IMS (IP Multimedia Subsystem) subscribers may be issued with an ISIM for the operator or carrier supporting the IMS service. This is similar in nature to the SIM (Subscriber Identity Module) used in GSM and GPRS and the USIM (Universal SIM) employed in UMTS. It holds information regarding a user’s subscription level as well as authentication, security information and their IMS private identity held in the form of a NAI (Network Access Identifier). It should be noted that if the subscriber does not have an ISIM then the USIM may be employed as the security algorithms held on the USIM are the same as those held on the ISIM. An IMS private identity however will have to be resolved from the user’s IMSI (International Mobile Subscriber Identity).