First-generation cellular systems (1G) were analog, insecure, and lacked international roaming standards. There was no unified digital cellular standard for Europe and the world.
GSM is the 2G digital cellular standard that introduced digital encryption, international roaming, and SIM cards. It defines the complete architecture for digital mobile communication.
GSM operates in the 900 MHz and 1800 MHz bands (different regions vary). The system uses:
- TDMA/FDMA hybrid — time division and frequency division multiplexing
- GMSK modulation — Gaussian Minimum Shift Keying for spectrally efficient transmission
- Three subsystems: RSS (radio), NSS (network switching), OSS (operations)
- SIM cards — removable subscriber identity module for portability
Key processes: location updates, handoff, authentication, and encryption all managed by the NSS.
- Digital encryption (A5/1, A5/2, A5/3) for voice and data privacy
- SIM cards enable subscriber portability across devices
- International roaming through HLR/VLR architecture
- GPRS (2.5G) adds packet-switched data on top of GSM’s circuit-switched foundation
- Uses FDD (Frequency Division Duplex) for uplink/downlink separation
- Built from: Wireless Network — GSM operates over wireless radio
- Built from: Frequency Shift Keying — GSM uses GMSK (Gaussian MSK)
- Builds into: GSM Architecture — detailed subsystem breakdown
- Builds into: GSM Services — telephony, data, supplementary services
- Contrasts with: CDMA — GSM uses TDMA, CDMA uses spread spectrum
- Related: GPRS — packet-switched extension to GSM (2.5G)
- A5/1 encryption was broken — modern attacks can decrypt GSM traffic in real-time
- GSM operates in multiple frequency bands globally (850/900/1800/1900 MHz) — devices must support regional bands
- Circuit-switched nature makes GSM inefficient for data — GPRS/EDGE added later