 are on the one hand authentication methods that are based on shared secrets and GBA, specifies a mechanism to provide communicating parties with such a shared secret. On the other hand, there are authentication methods that rely on (public, private) key pairs and digital certificates and SSC, specifies how to issue certificates to mobile subscribers. The key benefits of GAA are as follows: Application developers can save development time by using GAA instead of designing and imple- menting application-specific authentication mechanisms. The advantage of the mechanisms of GAA is that they can provide global coverage, inherited from the GSM/UMTS coverage. It also aims to enhance GSM security by addressing the known GSM vulnerabilities when using 2G GBA without requiring any changes to the existing SIM specification. TR 33.919 Shared secret Certificates TR 33.220 TR 33.221 Figure 8.16 Illustration of mechanisms to issue authentication credentials. Reproduced with permission from 2007 3GPP [18] Core Networks and Operations Support Systems 8.2.5.3 Common IMS In 2007, it was agreed that 3GPP will take overall responsibility of IMS specifications to avoid frag- mentation of the same. The ETSI TISPAN (Telecoms and Internet converged Services and Protocols for Advanced Networks) took the first step in migrating fixed IMS requirements to 3GPP. The 3GPP2 MMD (Multi Media Domain) requirement specifications have also been merged with Release 8. The 3GPP SA1 (Service and Systems Aspects) has completed common IMS harmonization for Release 8 (from ETSI TISPAN and 3GPP2). From Release 9 onwards, 3GPP will maintain one common set of IMS specifications. IMS enables real time multimedia connections over IP without defining any particular service. The applications implemented on terminal and/or in network make use of these connections. The Common IMS specifications will expedite the development of services and interoperability of these services. The topic of IMS is discussed in detail in Chapter 9. Operations Support Systems (OSS) O