clean SIP traffic to your enterprise. In model (c) two firewalls are on either side of the border element. Some refer to this model as the one for the truly paranoid, but this is the classic design of a DMZ (demilitarized zone). It is not an uncommon design, especially in large financial, educational, and government institutions, or any other business particularly attrac- tive to hackers. Model (d) is a variation of model (c), where there are two virtual firewalls on either side of the border element, but one physical firewall device is used for the function, routing the unified communications (UC) traffic twice. This is a virtual DMZ design often used in video deployments where the CUBE is not only fronting a SIP trunk, but is also bringing in H.323 Internet video traffic and acting as a Cisco IOS Gatekeeper. Model (e) provides a more cost-effective integrated deployment model for smaller sites or businesses where a separate firewall appliance does not already exist, is not desirable, or the cost is not justified. In this model the Cisco IOS router acts as both the CUBE and the firewall. Traffic flowing through this router is inspected first by the firewall and then handed to the border element for further processing. It is there- fore conceptually similar to model (a). Model (f) provides a lower end offering for commercial or small businesses (without IT departments) that do not want to carry the cost or the management of either a bor- der element or a firewall. In this model, an integrated service from a service provider is purchased, and all security and demarcation issues is handled by the service provider. The service provider puts an IAD at the customer premises to connect to its IP-PBX or key system, such as CUCM Express. The IAD device will likely do NAT, perhaps basic firewalling, but essentially all the service provider's network and secu- rity are delivered as a managed service. Security Protection at the SIP Protocol Level SIP is a widely used and understood protocol and simple to create because it u