usiness poli- cies and rules over time 14.4.1.4 Georesiliency, Topology, and Scalability In order to maximize the use of resources, from a georesiliency standpoint, the following principles should be applied to virtualized IMS service platforms: Failure of a site should not result in a nationwide impact to subscribers. 2. Localized onsite software or hardware failures should be contained. 3. Localized onsite failures due to software defects or misconfiguration should not propagate to result in a nationwide impact. 4. VNFs should be deployed in N+K redundancy at local sites where possible. 5. Individual VNFs should have the ability to fail over to one or more georedundant sites. 6. It should be possible to deploy multiple VNFs for the same network function at a site. 7. Registration state and stable session state should be replicated across all core sites that are part of a georedundant site pool. 8. Each VNF must have high availability software architecture that supports geographic resiliency. Latency requirements must be met for postdial delay. This drives the geographic placement of some of the virtualized IMS service platform network functions. 10. Latency requirements must be met for mid-call features under failure scenarios using the session state from the state persistence database. A virtual zone contains a grouping of registration and session control virtualized network func- tions that supports a set of subscribers. A target view would call for deployment of many virtual zones of smaller capacity in each zone. A greater number of cloud sites will enable smaller failure domains and increased efficiency for resource usage in the future. Although each site is mapped to a zone, upon failure of a site, failover can be executed to wherever the infrastructure (VMs, network) Service Platforms SUBSCRIBER DATABASE (HSS) APP LAYER DATABASE (VoLTE AS) STATE PERSISTENCE DB STATE PERSISTENCE DB Core Site 1 Core Site 2 Core Site 3 Core Site 1 Core Site 2 Core Site 3 ZONE 1 ZONE 2 SBCs-Zone 1 SBCs-Zone 2 FIGURE 14.16 Ge