Global system for mobile communication (GSM) is a digital cellular communications system that primarily provides voice, low-speed data, and short messaging services (SMS). GSM technology involves communication between a mobile phone, using a radio interface, to the various parts of the GSM network. GSM technology allows for a wide range of services that are similar and compatible with different types of fixed networks including the public analog and public data telephone networks (such as the Internet).

One of the key attributes of the GSM technology is a standardized digital radio carrier channel that allows multiple users to simultaneously share the channel. In historical analog radio and cellular systems, one radio carrier frequency carries one conversation. Therefore, a channel for a conversation in an analog system is synonymous with and corresponds one-to-one to a carrier frequency. However, in a digital radio system, it is possible to carry more than one conversation, or a conversation and other types of digital information, by means of various types of multiplexing. GSM uses time division multiplexing (TDM) to share one modulated carrier frequency radio waveform among 8 to 16 conversations. Therefore, documents related to GSM are careful to distinguish between a radio carrier and a communication channel.