Network Architectures 97 NETWORK ARCHITECTURES As with all networks, industrial networks vary considerably. However, because many common functions within industrial systems vary widely--from automation systems, to supervisory and control systems, to business systems--there are natu- ral demarcations within the network where these systems intersect. Table 5.1 indi- cates some of the major difference between these functional groups. The primary requirement of an industrial automation system is real-time operation and reliability, while the primary requirement of a business network might be high bandwidth and low operation costs. These requirements drive the use of real-time fieldbus protocols within control system processes and control loops, while business networks utilize fast, low-cost Ethernet networks and TCP/IP. SCADA systems sit between these two very different networks. In many ways, SCADA systems share the requirements of the control system itself--they need to be able to operate in real time, for example. However, they must also communicate with business systems over TCP/IP. For this reason, a DMZ is recommended for supervisory systems. The SCADA DMZ sits between the operational and automation systems that they are supervising and controlling, and the business networks and business information systems. The