GSM is based on FDMA : the frequency band is divided into channels of 200kHz each, with channels in downlink and channels in uplink.
A pair of channels UL+DL is represented by a number “ARFCN”. In GSM900 there are 124 channels, from ARFCN=1 to ARFCN=124.
ARFCN=1 correponds to UL=890.2 MHz and DL=935.2 MHz.
When a MS setups a call, it is allocated on one timeslot of one ARFCN (in case of NO HOPPING). The allocation is usually decided by the BSC, which chooses which ARFCN and which timeslot.