n Chapter 4, "The PSTN.") PSTN local exchanges are interconnected into a hierarchy. For a local exchange to call a neighbor who resides 10 miles (16 km) away and who draws a dial tone from a different local exchange, the connection between those two different exchanges is accomplished through the second part of the hierarchy-a tandem switch (also called a junction exchange). The tandem switch is used to connect local exchanges throughout the metropolitan area. When it's time to make a long- distance toll call, another switching center is called into action-the toll center (also called the Class 4 office, transit switch, or trunk exchange). The toll center is responsi- ble for establishing and completing national long-distance communications. Transmission Lines The top of the hierarchy is the international gateway, whose exchanges are spe- cifically designed to connect calls between different countries. A trunk supplies the connections between the numerous switches within the PSTN, between customer-owned switches such as the PBX, and between the PBXs and the PSTN. On the other hand, a line supports a single user in the form of a subscriber line in the PSTN or an extension provisioned from the PBX. (Chapter 4 describes in detail the entities involved in managing local, tandem, and toll exchanges.) Virtual Circuits Today, because of the great interest in and increased use of packet switching, most networks use virtual circuits. Unlike a physical circuit, which terminates on specific physical ports, a virtual circuit is a series of logical connections between sending and receiving devices (see Figure 1.5). A virtual circuit is a connection between devices that acts as though it's a direct connection, but it may, in fact, be composed of a variety of different routes. The routes might change at any time, and the incom- ing return route doesn't have to mirror the outgoing route. These connections are defined by table entries inside the packet-switching device. A connection is estab- lished after the two devices agree on