GSM (originally Groupe Speciale Mobile, later Global System for Mobile Communication) was first standardized in 1991 as the dominant 2nd generation (2G) fully digital mobile network.

Key properties:

• First fully digital mobile network standard — this was the major leap from 1G (analog)
• Primarily designed for voice, but from mid-1995 added data services: SMS, fax, and even notebook connectivity
• GPRS extension (around 2000) enabled mobile internet, though at very low data rates

GSM frequency bands:

GSM-900 system specifics:

• 124 frequency pairs (channels), each 200 kHz wide
• Uses TDMA (Time Division Multiple Access) with 8 time slots per channel
• This means one GSM-900 carrier can handle 8 simultaneous calls

Tip: The switch from analog (1G) to digital (2G/GSM) was like the switch from vinyl records to CDs — digital enables error correction, encryption, and multiplexing that are simply impossible with analog signals.

Go deeper:

• GSM (Wikipedia) — history, architecture, and why 2G became the global standard.
• GSM frequency bands (Wikipedia) — the full band plan behind the P-GSM-900 / DCS-1800 / PCS-1900 table.