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 system 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.