You can use the ClearSCADA Server Configuration Tool to define how ClearSCADA determines which of the available PSTN channels are used for communications with a PSTN outstation.
There are two methods for deciding which PSTN channel is used to make a call to an outstation:
- Use Channel with Fewest Total Calls—ClearSCADA will attempt to make a call to an outstation via all available channels. The load is distributed evenly between the available channels, and only those channels that have failed (if any) are not used.
- The main advantage of the fewest total calls method is that all available channels are used regularly—ClearSCADA can detect the quality of calls on each channel on a regular basis.
- A disadvantage of the fewest total calls method is that a channel that has a history of failing regularly will still be used (if it is available) even though it may not be as reliable as other available channels.
- Use Channel with Highest Quality—ClearSCADA will call an outstation via the channel that has the highest quality factor (of the channels that are currently available).
- When PSTN channels are created, they are allocated a quality factor. This is a number that is defined by the Quality Factor setting (in the Channels settings on the Server Configuration Tool).
- If a PSTN channel is used for a successful call, its quality factor rises by 1 (up to the maximum limit, which is defined by the Quality Factor setting); if it is used for a failed call, its quality factor drops by 1 (it cannot drop below 1).
- When ClearSCADA has to call an outstation or similar device via a PSTN channel, it uses the channels’ quality factor to determine which of the available channels should be used. To do this, it examines the quality factor of each available channel - the channel with the highest quality factor is deemed to be the most reliable channel and so is used. If there are multiple channels all with the same highest quality factor, ClearSCADA uses these channels sequentially. This means that ClearSCADA uses a variety of ‘good’ channels rather than continuously using the same ‘good’ channel each time.
- The Quality Factor setting is only available for use when you select the Use Channel with Highest Quality setting. By default, it is set to 5 which is regarded as suitable for many systems.
- If you set a higher number for the Quality Factor, you raise the maximum number of consecutive failed calls that can occur before a different channel is used. Similarly, if you set a lower number for the Quality Factor, you lower the maximum number of consecutive failed calls that can occur before another channel is used.
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The current quality factor for a channel can also be reset to the Quality Factor that is defined in the server configuration. To do this, you need to use the ResetCallQualityFactor action for the PSTN Channel.
To define the method used for selecting PSTN channels:
- Access the ClearSCADA Server Configuration Tool (see Accessing the ClearSCADA Server Configuration Tool).
- Expand the Global Parameters branch of the tree structure.
- Select Channels to access the Channels section.
- Use the option buttons in the PSTN section to choose the required method of PSTN channel selection:
- Select the Use channel with fewest total calls option button to enable balanced load sharing. We recommend that you use this method unless there are known issues with the communication lines.
- Select the Use channel with highest quality option button to set ClearSCADA to choose a channel based on its availability and quality factor. We recommend that you only use this method if there are known issues with the communication lines.
- When you choose the Use channel with highest quality option, the Quality Factor field becomes available. In the Quality Factor field, enter the number that will be used as:
- The quality factor that will be allocated to every new PSTN channel
- The highest quality factor that any PSTN channel can have. Each PSTN channel has its own quality factor that rises and falls when it is used for good calls and failed calls respectively. But the quality factor of a channel cannot drop below 1 or rise above the amount you define in the Quality Factor field in the Server Configuration Tool’s Channels settings.
- Right-click on the system icon in the tree-structure, and select the Apply Changes option to apply the changes.
When you have finished defining how ClearSCADA should choose a PSTN channel, you can either configure other Channels settings. For more information, see the topics that are listed in the gray footer section at the bottom of this topic. Select the relevant entry to display the topic that you require.
Or:
Continue with the server configuration. If you are unfamiliar with the server configuration process, we recommend that you proceed to learn about Driver Settings.