If you are using two SR5500 to do the downlink and uplink fading simutanously, make sure before the data call is up, you have finished switch by-pass mode to AWGN mode, if the data call is active and you switch bypass mode and AWGN mode, you will experience data call drop as we will have several ms delay to switch between different mode.
The best way to do that is before the data call is active, you have already switched between different mode.