d with that foreign local exchange, the PBX automatically selects the FX line. The dial tone the caller receives is actu- ally coming from the distant local exchange, and the call proceeds as if it were a local call. The tradeoff with FX lines is that although you are not charged per call for your long-distance calls to the specified exchange, you pay a flat monthly fee Chapter 4 The PSTN for the leased line, and you have to apply some traffic engineering to ensure you're not making people wait for the FX line to become available. So with lines, you need to find the right balance between reducing costs and ensuring a high level of service. OPXs are used in distributed environments, such as a city government. Say that the city government has public works stations, libraries, fire stations, and parks and recreation facilities that are too far from the PBX to be served by the normal cabling. The city uses an OPX setup: It connects a leased circuit from the PBX to the off- premises location and ties it in as if it were part of that PBX. City government employ- ees can then call one another, using their normal extension plan, their call accounting information can be accumulated SO that cost allocations can be performed, and the employees can access the full suite of features that a business PBX offers. Although you might think that VPNs are related to the Internet or to Internet Proto- col (IP) and are a somewhat new development, they actually originated in the circuit- switched network environment, with AT&T's software-defined network (SDN) in the early 1980s. A VPN is a concept, not a technology platform or a set of network- ing techniques. A VPN defines a network in which customer traffic is isolated over shared-service provider facilities; as more customers share the same facilities, their costs go down. The purpose of a VPN, then, is to reduce the high cost of leased lines while still providing high quality of service and guaranteeing that private traffic has capacity between locations. Figure 4.2 shows an exampl