es on the existence of a PKI under the control of a trusted CA. S/MIME is fundamentally used for point-to-point security. Security Security mail gateway Internal mail Intranet user server Corporate PKI server Corporate network Mail server Firewall (IMAP/POP) Firewall IMAP/POP Internet Mobile user Extranet or mail service provider Email in plain text Email protected with S/MIME PKI traffic cert processing Figure 6.26 Corporate e-mail architecture example - The Internet and corporate sections fire- wall-security mail gateway protected by S/MIME 6.14 APPLICATIONS AND SOLUTIONS 6.14.1 VPN As mentioned before, the main VPN idea is to create a secure path through the insecure Internet environment by using tunnelling, encryption, keys and other security enablers. VPN technology allows an organization to securely extend its networking services over the Internet to remote users, branch offices and partner companies, allowing 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 Secu