A SCADA (Supervisory Control and Data Acquisition) system is a common process automation system which is used to gather data from sensors and instruments located at remote sites and to transmit data at a central site for either controlor monitoring purposes. The collected data is usually viewed on one or more SCADA host computers located at the central or master site. Based on information received from remote stations, automated or operator-driven supervisory commands can be pushed to remote station control devices, which are often referred to as field devices.
Generally, a SCADA system includes the following components:
- Instruments that sense process variables
- Operating equipment connected to instruments
- Local processors that collect data and communicate with the site’s instruments and operating equipment called Programmable Logic Controller (PLC), Remote Terminal Unit (RTU), Intelligent Electronic Device (IED), or Process Automation Controller (PAC)
- Short range communications between local processors, instruments, and operating equipment
- Host computers as central point of human monitoring and control of the processes, storing databases, and display of statistical control charts, and reports. Host computers are also known as Master Terminal Unit (MTU), the SCADA server, or a PC with Human Machine Interface (HMI)
- Long range communications between local processors and host computers using wiredand/or wireless network connections.
SCADA systems differ from DCSs (Distributed Control Systems) which are generally found inplant sites. While DCSs cover the plant site, SCADA systems cover much larger geographic areas.Also, due to the remoteness many of these often require the use of wireless communications. Figure 1 shows an integrated SCADA architecture. SCADA architecture supports TCP/IP, UDP or other IP-based communications protocols as wellas strictly industrial protocols such as Modbus TCP, Modbus over TCP or Modbus over UDP, all working over private radio, cellular or satellite networks.
Figure 1. Integrated SCADA Architecture
In complex SCADA architectures, there is a variety of both wired and wireless media and protocols involved in getting data back to the central monitoring site. This enables implementation of powerful IP-based SCADA networks over mixed cellular, satellite, and landline systems. SCADA communications can employ a diverse range of both wired (lease line, dialup line, fiber, ADSL,cable) and wireless media (licensed radio, spread spectrum, cellular, WLAN, or satellite). The choice depends on a number of factors that characterize the existing communication infrastructure. Factors such as existing communications infrastructure, available communications at the remote sites, data rates and polling frequency, remoteness of site, installation budget and ability to accommodate future needs, all impact the final decision for SCADA architecture. In the following section, we discuss key issues in the development of SCADA systems. Therefore, a review of SCADA systems evolution allows us to better understand many security concerns.