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 [ 24 ].
Figure 2.3 depicts typical second generation SCADA architecture.
Figure 2. 3 : Second Generation SCADA ArchitectureDistribution 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 [24]. 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 [ 24 ].