Designing effective automation networks is a difficult task, not because the physical network is hard to design, but because of the number of applications and services that must operate within it. The physical network is made up of routers, switches, and firewalls, usually configured in a redundant tree structure. The tree has high-performance redundant core switches at the root, redundant distribution switches at sites or buildings, and access switches for each floor or area.

The NIST (National Institute of Standards and Technology) “Guide to Supervisory Control and Data Acquisition (SCADA) and Industrial Control Systems Security” provides models for the firewalls and routers in an automation physical network. However, designing an effective automation network also requires knowledge of all the soft networks (softnets) of applications and servers that use the physical network and that influence the structure of the physical network.

Softnets define services for authentication, data sharing, anti-virus, O/S (operating system) patches, application patches, DNS (domain name service), security certification, e-mail, printer servers, router/switch administration, and Microsoft Vista activation. Many softnet structures do not mirror the tree structure of the physical network.

One of the most important softnets provides authentication services. These services use LDAP (Lightweight Directory Access Protocol) services or Microsoft Active Directory (AD) servers to handle user authentication and security rights assignment. The authentication softnets are a hierarchy of replicated AD databases or a network of “trusted” LDAP servers with replicated servers on both sides of the automation firewalls. Unless carefully planned, the network traffic required to replicate the authentication databases may impact automation communication.

The data sharing softnet defines the applications and servers involved in exchanging data. This may be through simple file exchanges, transaction messages, or database operations. Data sharing across separate shop floor areas, such as from a production line MES to a laboratory LIMS (laboratory information management system) in another building, often requires ports through the protecting firewalls. One softnet service that directly impacts the physical network is a data historian replication network. Data historian databases often collect data from a protected area and replicate them in a more open area to limit real-time access through firewalls. Replication can slow network traffic and needs to be factored into the physical network design.

Anti-virus server networks and patch server networks are two additional softnets that often exist on both sides of an automation firewall. Fortunately, these softnets have little impact on network traffic, but they do require their own paths through automation firewalls. If you plan to use OPC-UA (Unified Architecture) or any secure communication protocol that uses X509 or any public key certificates, then you may need a softnet of security certification servers with their own paths through the firewall. Most automation networks will also contain a local DNS server that is connected to a corporate DNS softnet to allow local operation if communication to the corporate network is lost. The DNS softnet will need access through the firewalls. If you plan to use Microsoft Vista, do not forget about local Vista activation servers and their connections to corporate Vista activation servers.

Finally, to round out automation softnets, do not forget any e-mail servers that must cross the firewalls' print server networks to allow remote printing outside the protected area, and a method for IT maintenance to administer the various routers and switches on both sides of the firewalls. All the soft networks that layer on top of a physical network must be considered when designing an Ethernet TCP/IP-based automation network. Designing the automation softnets before finalizing the physical network design will result in faster startups and fewer security risks.