GSM and UMTS Mobile Communications

GSM, the Global System for Mobile Communications, is the digital cellular standard that became the dominant technology for mobile telephony as a second-generation (2G) system. It divides a geographic area into cells, each served by a base station, and allows a call to be handed from one cell to the next as a user moves. GSM carries speech in digital form, which improved quality and security over the earlier analogue systems and allowed many users to share the radio spectrum efficiently.

To let many phones share the available radio channels, GSM combines two methods. Frequency-division multiple access (FDMA) splits the spectrum into separate carrier frequencies, and time-division multiple access (TDMA) divides each carrier into time slots so several calls share one frequency by taking turns. GSM also introduced the subscriber identity module (SIM) card, which holds the user's identity separately from the handset, and the Short Message Service (SMS) for sending brief text messages between phones.

UMTS, the Universal Mobile Telecommunications System, is the third-generation (3G) successor to GSM, designed to carry much higher data rates for mobile internet access and multimedia. It uses a different radio technique based on wideband code-division multiple access, in which users share the same frequency band and are separated by unique codes rather than by time slots. UMTS networks were built to coexist with and evolve from GSM, so operators could upgrade gradually while still supporting existing phones, and the two technologies together carried mobile communication for many years.

Frequently asked questions

What is GSM?

It is the second-generation digital cellular standard that became the most widespread mobile telephone technology, carrying speech digitally and dividing coverage into cells.

How does GSM let many users share the spectrum?

It combines frequency-division multiple access, which splits the spectrum into separate carriers, with time-division multiple access, which divides each carrier into time slots.

How does UMTS differ from GSM?

UMTS is the third-generation successor offering much higher data rates and uses wideband code-division multiple access, separating users by codes rather than time slots.