Previously I had implemented a project that needed to be always-connected, and in order to take network outage into account, we would write to a buffer (a sqlite database actually) if we couldn't detect network. We will sync with server once online and sync the related information as necessary. Simcity can use similar technique because I don't see there is any hard real-time interactions between players or the server. In offline mode, the simulation will run with a default parameter set, or the data from the previous sync, until the next sync happens. In fact, my gut feeling tells me the simulation engine should have an offline switch to let it run stand-alone, or else it would be hell to debug a system that keeps on changing states.