SCADA systems have evolved in parallel with the growth and sophistication of modern computing technology. The following sections will provide a description of the following three Architectures of SCADA systems:
• First Generation – Monolithic
• Second Generation – Distributed
• Third Generation – Networked
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1.Monolithic SCADA Systems
When SCADA systems were developed for the first time, the concept of computing in general focused on mainframe systems. Networks generally did not exist, and each centralized system was alone. As a result, SCADA systems were independent systems that had virtually no connectivity to other systems.
The wide area networks (WAN) that were implemented to communicate with remote terminal units (RTU) were designed with one sole purpose in mind: to communicate with RTU in the field and nothing else. In addition, the WAN protocols in use today were largely unknown at the time.
The communication protocols in use in SCADA networks were developed by RTU equipment suppliers and were often exclusively owned. In addition, these protocols were generally very “thin”, and did not admit practically any functionality beyond the scanning and control points required within the remote device. Also, in general it was not feasible intermix other types of data traffic with RTU communications in the network. The connectivity with the SCADA master station was very limited by the system provider. The connections to the master were usually made at the bus level through a patented adapter or controller connected to the backplate of the Central Processing Unit (CPU).
Redundancy in these first generation systems was accomplished by the use of two identically equipped mainframe systems, a primary and a backup, connected at the bus level. The standby system’s primary function was to monitor the primary and take over in the event of a detected failure. This type of standby operation meant that little or no processing was done on the standby system. figure shows a typical first generation SCADA architecture.
2.Distributed SCADA Systems
The next generation of SCADA systems took advantage of the development and improvement in system miniaturization and local area network (LAN) technology to distribute processing across multiple systems. Multiple stations, each with a specific function, connected to a LAN and shared information with each other in real time.
These stations were typically of the mini-computer class, smaller and less expensive than their first-generation processors. Some of these distributed stations served as communication processors, mainly communicating with field devices such as RTU. Some served as operator interfaces, providing the man-machine interface (HMI) for system operators. Others served as calculation processors or database servers. The distribution of the individual functions of the SCADA system in multiple systems provided more processing power for the system as a whole than would have been available in a single processor. The networks that connected these individual systems were generally based on LAN protocols and could not reach beyond the limits of the local environment.
Some of the LAN protocols that were used were patented in nature, in which the provider created its own network protocol or a version of it instead of extracting an existing one from the shelf. This allowed a provider to optimize its LAN protocol for real-time traffic, but limited (or effectively eliminated) the network connection of other providers to the SCADA LAN. The figure shows the typical second generation SCADA architecture.
The distribution of the functionality of the system through the systems connected to the network serves not only to increase the processing power, but also to improve the redundancy and reliability of the system as a whole. Instead of the simple primary / standby switching scheme that was used in many first-generation systems, the distributed architecture often kept all stations on the LAN in an online state all the time. For example, if an HMI station fails, another HMI station could be used to operate the system, without waiting for failover from the primary to the secondary system. The WAN used to communicate with devices in the field was not greatly modified by the development of LAN connectivity between local stations in the SCADA master. These external communication networks were still limited to the RTU protocols and were not available for other types of network traffic.
As was the case with the first generation of systems, the second generation of SCADA systems was also limited to hardware, software and peripheral devices that were provided or at least selected by the provider.
3.Networked SCADA Systems
The current generation of the architecture of the SCADA master station is closely related to that of the second generation, with the main difference being an open system architecture instead of a patented environment controlled by the provider. There are still several networked systems, which share master station functions. There are still RTUs using protocols that are owned by the provider. The main improvement in the third generation is to open the architecture of the system, using standards and open protocols, and enabling the distribution of SCADA functionality through a WAN and not just a LAN.
Open standards eliminate a series of limitations of previous generations of SCADA systems. The use of ready-to-use systems makes it easy for the user to connect peripheral devices of third parties (such as monitors, printers, disk drives, tape drives, etc.) to the system or the network. As they have moved to “open” or “commercially available” systems, SCADA providers have gradually moved out of the hardware development business. These providers have turned to systems providers such as Compaq, Hewlett-Packard and Sun Microsystems for their experience in developing basic computing platforms and operating system software. This allows SCADA vendors to concentrate their development in an area where they can add a specific value to the system: that of the software of the SCADA master station.
The main improvement in third-generation SCADA systems comes from the use of WAN protocols, such as Internet Protocol (IP) for communication between the master station and communications equipment. This allows the part of the master station that is responsible for communications with the field devices to be separated from the master station “properly” through a WAN. Vendors are now producing RTUs that can communicate with the master station through an Ethernet connection. The figure represents a SCADA network system.
Another advantage brought about by the distribution of SCADA functionality over a WAN is that of disaster survivability. The distribution of SCADA processing across a LAN in second-generation systems improves reliability, but in the event of a total loss of the facility housing the SCADA master, the entire system could be lost as well. By
distributing the processing across physically separate locations, it becomes possible to build a SCADA system that can survive a total loss of any one location. For some organizations that see SCADA as a super-critical function, this is a real benefit.