The up and down-link frequency channel pair allocation has been arranged such that the two frequencies comprising a channel pair are 45 MHz apart.
Each of these frequency pairs is identified by an Absolute Radio Frequency Carrier Number (ARFCN).
GSM Physical Channel Structure
Absolute Radio Frequency Channel Number (ARFCN)
Within the spectrum allocated for cellular mobile communications, the radio channels are identified by ARFCN. When the system is operating in Frequency Division Duplex (FDD) mode then the channel number is associated with both the uplink and downlink radio channels.
