therefore, could control the QoS mark- ings on both the signaling and media VoIP packets entering your network for calls from the PSTN. With an end-to-end VoIP call over a SIP trunk, it's quite possible that the serv- ice provider preferred QoS markings are different from the ones you prefer, and, there- fore, packets have to be remarked in both directions as the packets cross the border. The CUBE is a back-to-back user agent and, therefore, has full control over packet mark- ing in both directions and can be set either globally or based on destination. For exam- ple, if you have two SIP trunks to different providers and their choices of marking is dif- ferent from each other and from your choice in the enterprise, the border element can remark these packets on a per-flow basis. 112 SIP Trunking VoIP endpoints and call agents such as CUCM and CUCMExpress also have facilities to control and mark packets. These can be used directly if the enterprise markings are the same as the SP UNI markings, and an SBC can be used if markings need to be translated between the enterprise and the SP networks. Security Considerations The security concerns of TDM trunking, primarily toll fraud, exist equally on SIP trunk- ing. In addition, SIP trunking exposes your network to IP level threats similar to data WAN or Internet access, such as denial of service (DOS). For a hacker to gain access to your enterprise IP network via a TDM voice trunk is virtu- ally impossible to do unless the TDM connection is specifically configured for modem dial-up access-and most voice trunks are not. Perpetrating a DOS attack on a TDM trunk is also highly unlikely as it is both expensive to do and requires large-scale auto- dialer equipment the average Internet hacker does not have access to. Launching these same attacks on IP addresses is significantly easier and open to a much larger pool of perpetrators because no sophisticated equipment is necessary, and the attacks can be launched for free from any Internet access connection. When considering secu