Tuesday, January 18, 2011

GPRS Communications for SCADA

The GPRS service offered worldwide is attractive for SCADA systems as operators network charge only fot he data volume and not for the connection time as in a circuit switch connection GSM. All sites shall ideally have their own pre-assigned Static or Fixed IP address in a SCADA system it is required to create a sort of private IP based link between SCADA control center and RTUs.

However due to the shortage of IP address and the IP address space (4 bytes as per IPv2 standard GPRS/GSM operators are reluctant to allocate a pre-assigned IP address to each RTU. One possible solution is that data communications over GRS can be achieved by using Dynamic IP addresses which are assigned on demand to subscribers for a short session.

A master SCADA site typically is allocated a fixed IP address, and all RTUs have temporary IP addresses that are usually dropped after a pre-defined timeout. This operating mode allows the RTU subscribers to initiate a session with the SCADA control center but not vice versa. Once the session is complete or the connection fails, the allocated IP address is released and returned to the common NAT (Network Address Translation) server, holding the available IP addresses. When polling of an RTU is required by the SCADA control center, a solution to obtain and maintain a Dynamic IP Address for each RTU can be achieved by using an RTU initiated periodic contention message.

SCADA system based on GPRS communication technology may work well fairly, however this solution can be applied only for simple and non time critical applications. The method described above is not suitable for demanding SCADA applications, as RTUs can not be polled frequently. This method also does not allow RTU to RTU communication. Under the classification of demanding or critical SCADA applications:
• Real time SCADA system designed for mission critical systems.
• Near real time operating SCADA applications, etc.


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