e of a VPN. London New York IXC POP Switch IXC POP Switch Dubai IXC Network Switches and Transmission Facilities Washington, DC Singapore San Francisco Sydney Figure 4.2 An example of a VPN The PSTN Infrastructure The underlying facilities of a VPN include the carrier public network, aug- mented by network control programs (NCPs) and service management systems (SMSs). Under computer control, the traffic is then routed through the public net- work in a manner that makes the VPN service seem like a facilities-based private network. Access to the VPN can occur via dedicated access, leased lines, or carrier- switched access, using either an analog or a digital carrier. The NCP represents a centralized database that stores a subscriber's unique VPN information. The NCP screens every call and then applies call processing in accordance with the customer-defined requirements. A common-channel signaling network connects the various network elements so that they can exchange infor- mation with each other in real-time. (Common-channel signaling is discussed later in this chapter, in the section "Signaling Systems.") An SMS is used to build and maintain the VPN database. It allows customers to program specific functions to accommodate their particular business applications. It transmits information to the NCPs, with important instructions on a customer- by-customer basis. Thus, VPNs introduce to the realm of the PSTN a lower-cost alternative to building a private voice network. PSTN Architecture The PSTN includes a number of transmission links and nodes. There are basically four types of nodes: CPE nodes, switching nodes, transmission nodes, and service nodes. CPE Nodes The term CPE node generally refers to equipment located at the customer site. The main function of CPE nodes is to transmit and receive user information. The other key function is to exchange control information with the network. In the tradi- tional realm, this equipment includes PBXs, key telephone systems, and single-line telephones. Switching Nodes Switch