ch means that each SCSCF needs to be modified to Service Orchestration in the IP Multimedia Subsystem include the SCIM, and it is no longer possible to deploy a SCIM in an existing IMS network without additional work. As the IMS architecture stands, the SCSCF does the most work on the control signaling as it is responsible for passing every message through the network and determining where to route the initial requests. Therefore, any steps that increase its load need to be carefully examined. Nakajima et al. [13] agree with the placement of the SCIM outside the SCSCF. They propose that while placing the SCIM inside the SCSCF would also fall within the 3GPP-proposed standards, there is a scalability benefit and increase to failure tolerance to be gained by placing it outside as a stand-alone function. They state that the "SCSCF is considered to be busy handling basic calls. For scaling, the SCSCF has to concentrate on handling basic calls by delegating the proposed func- tions to the SCIM" [13]. In terms of failure impact, they classify the different types of calls or sessions into three distinct groups. These three groups are: basic calls, emergency calls, and enriched services. If the SCIM is separated from the SCSCF, and the SCIM fails, only enriched services will fail to operate. Most importantly, the emergency calls will continue to operate successfully, together with the basic calls. If a combined SCSCF/SCIM fails, then all three groups of sessions will fail. The latest IMS standards have introduced another element called the Emergency Call Session Control Function (ECSCF) to provide a dedicated entity to deal with the establishment of emergency calls, and thus, the concern about the emergency calls being affected by the combined SCSCF/SCIM falls away. However, it is still preferred to only have enriched services failing with a SCIM failure. In both sce- narios, if the SCSCF fails, all groups of sessions will fail as there would no longer be a functional entity passing messages from the end user to the inte