GSM

GSM is a digital cellular standard that operates on 900 MHz and 1800 MHz frequency bands (850 MHz and 1900 MHz in North America). It uses time division multiple access, or TDMA, to split each radio channel into eight time slots so that eight calls can occupy a single frequency simultaneously. This architecture made GSM efficient enough to become the dominant mobile standard worldwide from the 1990s onward.

The standard defines everything from the radio interface and network architecture to the SIM card format and security protocols. A GSM phone connects to a base station, which routes the call through a mobile switching center and then to either the public switched telephone network or another mobile network. The system includes specifications for voice compression (using the AMR codec in later versions) and data transmission, originally at 9.6 kbps but eventually reaching 115 kbps with GPRS overlay.

Before GSM, mobile standards were fragmented by country and region, making roaming nearly impossible and forcing manufacturers to produce handsets for each local market. GSM changed this by establishing a single global specification. A GSM phone with a SIM card could be used in any country that deployed GSM infrastructure, a capability that drove massive adoption in Europe, Asia, Africa, and eventually worldwide. The removable SIM card itself became a defining feature, separating the phone hardware from the subscriber identity.

GSM security relies on a shared secret key stored on the SIM and in the operator's authentication center, plus the A5 stream cipher for call encryption. The A5/1 variant (128-bit) became standard, though weaknesses were discovered later. Network operators still use GSM infrastructure for fallback, even though newer technologies like UMTS and LTE have replaced it for primary service.

Though 2G GSM networks are being shut down in many countries as of 2022 onwards, the standard's engineering principles influenced every mobile generation that followed. Its impact on standardization and interoperability extended far beyond telephony into industrial applications where reliable, standardized wireless communication became essential.