GPRS application IN SCADA

There are some drawbacks in the IP/Ethernet SCADA systems world such as network vulnerability and stochastic network (unpredictable latencies). These disadvantages can be resolved by using networking solutions that are designed and optimized for SCADA over GPRS. SCADA systems serving critical applications must utilize highly reliable communications and shall have at least the important features as below:
• Communication networks should support immediate availability and be ready on demand sessions in both direction between RTUs and SCADA control center.
• RTUs should be instantly and easily accessible from the SCADA control center or from another RTU connected to the system.
• To maintain lengthy messages, data reliability, and file transfer reliably, end to end acknowledgement within the protocol layers and not just via the application.
• The system must be able to limit the time of communicated message validity. If a message can not be delivered within a pre-defined time slot, it shall be canceled with indication on non delivery to the sending device.
• SCADA wireless network should be secure in order to eliminate illegal access by hackers, injection of harmful viruses and electronic abuse attacks.
• The system must allow remote diagnostics upload and download of application and the flexibility to use alternate routing when the main link or networks fails.
• Use advanced communication protocols suitable for operation over fading radio links, by using advanced error detection and error correction schemes which are efficient for SCADA.

When the SCADA uses a public GSM or GPRS networks, users should set realistic expectation and remember that there is a compromise between cost and performance, since the operating parameters relating to traffic of data are affected by the factors as below:
• Network loading might be uncontrollable and unpredictable.
• Re-establishing a disconnected IP link might take up to 10 seconds.
• Often over loaded networks.
• SCADA users in GPRS have no priority over regular cell phone.

GPRS

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 theSCADA 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.