SIP AS SIP AS SCSCF SCSCF Centralized Distributed architecture architecture Figure 8.3 Difference between centralized or distributed SCIM architectures. (in terms of how it connects to the AS). The following sections discuss the approaches taken by existing research in this area (Figure 8.3). 8.3.4.1 Architecture Gouya et al. [12] discuss a centralized hybrid approach, where the SCIM is placed outside the SCSCF and it is viewed as an AS that communicates with the individ- ual service capabilities [12]. They also propose a second approach that can be classed as a distributed hybrid architecture. In this architecture, they have multiple SCIMs and multiple SCSCFs, with each SCIM integrated into a single SCSCF. They claim that their centralized network architecture is not scalable as each SCIM creates a bottle neck, whereas their distributed approach is scalable as they place each SCIM within a SCSCF. However, in their centralized architecture they have one SCSCF talking to one SCIM, and therefore, they can have the same number of SCIMs in both architectures. Thus, the only real difference between their architectures is the placement of the SCIM within the SCSCF or integrated with the AS. In terms of managing the fault tolerance of the system, they claim that having the SCIM integrated with the SCSCF is a better approach as each SCIM/SCSCF has a smaller impact when it fails. However, if the SCIM fails, the SCSCF will fail and vice versa as they are now a single functional entity. If this integration did not take place (i.e., the SCIM was a separate entity but still connected to the SCSCF) the network could carry on operating, and only services requiring a SCIM would fail. The SCSCF will carry on handling basic session establishment. Thus, it is more robust to have the SCSCF and SCIM as separate entities [13]. By integrating the SCIM and SCSCF it is possible to reduce the complexity of the SCIM as it can now take advantage of the existing diameter connection to the HSS, and does not need its own interface. This approa