ing access e.g. through user ID and passwords. VPNs always include encryption of some type, typically IPsec is employed. The IP-VPN tunnel can be regarded as a competitor to a frame relay or leased line connection for creation of wide area corporate networks. The tariff is based on IP with no distance-dependent fee, as opposed to frame relay and leased lines. The solution is cheaper, global and flexible. A leased line, a PSTN/ISDN VPN or maybe a frame relay/ATM connection can be used for intra-company voice, apart from public voice networks. See Figure 6.27. The tunnels form VPNs. (Note: there is another VPN type for voice, mentioned above.) A VPN is a private data network that makes use of the public telecommunications infras- tructure. Typically, a VPN uses a private numbering plan. There are three types of VPN: Corporate-built VPN, totally handled by the company. Telecommuters can be connected by means of L2TP. (layer 2 tunnelling protocol). The tunnel is set up by NAS at an ISP. Understanding Changing Telecommunications - Building a Successful Telecom Business Insecure network (Internet) Safe way Security gateway Figure 6.27 A possible VPN architecture Customer premises based IP-VPN. About the same solution but with central VPN management. Network-based IP-VPN. Everything is done between edge routers. One of the driving factors in this development of enterprise networks is the minimiza- tion of costs for both equipment and maintenance. At the same time adequate security measures must be employed. The use of VPN-based tunnelled remote access sessions substantially decreases the operational costs for providing access to home workers and road warriors. This is due to the reduced need to rely on long-distance modem calls. Flexible configuration options in enterprise NASs make it possible to allow both PC-to- enterprise and operator-to-enterprise tunnels, thereby allowing the enterprises to outsource and contract remote access services from operators. VPNs can also provide gaps in security when the end node is con