The Specification states that the IED Configurator tool (or the IED) must determine which communications service shall be used to bind with an external signal (ExtRef element).
This statement is easily satisfied when the signal for binding is published using a single service only. However, if the signal is available through more than one service (i.e. GSE and Reporting), un-deterministic behaviour in the 'service selection' algorithm will be introduced, i.e. which service will be used and does any one in particulare take preference?
Given a scenario where an IED wants to subscribe to a time critical signal; How does the configurator tool (or IED) identify this signal as being 'time critical', such that it can select the most suitable communications service available e.g. selecting GSE for protection signalling rather than the Reporting service(RCB's). It could also be argued that selecting the Reporting service in this instance could potentially be a failure in system configuration.
The same can also apply for lower priority signals, where GSE maybe selected (overkill!) in favour of Reporting (preffered), hence potentially wasting valuable bandwidth.
I propose this issue is resolved by adding an optional attribute "source" to the ExtRef element allowing the user to state the required communication service to bind with this signal. Possible values for the "source" attribute could be:
gse - For GOOSE communications service
rcb - For Reporting communications service
any - For all/any communications service (Default value)
(I'm sure there are others too...)
The default operation when the attribute is not provided would be vendor/algorithm specific and should be documented in the appropriate conformance statement.
This solution will also provide additional integrity tests on a system configuration - If the specified service does not publish the required signal there is probably a configuration error that requires attention.
If the IED tool has a choice to select a service, then it needs additional information, in above case if the signal reception is real time critical or not, further if the time stamp is needed or not, and finally, which services (as a client) the IED really supports. Therefore it is a local issue of the IED tool based on information not available in the SCD file to decide, via which service it takes in the signal. In any case it is no additional bus load, because the GOOSE as well as the report are already configured to be sent to the IED - it just has the choice, from where it takes it in for which purpose...