Comparing GPRS with circuit switched
In a GSM network the BSC governs the cell selection behavior of the MS when in idle and active mode by different methods. Idle mode MSs autonomously performs cell reselection by using the C1/C2 criteria.
In active mode, non-GPRS MSs are steered by the locating functionality implemented in the BSC. So this means that the BSC initiates the handovers to other cells. With GPRS, the MS determines the base station with which it will communicate, Figure 28 shows the handover procedures for both Circuit Switched and Packet Switched. The GPRS MS manages both the idle packet and transfer packet mode behaviors.
The cell selection and reselection algorithm used for controlling the idle/transfer mode behaviors are governed by the GPRS cell selection and reselection parameter setting broadcast in the packet system information on the PBCCH in each GPRS capable cell with an allocated PBCCH (MPDCH). If no PBCCH has been allocated in a cell, the GPRS MS will read the system information broadcast on BCCH and use the C1/C2 criteria for cell selection and reselection as in the circuit switched idle mode case.
So as you can see the GPRS cell selection and reselection algorithms are governed by parameter settings. These parameters C31 and C32 are different to the corresponding parameters for the circuit switched system. However with some GPRS systems GPRS cell section parameters are automatically mapped on those for cell selection/locating known from the circuit switched case. The reason for this is to achieve the same cell selection behavior for GPRS, as with GSM, this will enable an easy rollout of GPRS in the network.
The GPRS standard allow the network to take over cell reselection for a specific MS or for all MSs. This is called Network Controlled Cell Reselection and have not yet been implemented in any UK or European GPRS systems.