As with all telecommunications systems, GSM has a hierarchical and complex system architecture comprising of many entities, interfaces and acronyms figure 4 shows a simplified overview of the GSM system as specified in the ETSI (TS 101.622). GSM systems consist of three subsystems, the radio subsystem (RSS), the network and switching subsystem (NSS), and the operation subsystem (OSS). Generally a GSM user will only notice a very small portion of the whole network, commonly the mobile stations (MS) and some antenna masts of the base transceiver stations (BTS).
