SCADA architecture, legacy to modern structure

Distributed SCADA systems

The next generation of SCADA systems took advantage of developments and improvements in system miniaturisation and local area Networking (LAN) technology to distribute processing across multiple systems. Multiple stations, each with a specific function, were linked to a LAN in real-time and information become shared.

These stations were typical of the mini-computer class, smaller and much less expensive than their first-generation processors.

Some of these distributed stations served as communications processors, communicating mainly with field devices such as RTUs. Some served as operator interfaces to provide human-machine interfaces (HMI) for network operators. still, others served as computation processors or database servers.

Distributing the individual SCADA system functions across multiple systems provides more computing power to the system as an entire than would were available in a single processor.

The networks connecting those individual systems were generally based totally on LAN protocols and could not cross the boundaries of the local environment.

Some of the LAN protocols used were of a proprietary nature, where the vendor created their own network protocol or version instead of pulling an existing protocol off the shelf. This allowed a provider to optimize their LAN protocol for real-time traffic. However, the connection of third-party networks to the SCADA LAN has been restricted (or efficaciously eliminated)

The distribution of the system functionality to networked structures not only served to increase the computing power however also to increase the computing power to improve the redundancy and reliability of the system as an entire. Rather than the simple primary/standby failover scheme used in many first-generation systems, the distributed architecture always kept all stations on the LAN online.

For instance, if one HMI station fails, another HMI station could be used to run the system without waiting for failover from the primary system to the secondary system.

The WAN used to communicate with devices in the field was largely unchanged by the development of LAN connectivity between local stations at the SCADA master. These external communication networks were still limited to RTU protocols and have been unavailable for other types of network traffic.

As with the first generation of systems, the second generation of SCADA systems was also restricted to hardware, software, and peripherals that were provided or at least selected by the supplier.