With Logix PLC’s, and actually, almost all PLC’s, you have to actively poll them to extract the data; they don’t just pump it out on a schedule.
So, you have “Hey, PLC, What is the value of SomeTagInThePLC” written, then “My Value for SomeTagInThePLC is 12345”.
You can make logix communications more efficient if you are polling for large arrays, so you ask once and receive 1000 items back
From your question, you are thinking in terms of “Upload” meaning writing a value to the PLC, but as far as your modem is concerned, just asking for a value is an upload, and receiving it is a download.