Global System for Mobile Communications
GSM is presently the most popular standard of cellular telephony, in which signalling as well as speech channel are digital (numeric).
From the consumers point of view, the GSM's biggest advantage is high quality of voice and text message transmission. The advantage for operators is the possibility of using different producers' devices (it wouldn't be possible without an open standard). Besides, standardization has enabled operators offering roaming services, which means that phone can be used worldwide. GSM holds down comparability backwards, i.e. old models of phones work trouble-free in today's network, in spite of adding many new functions to the GSM standard.