PSTN Outstations communicate with the ClearSCADA server using a dial-up modem, and are grouped into PSTN Outstation Sets.
The ClearSCADA server and the PSTN outstations communicate via the Public Switched Telephone Network. This means that communications are only established under certain circumstances—in order for the outstations to report (dial in) to the ClearSCADA server when an alarm condition occurs, according to a schedule, or upon request.
With some protocols, you can associate a number of PSTN Channels with each PSTN outstation, to enable that outstation to dial out to the server when an alarm occurs (see the driver-specific guide for information on how many channels you can associate with each outstation). Ideally, some of these channels should be dedicated Dial In lines, to improve the likelihood of the outstation being able to contact the server if an alarm situation occurs. You specify whether a channel is Dial In, Dial Out, or Dial In and Out, on the Form for the channel (see Defining PSTN Settings for a Channel). PSTN channels are not dedicated to individual outstations—they can be used by any outstation that uses the same connection type to communicate with the server.
PSTN outstations store the data from the measuring instruments and control devices to which they are attached. The data is available to the ClearSCADA server upon request.
When the ClearSCADA server makes a request for the logged data, it typically calls the outstation, the outstation reports the data, and then the communications line is typically closed. This type of request is called a One Shot request.
A similar procedure occurs when controls are sent from the ClearSCADA server—the ClearSCADA server calls the outstation, sends the control, retrieves any data, and then closes the line.
If required, a Queue feature can be enabled on your system to allow users to optionally queue certain pick action requests on PSTN outstations, rather than send those requests to the outstations immediately. For more information, see Define whether the Queue Parameter is Available, and see Optionally Queue Pick Action Requests on PSTN and ‘On Demand’Outstations.
PSTN communications are used when:
- A direct line connection is impractical due to the geographical location of the outstations (for example, radio communications may be impractical due to interference, or regulations regarding the erection of communication masts.)
- A direct line connection is not cost-effective (for example, it can be expensive to lay direct lines over long distances).
Additionally, outstations that usually use a permanent connection to communicate with the ClearSCADA server can have a dial-up modem to use PSTN as a ‘fallback’ means of communication. Such outstations typically only use PSTN to communicate with the server if ClearSCADA detects that the direct channel or channels via which they normally communicate have failed. Once permanent communications is re-established, the outstations return to using the direct channels (see Direct - PSTN Fallback).
For an example that demonstrates how PSTN Channels might be used for inbound and outbound PSTN communications, see Define Dial In Line Properties on a PSTN Outstation.
Further Information