You posted your question back in January, so this response may no longer be of interest.
The traffic model for a SCADA system depends critically on the nature of the scan process.
There are three alternatives commonly used. Each requires a different model.
a. Periodic scans with all data returned with each scan response. (For example, scan all status points every 5 seconds and all analog points every 30 seconds.)
b. Periodic scans with only changed data returned (“report by exception”)
c. No scanning, with remote terminals reporting changed data on detection (“unsolicited data reporting.”)
The traffic with the third alternative is the closest to a Poisson distribution, where the Erlang calculations can be applied.
If otherwise, you can drop me e-mail, and I’ll see if I can help.