GSM operates in four primary bands:

GSM uses a complex timing structure: bursts (0.577 ms) -> frames (4.615 ms, 8 bursts) -> multiframes (120 ms) -> superframes -> hyperframes. This structure allows 8 simultaneous calls on a single radio frequency.

GSM introduced:

Voice signals are digitized and compressed into a data stream operating at 9.6 kbps . Common GSM Frequency Bands