WHEN selecting a satellite communications solution for SCADA (Supervisory Control and Data Acquisition) applications, factors to consider include security, reliability, types of connectivity and recurring communications costs. Additional aspects are: dedicated networks, communications protocols and user control.
FOR greater security against hacker attacks and virus infestation, any mission critical SCADA communications infrastructure must be isolated from public communications networks.
DUE to the critical nature of most SCADA operations, reliable communication is crucial. Satellite transponder availability is typically in the 99.99 percentile range, which outperforms terrestrial networks.
The TSAT 2150, a satellite communications network tailored for SCADA applications, deploys a miniature satellite hub at the user’s SCADA control centre. Data to and from remote terminal units (RTUs) goes directly into the SCADA system. There is no vulnerable terrestrial back-haul from a communication service provider’s facility, which can cause the entire network to crash if cut during public works i.e. through digging.
To increase reliability, the TSAT hub is frequently deployed in a redundant/load sharing configuration. This ensures the hub is available more than 100 percent of the time.
TYPES OF CONNECTIVITY
CONTRARY to enterprise related communications, which occur randomly, SCADA communication is predictable. It is also continuous, polling RTUs at regular intervals.
The outgoing poll request is a short datagram (packet), containing as little as 10 bytes. The returned data is also in datagram format, with the message size ranging from 10-250 bytes.
A satellite solution based upon dial-up connectivity such as Inmarsat, Iridium or Globalstar would suit this application environment.
However, SCADA is not just about data collection but also control (which at times can be of an emergency nature).
Here you can’t simply wait for the system to encounter a busy connection. Instead, the system must provide an “always-on” connection, called “leased line connectivity”.
The TSAT network supports both circuit switched (leased line and multidrop) and packet switched (TCP/IP) applications concurrently.
RECURRING COMMUNICATIONS COSTS
SINCE a typical SCADA application runs for at least 10 years, the cost of satellite communications can become astronomical unless a proper solution is selected. Commercial operators provide leased line connectivity, based on a business model not unlike that of mobile phone companies i.e. they provide an inexpensive satellite terminal but tie the user to their service exclusively.
TSAT’s solution utilises satellite transponder bandwidth effectively and operates at a low cost.
Since the typical SCADA RTU transmits at low data rates (2400-9600bps), TSAT minimises the use of bandwidth. A typical TSAT network operates with as little as 50 kHz.
Because the TSAT network is compatible with all commercial fixed stationary satellites (FSS), having Ku- or C-band transponders, the user is able to select the satellite operator that has the best coverage and the best price.
TSAT makes transponder lease arrangements on the user’s behalf. It is not unusual for TSAT networks to recur monthly costs of less than US$1000 for the entire network.
WITH shared hub services, multiple customers share the same frequency spectrum. From a security perspective, this may not be desirable.
Furthermore, as previously mentioned, data to/from RTUs is routed through the service provider’s hub facility - in most cases through a dedicated (leased) line.
The cost for this line can be staggering, since in many cases the connection will be going to a different city, state and or even country.
To eliminate terrestrial back-haul, the service provider may offer an alternative connection called a Double-Hop. This involves installing a satellite terminal at the user’s SCADA control centre. Here, the data has to travel from the centre to the satellite, then to the hub, back to the satellite and finally to the remote terminal. The return reply from the RTU makes the same trip in reverse. The implication of this Double-Hop is a time delay. For time-sensitive control applications, this additional delay may not be acceptable.
The TSAT hub pipes the data directly into the SCADA computer without terrestrial back-haul or Double-Hop.
SOME service providers only support IP-based application traffic. In reality, most of the SCADA world deals with legacy hardware that’s based on asynchronous RS232 serial communication protocols.
To complicate matters, a typical SCADA network may include RTUs from several manufacturers, each utilising their own set of communications protocol.
TSAT is designed to manage multiple communications protocols concurrently and transmit data transparently over the network.
Older RTUs generally do not have “report by exception” (RBE) capabilities, which increases a SCADA system’s responsiveness to real-time events.
Apart from gaining the desired responsiveness, traffic over the satellite link drops, increasing the network’s throughput. Here, throughput increases of up to 150 percent have been realised.
SINCE no two SCADA systems are alike, it is imperative that any solution allows the user to optimise network design.
This is achieved by logically grouping RTUs to a specific serial port on the SCADA system’s front end processor, assigning bandwidth resources to individual RTUs or group of RTUs to optimise system performance, and defining the optimal satellite link access scheme for the application.
The TSAT network management system is provided with a graphical user interface that makes configuration, system management and support easy for the non-satellite professional SCADA engineer.
Con Kondilis, business development executive, Australian Satellite Services