n) may comprise many VFs, such as signaling gateways, call process- ing applications, and location databases. Each of these VFs may comprise multiple modules in turn comprising one or more VMs. 5.3.1 FAULT TOLERANT VM DESIGNS Using the principles of fault-tolerant design, there are three types of replication that can be used for the VMs within a VF. 5.3.1.1 Hot VM Replication Load is either provided to all the replicas, or state information of the primary replica is frequently transmitted to the backup replicas. The primaries as well as the backup replicas execute all the instructions, but only the output generated by the primary replica is made available to the user VM application. The output messages of backup replicas are logged by the VM application. In case the primary replica fails, one of the backup replicas can readily resume the service execution. A control scheme for designating the backup replica is needed to ensure stability. For each replica failure, an equivalent replica (VM instance) should be created on another host and its state updated. The availability obtained by using this technique can be very high, but it comes at the expense of high resource consumption. This replication design is typically reserved for applications designed for low tolerance to downtime and transaction defects. Architecting the Network Cloud for High Availability 5.3.1.2 Warm VM Replication The state information is obtained by frequently check pointing the primary replica and buffer- ing the input parameters between each checkpoint; replication is performed by transferring the state information to the backup replicas. The backup replicas do not execute the instructions, but save the latest state obtained from the primary replica. In case the primary replica fails, a backup replica is initiated and updated to the current state, incurring some loss in the present execution cycle and limited downtime. The availability obtained from this technique is less than that from hot replication, but the resource consumption costs are 