The GPRS radio interface uses a 52 frame multiframe, which is identical in length to the 2*26 frame traffic multiframe used by GSM.
The GPRS hyperframe can therefore either have its own ARFCN, or it can use a GSM traffic or control hyperframe ARFCN. It can do this because not all time slots in the radio block need to be used, and can be allocated to GSM instead.
Once you have located the BCCH within the cell, it tells you the ARFC and frame number the PBCCH within the hyperframe modulo the length of a multiframe. You can then attach to the GPRS network over the PRACH channel indicated in the PBCCH. Similarly, you can attach to the GSM network over the RACH indicated in the BCCH. The BSC knows where to forward these channels, MSC or PCU. You can be connected to both networks at the same time.
EDGE is simply the use of the MCS-1–9 coding and modulation schemes on the radio interface instead of CS-1—4 defined for GPRS. It does this by using the CS-1 encoding to perform a request of EGPRS Packet Channel Request RLC message type on the PRACH, and the PDTCH assigned by the PCU will now use the assigned MCS modulation scheme, which uses different RLC/MAC headers and coding, a different L1 slot and burst format, and a different modulation scheme, either GMSK or 8-PSK depending on the MCS version selected. EDGE also encompasses ESCD which enhances the existing HSCSD (data over GSM TCHs), but this is never used.