ng a steady growth in the pseudoprivate realm of the VPN (see Figure 9.10). A VPN is a logical net- work that isolates customer traffic on shared service provider facilities. In other words, the enterprise's traffic is aggregated with the traffic of other companies. VPNs have been around for quite some time-since X.25 closed user groups on the packet-switched network and the AT&T Software-Defined Network (SDN) on the circuit-switched networks. A VPN looks like a private network, but it runs across either the public circuit-switched network or public packet-switched data net- Hong Kong Partner Telecommuters Internet Dialup Dialup, Leased Lines Singapore Branch Office Leased Line Dialup San Francisco Headquarters Internet London Mobile Branch Office Users Figure 9.9 An enterprise network based on leased lines Singapore Hong Kong Branch Office Partner Telecommuters Internet, IP Backbone, FR, or ATM Mobile London San Francisco Users Branch Office Headquarters Figure 9.10 An enterprise network using a VPN works. Thus, a VPN is not just a solution within the IP realm; a VPN is a concept, not a specific set of technologies, and it can be deployed over a wide range of net- work technologies, including circuit-switched networks, X.25, IP, Frame Relay, ATM, and MPLS. With the advances in VoIP and Video over IP, consumers have realized that they can converge real-time applications such as voice, video, and data into one access circuit. Carriers have realized that with the evolution of converged technol- logies, they no longer need to deploy, maintain, and support both TDM and IP backbones. However, carriers' IP backbones provide neither segmentation of cus- tomer traffic nor the security normally associated with that segmentation. Under the initiative of service providers, the standards organizations have recognized the value of starting work on the standardization of VPN technology, through ITU-T Study Group 13 and the IETF Provider Provisioned VPNs (PPVPNs) group. These standardization efforts have focused on the provider