In the days and age of LTE it seems to be a bit outdated perhaps to write a technical post on a GPRS topic. But anyway here we go since I looked up the following stuff on GPRS network control order in the 3GPP specs lately:
When GPRS was initially launched, mobile devices performed cell reselections even during data transfer on their own and without any information from the network on the parameters of the target cell. Consequently, there was an outage of several seconds during the cell change. Over time networks adopted a feature referred to "Network Assisted Cell Change" (NACC) which comes in a couple of flavors depending on the network control (NC) order.
From what I can tell, most GPRS and EDGE networks today use Network Control Order 0 (NC0). That means that the UE performs neighboring cell measurements during the data transfer and reselect to a new cell on their ow, i.e. without informing the network. If the network indicates that it supports Cell Change Notification (CCN) (in SIB-13), the UE can ask for support for the cell change by sending a Packet Cell Change Notification request message to the network. The network then supplies the system parameters of the new cell to the UE that can then perform the cell reselection much quicker. That's the NACC mode that's pretty much common now in networks.
But there is more. If the network indicates in SIB-13 that the Network Control Order equals 1 (NC1), the UE has to send measurement reports to the network. Cell reselections are still performed autonomously by the UE when required, again by using NACC if the CCN feature is indicated as active by the network.
And finally, there's Network Control Order 2 (NC2) in which the UE has to send measurement reports to the network and only reslects to another cell when told by the network with a Packet Cell Change Order.
I haven't seen NC1 or NC2 in live networks yet but perhaps some of you have. If so, I'd be happy to hear about it.
For the details have a look in 3GPP TS 44.018 and 44.060.