The GSM network is divided into the following three major systems:
Answer : SS ,BSS ,OSS
GSM describe the protocols for second-generation (2G) digital cellular networks used by mobile phones .GSM supports voice calls and data transfer speeds of up to 9.6 kbps. It is based on the time division multiple access (TDMA) system. Currently GSM networks operate on the 850MHz, 900MHz, 1800MHz, and 1900MHz frequency bands.
Reference : GSM - Architecture