SCADA systems have evolved with the growth of moderncomputing technology. The first type of scada system used was Monolithic scada system (first generation). With a little more improvements came Distributed scada system (second generation). The third type of scada system, which is presently being used, is called the Networked scada system (third generation).
The first type of SCADA system is called Monolithic SCADA system...When scada system was first introduced, networks did not exist. So each centralized system stood alone with no connectivity to the other systems.Communication protocols used in the SCADA systems to communicate with the other units weredeveloped by the vendors. These protocols were generallyvery lean.That is, they supportedno functionality beyond that required.
Also, in Monolithic SCADA system, it was 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.The Monolithic SCADA system uses two identically equipped mainframe systems, a primary and a backup. 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 noprocessing was done on the standby system. This was the main disadvantage of Monolithic scada system.
As you can see from the diagram, every Remote Terminal Unit(RTU) is connected to the SCADA master system individually through a WAN.
The second type of SCADA system is called Distributed SCADA system...This generation of SCADA systems took advantage of the improvement in system miniaturization and Local Area Networking (LAN) technology.LAN technology was used to distribute the system functions across multiple systems. This increased the processing speed of the system operation.Multiple stations were typically of the mini-computer class, smaller and less expensive thantheir first generation processors.
The networks that connectedthese individual systems were generally based on LAN protocols and were not capable ofreaching beyond the limits of the local environment.Some of these LAN protocols that were used were of a proprietary nature, where the vendor created its own network protocol.This allowed a vendor to optimize its LAN protocol for real-time traffic, but itlimited the connection of network from other vendors to theSCADA LAN.Distribution of system functionality also improved the redundancy and reliability of thesystem as a whole.Rather than the simple primary/standby failover scheme that wasutilized in many first generation systems, the distributed architecture kept allstations on the LAN in an online state all of the time.So, in case of a failure of one system, another system could be used to operate it.
As you can see, every system is connected to the Local Area Network (LAN). A communication server is used to distribute the processing across multiple operating systems.
The third type of SCADA system is called Networked SCADA system...The primary difference between this type and the second generation is being that of an open systemarchitecture rather than a vendor controlled environment.Multiple networked systems are also used, sharing master station functions.The major improvement in the thirdgeneration 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 SCADAsystems. The utilization of these systems makes it easier for the user to connectthird party peripheral devices tothe system and/or the network.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 across a WAN.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.
As you can see, the communication servers and the RTUs are connected to the Wide Area Network (WAN). Scada Master controls the whole network through the WAN.