The BlackBerry will automatically switch to mobile network if it detects some level of wifi degradation that is too much. If you are on a router with many other users and the bandwidth available to you drops too low, the BlackBerry could switch to the mobile network. And there could be other reasons, such as if you are on a public hotspot and the wifi service begins to degrade or is dropping in and out. It has nothing to do with the quality of the BlackBerry. And as we are saying, the only way to assure yourself that this doesn't happen is to disable mobile network when you want to use wifi data exclusively.
I don't know if this is archaic or not. To me it is a good thing that the device will maintain service by switching to mobile if there is a problem with WiFi connection or throughput. The question as I always perceive it when it is asked is, How to I absolutley, positively, without doubt make sure the BlackBerry is only using wifi. And the answer is to disable mobile network.
GPRS and EDGE (and 3G) in all caps are the indication that you are connected to the RIM/BlackBerry network and therefore should have BlackBerry data services operating. If you did not have a BlackBerry data plan and instead had a regular mobile data plan through your carrier and you were using a third-party browser, you would not see GPRS or EDGE in all caps. See
KB02334-Network status indicators from the GSM/GPRS/EDGE/3G networks.
As an aside in case it is not clear, you will have access to the BlackBerry network on wifi with mobile network disabled.
Finally, you probably do not have UMA. First, the carrier must support it, and you must have a handset that is capable. What it would give you, if you had it, is the ability to make and receive phone calls on wifi with the mobile network disabled. So, that's why we say a disadvantage for you of turning off the mobile network when you are using wifi is that you will be disabling the phone, i.e. voice communication.
Posted via BlackBerryForums.com Mobile