not received, the trunk would be marked as DOWN and action taken to resolve the issue. In a SIP trunk solution, because there is no permanent connec- tion when a call is not active, an alternative method to determine the status of a trunk needed to be developed. The solution that is most widely deployed is the OPTIONS PING method. In this case, a SIP OPTIONS request is sent outside of an active call dialog (often called Out of Dialog [OOD] OPTIONS PING) to determine if the entity on the other side is responding. If the SIP target of the OPTIONS message is not responding, an alarm condition is met, and a response would be required. Utilization of the OPTIONS PING message is a distinct method of monitoring a SIP trunk solution outside of the standard IP-based services monitored for ping, power, and path. In summary, device and solution monitoring in a SIP trunk solution, as with most solu- tions, is essential, and SP networks need to ensure they have in place methods and proce- dures to monitor all the components of the network. Enterprise Network Components Enterprise networks have many of the same components as SP networks with respect to SIP trunks. The biggest difference is in the size and complexity of deployments, though many large-scale enterprise architectures for SIP trunks look like small-scale SP solutions. SP networks are optimized to have as little maintenance required for as profitable a serv- ice as possible. Whereas enterprise networks are more about offering enhanced services and value to the end customer that can distinguish one enterprise from another. In this 76 SIP Trunking section, the focus is on features and requirements that are different and distinct from the components of a SP SIP trunking solution. Enterprise Networks-SP Interconnecting Session Border Controllers Enterprise networks have many of the same issues to resolve as SP networks, and as such, many of the same solutions are appropriate. An SBC solves enterprise problems with respect to: Session management: Ensures that sessions fr