When GSM was already in use the specifications were further developed and enhanced with functionality like SMS, GPRS, etc. These development steps are referred as releases by ETSI. Within this development cycles the SIM specification was enhanced as well: new voltage classes, formats and files were introduced. In GSM-only times, the SIM consisted of the hardware and the software. With the advent of UMTS this naming was split: the SIM was now an application and hence only software. The hardware part was called UICC. This split was necessary because UMTS introduced a new application, the Universal Subscriber Identity Module (USIM). The USIM brought among other things security improvements like the mutual authentication and longer encryption keys and an improved address book.
