SUPL, GPSOne (Xtra) and GPS Modes

I’m a smidgen confused about this.

I have set my SUPL url to the “standard”
I have checked and verified the settings for GPSOne (XtraDataUrls etc.).
If the modem is in standalone mode, the GPSXtra Status is all good - has time and current data files. It takes a while to initialize because it still has to begin integration of satellites and determine SNR to determine which birds to use and where it is, but it comes up fairly quickly once it sees open sky.

If the modem is in A-GPS mode (we use #2), the GPSXtra Time is good, but GPSXtraData doesn’t update. If the unit starts out with a sky view, most of the time (but not always), it figures things out amazingly quickly and off it goes. If it doesn’t figure it out right away, in many cases examining the NMEA data stream, it claims 0 satellites and will stay like that for hours. The next time it gets power cycled or if I can get onto it remotely and do an AT!reset, it comes up pretty quickly if not right away.

In both cases, the modem has automatically established a data connection to the carrier and is exchanging data with our server. I have read statements which either state or deny that A-GPS can work with a data connection established. I would also like verification on that as well.

So are these mutually-exclusive technologies, and what exactly is SUPL supposed to provide? Does it provide ephemeris / constellation data? Does it provide accurate time (or does the modem get that from the network)? Does it actually provide the location assistance, and if so, how much information does the modem have to provide to the system to get assisted location? (I have read that it has to provide at least part of the data stream that it has integrated for the server to assist, but if it has 0 satellites in the list, never mind integrated, it has nothing to give the server!)

I could ask the same for GPSOne. It seems to provide solely time and ephemeris / constellation data so that the modem has a “kick-start” and doesn’t have to download that from a satellite before attempting to see who it can see and integrate data.

Thanks in advance for any information / clarification that you can provide.

PS: I have read a lot of the Wiki data and think I have a pretty good grasp of how native GPS works (I first worked with it professionally in 1988), however it is the enhancements available over cellular networks that I’m not entirely clear on.