The implications of a secure automation from anywhere in the world extend beyond nightmare troubleshooting scenarios, such as a remote PLC in the middle of a desert with only basic cellular coverage connecting to the primary information center (SCADA PC or database server) across the Internet. How is this accomplished in a secure, simple fashion?
The answer is the humble Virtual Private networking (VPN) connection, sometimes referred to as a VPN tunnel. This particular piece of technology allows two disparate networks separated by the cloud to connect to each other as though they are on the same local networking, albeit with a substantial penalty in connection speed. This penalty is not particularly relevant when passing small pieces of data, such as a SQL command or a Modbus function call. It is now even possible to send programs to PLCs and monitor visualizations of industrial networks across such tunnels. These capabilities allow for truly unparalleled possibilities.
VPNs are typically established between two pieces of network infrastructure, such as a commercial router and an industrial hardware VPN solution. Alternatively, software VPNs are available that would allow a PC to connect to the remote networking. Software VPNs are particularly useful for the traveling technician, allowing remote troubleshooting to one customer’s site while at another site for long-term support. This sort of flexibility means that a technician can, quite literally, be in two places at once.
refer to:
http://www.automation.com/leveraging-it-technology-for-industrial-controls-applications