Also known as: GSM, 2G, Global System for Mobile Communications, Groupe Spécial Mobile
GSM (Global System for Mobile Communications) is the second-generation (2G) digital cellular standard that carried mobile voice and text to billions of users worldwide. It combines frequency-division and time-division multiple access — eight time slots on each 200 kHz carrier — with GMSK modulation, and it introduced the removable SIM card that decoupled a subscriber identity from any one handset.1
Overview
GSM replaced the incompatible analogue 1G systems of the 1980s with a single pan-European — and ultimately global — digital standard. A phone tunes a 200 kHz carrier and is assigned one of eight repeating TDMA slots, so eight calls share each frequency pair (uplink and downlink are separated by a fixed duplex spacing). Speech is digitised, compressed by a low-bit-rate vocoder, protected with channel coding, and sent as short bursts. The design also standardised SMS text messaging and the SIM smart card.
Technical characteristics
| Property | Value |
|---|---|
| Generation | 2G |
| Access | TDMA over FDMA, 8 slots per carrier |
| Carrier spacing | 200 kHz |
| Modulation | GMSK (0.3 Gaussian filter, 270.833 kbit/s gross) |
| Bands | 900 / 1800 MHz (Europe), 850 / 1900 MHz (Americas) |
| Vocoder | Full Rate, Half Rate, Enhanced Full Rate |
| Frame | 4.615 ms, eight bursts |
| Security | A5/1 or A5/2 stream cipher, SIM-based authentication |
The constant-envelope GMSK waveform lets handsets use efficient, non-linear power amplifiers, which helped battery life and kept terminals cheap.
History
Work began in 1982 under the Groupe Spécial Mobile of CEPT; the standard later moved to ETSI and then to 3GPP, which maintains it alongside later generations. The first commercial GSM call was placed in Finland in
- Through the 1990s and 2000s GSM became the dominant cellular technology on most continents, eventually serving well over four billion connections and making international roaming routine.
Deployment
GSM spawned a family of enhancements: packet data via GPRS and faster EDGE, both bolted onto the same carriers. Operators have since refarmed much GSM spectrum for LTE and 5G, and several countries have switched 2G off entirely. Even so, GSM lingers as a fallback layer and underpins low-power machine-to-machine devices in regions where it remains licensed.
Decoding it with GopherTrunk
GopherTrunk is a land-mobile and utility-signal trunking scanner; cellular telephony such as GSM is out of scope and is not decoded. GSM carries private, operator-licensed traffic that is authenticated and usually ciphered, and intercepting it is illegal in most jurisdictions. GSM appears here for reference and to contrast its GMSK / TDMA air interface with the land-mobile protocols GopherTrunk does handle.