Versatile protocols
May 2009
Fieldbus & Industrial Networking
Jaco Hoogenboezem, SCADAGroup
Distributed network protocol (DNP3) is a protocol that was developed to allow for flexible, secure communications between devices
To use the GPRS (GPRS/EDGE 3.5G) platform as a communication medium in machine to machine (M2M) applications is often the only cost viable option available. The costs and time delays involved in trenching, radio repeater stations and the theft of cables has forced the recognition of this as a viable and cost-effective alternative.
Applications where M2M via GPRS has been implemented very effectively are:
* Borehole monitoring and control.
* Remote pump stations and pipe lines.
* Remote automated meter reading.
* Environmental monitoring.
One very interesting application used two different network operators at each point for redundancy, using the most available operator at each location to operate a pipeline over country borders with zero downtime of data.
GPRS is an enhancement of the GSM mobile phone network that natively supports the Internet Protocol (IP). Service providers charge for GPRS usage based on the amount of data transferred rather than the duration of the session. This type of technology if correctly used, allows monitoring and control of remote sites at a very low monthly cost – the service charge.
However, most M2M applications using the GPRS platform use non-standard programmed methods to contain the amount of data transmitted for the obvious reason of controlling the data service charge. Especially if analogue values are transmitted, a lot of the options available in the DNP 3 protocol are programmed by hand. Some implementations are more user friendly, but some lead to under and/or over reporting of data. These methods can often not handle variable reporting speeds. Certain analogues, such as vibration, pressure and flow for instance, will be unchanging for a long period of time, but when it really needs reporting, will probably be under-reported.
The DNP 3 protocol is one of the most complete and versatile protocols for communicating among devices for the utility market (ie, water, electricity and gas). Combining these two will decrease service costs, enhance efficiency and increase transmission speed, and turn-around time. For example, how long will it typically take to remotely start/initiate a device or event over GPRS, and get the condition/result feedback?
Some of the requirements for devices supporting DNP 3 protocol are data storage and time stamping. This is just perfect for GPRS communication as delays due to network overloading will not result in any data loss, and the data is buffered in the source device until being accepted by the destination device.
A large amount of devices today support the DNP 3 protocol.
The device capturing the information, on which the DNP 3 protocol is then applied, is normally handled in one of the following categories:
* PLC or RTU (with direct I/O).
* Fault recorders.
* Transducers.
* Meters (ie, electricity meters).
* Relays (ie, protection relays).
* Speciality devices such as battery chargers and protocol analysers.
* Scada’s and scada master.
Telemetry applications, be it for the utility markets, environmental monitoring or control, sub-station monitoring and control, remote metering or remote plants like pump stations, pipe lines or railway lines, are all applications that can leverage these technologies.
For more information contact Jaco Hoogenboezem, SCADAGroup, +27 (0)83 282 5706, [email protected], www.scadagroup.co.za
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