SCADA Architectures

3.0    SCADA Architectures

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 generations of SCADA systems:

•       First Generation – Monolithic

•       Second Generation – Distributed

•       Third Generation – Networked 

3.1 Monolithic SCADA Systems

 When SCADA systems were first developed, the concept of computing in general centered on “mainframe” systems. Networks were generally non-existent, and each centralized system stood alone. As a result, SCADA systems were standalone systems with virtually no connectivity to other systems.

The Wide Area Networks (WANs) that were implemented to communicate with remote terminal units (RTUs) were designed with a single purpose in mind–that of communicating with RTUs in the field and nothing else. In addition, WAN protocols in use today were largely unknown at the time.

The communication protocols in use on SCADA networks were developed by vendors of RTU equipment and were often proprietary.  In addition, these protocols were generally very “lean”, supporting virtually no functionality beyond that required scanning and controlling points within the remote device.  Also, it was generally not feasible to intermingle other types of data traffic with RTU communications on the network.

Connectivity to the SCADA master station itself was very limited by the system vendor. Connections to the master typically were done at the bus level via a proprietary adapter or controller plugged into the Central Processing Unit (CPU) backplane.  

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 3.1 shows a typical first generation SCADA architecture. 

3.2 Distributed SCADA Systems

The next generation of SCADA systems took advantage of developments and improvement in system miniaturization and Local Area Networking (LAN) technology to

Figure 3.1: First Generation SCADA Architecture [5]

Distribute the processing across multiple systems. Multiple stations, each with a specific function, were 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 communications processors, primarily communicating with field devices such as RTUs. Some served as operator interfaces, providing the human-machine interface (HMI) for system operators. Still others served as calculation processors or database servers. The distribution of individual SCADA system functions across 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 were not capable of reaching beyond the limits of the local environment.

 Some of the LAN protocols that were used were of a proprietary nature, where the vendor created its own network protocol or version thereof rather than pulling an existing one off the shelf. This allowed a vendor to optimize its LAN protocol for real-time traffic, but it limited (or effectively eliminated) the connection of network from other vendors to the SCADA LAN.  Figure 3.2 depicts typical second generation SCADA architecture.

Figure 3.2: Second Generation SCADA Architecture [5]

Distribution of system functionality across network-connected systems served not only to increase processing power, but also to improve the redundancy and reliability of the system as a whole. Rather than the simple primary/standby failover scheme that was utilized in many first generation systems, the distributed architecture often kept all stations on the LAN in an online state all of the time. For example, if an HMI station were to fail, another HMI station could be used to operate the system, without waiting for failover from the primary system to the secondary.

The WAN used to communicate with devices in the field were largely unchanged by the development of LAN connectivity between local stations at the SCADA master. These external communications networks were still limited to 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 vendor.

3.3 Networked SCADA Systems

The current generation of SCADA master station architecture is closely related to that of the second generation, with the primary difference being that of an open system architecture rather than a vendor controlled, proprietary environment. There are still multiple networked systems, sharing master station functions. There are still RTUs utilizing protocols that are vendor-proprietary. The major improvement in the third generation is that of opening the system architecture, utilizing open standards and protocols and making it possible to distribute SCADA functionality across a WAN and not just a LAN.

Open standards eliminate a number of the limitations of previous generations of SCADA systems. The utilization of off-the-shelf systems makes it easier for the user to connect third party peripheral devices (such as monitors, printers, disk drives, tape drives, etc.) to the system and/or the network.

 As they have moved to “open” or “off-the-shelf” systems, SCADA vendors have gradually gotten out of the hardware development business. These vendors have looked to system vendors such as Compaq, Hewlett-Packard, and Sun Microsystems for their expertise in developing the basic computer platforms and operating system software. This allows SCADA vendors to concentrate their development in an area where they can add specific value to the system–that of SCADA master station software.

The major improvement in third generation SCADA systems comes from the use of WAN protocols such as the Internet Protocol (IP) for communication between the master station and communications equipment. This allows the portion of the master station that is responsible for communications with the field devices to be separated from the master station “proper” across a WAN.  Vendors are now producing RTUs that can communicate with the master station using an Ethernet connection. Figure 3.3 represents a networked SCADA system.

Figure 3.3: Third Generation SCADA System [5]

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.