In Supervisory Control and Data Acquisition (SCADA) systems, the reliability of telecommunications is crucial for efficient operation and safety. To ensure that communications between various components in a SCADA system remain uninterrupted, it is important to follow best practices for telecommunications. This article outlines some of the key best practices that should be followed when implementing telecommunications in SCADA systems.
Communications Interface and Connectivity
The first step towards implementing a robust telecommunications infrastructure in a SCADA system is to ensure that Industrial Ethernet using a self-healing ring configuration topology is used for all new wired SCADA communication connectivity architecture. Additionally, Industrial Ethernet switches should be used to expand existing installations of fiber optics communication networks.
If Peer to Peer Communications between Emergency Shutdown (ESD) systems is required, the network design should follow the requirements outlined in 34-SAMSS-623.
Communication Channels
To ensure that communication data links are logically segregated from any other services such as Voice, CCTV, and all other IT services, segregation at the SCADA Host level should be done using separate network cards and switches.
In cases where the Remote Terminal Unit (RTU) protocol supports exception polling, the communication software should make use of it to optimize data communication throughput and to provide rapid alarm throughput and capture of multiple, rapid succession alarms.
When IP based data communication is used, the SCADA Host station communication subsystem should include functionality to limit the number of open communication ports. The number of open communication ports should not degrade the overall system performance.
SCADA communication channels should be available to the RTU continuously and without change to message routing to achieve adequate system response time. The system should verify the operation and periodically test and validate the integrity of the primary and backup communication ports and the communication channels, and shall alarm on any failure. The availability of the failed channel should be checked using retries at least once every minute.
The system should alarm when any RTU fails to respond to a message after three unsuccessful retries. Additionally, the system should be configured to generate an alarm when communication error rates exceed a predetermined limit.
The communication facility should provide bidirectional data transfer with maximum bit error rate (BER) of 10e-7 for wired links. For wireless communication, a bit error rate (BER) of 10e-6 is acceptable.
The SCADA system should be configured to collect and historize critical communications statistics covering the health and performance of each communication channel for each RTU connected to the system. A communications overview display should be built to enable engineers to quickly ascertain the health of the overall communications network.
Telecommunications Channels Redundancy
While redundant network interface in the RTU is generally not required, for applications where alternate communication routes are required per the Functional Safety Design (FSD), i.e., wired and wireless, the SCADA Host shall monitor the availability of both data channels.
When the SCADA Host declares any of the two communication channels inoperative or marginal, it should discontinue its use, issue an appropriate alarm to the local operator, and transmit all subsequent messages on the alternate channel.
Network Management
Recoverable and unrecoverable communications errors should be counted by the system for each communications channel and stored in a history file. A graphical display should be provided to show the health status of the Network infrastructure devices such as switches, routers, and gateways.
Conclusion
Telecommunications is an essential component of SCADA systems, and following best practices is critical for ensuring their reliability. By implementing the guidelines outlined in this article, SCADA system operators can ensure that their systems are designed to operate efficiently and safely, and are capable of withstanding unexpected interruptions.
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