fix it.

But apart from those whose lifeblood is radio communications, this can also present a major disadvantage of radio communications. A lot of utilities don’t have the expertise to troubleshoot and fix radio systems, or to replace radio repeaters in the middle of a storm. Even if they do, there’s the question over response time. In later posts we will look at possible benefits of cellular, but for now, the radio “ownership” problem and the low cost of cellular comms raised the possibility of using cellular as a backup.