ibility into any auto-scaling and auto-recovery action. During service designs, the set of events and per- formance data required for control loop automation is identified. Correlation and analytics at the virtual function level and at the service level (across virtual functions that are part of the service) is designed, and the appropriate policies at the virtual function and service levels are created for the recommended actions. 14.2.1.3 Deployment Considerations Real-time services such as VoLTE have an operational requirement to constrain failures to a local- ized region in the spirit of avoiding a disaster that impacts customers nationwide. The virtual service framework defines the concept of a virtual zone (see Figure 14.7) to enable using cloud capacity where it exists and moving from a regional to a national model. A virtual zone is a cloud availability zone that contains a grouping of virtualized registration and session control network functions that supports a set of subscribers from a localized region. For subscriber expan- sion at a localized region, the subscribers are typically homed to virtual zones within the localized region. If there are capacity constraints at cloud availability zones within the localized region, the new subscribers may be homed to virtual zones that are located outside the localized region tem- porarily, until the time when capacity is available at cloud availability zones within the localized region. This approach enables rapid expansion of subscribers while still meeting the operational require- ment of avoiding a nationwide impact to subscribers if a data center fails. It enables utilizing cloud capacity where it exists, avoids stranding available capacity and also enables a flexible design for georesiliency. With the implementation of virtual zones, sizing of the equipment at each zone will be small SO as to enable deployment of just the capacity that is needed. Also, with the concept of smaller virtual zones, we achieve smaller failure domains. The impact of a site failu