ing nodes interconnect transmission facilities at various locations and route traffic through a network. They set up the circuit connections for a signal path, based on the number dialed. To facilitate this type of switching, the ITU standard- ized a worldwide numbering plan (based on ITU E.164) that essentially acts as the routing instructions for how to complete a call through the PSTN. The switching nodes include the local exchanges, tandem exchanges (for routing calls between local exchanges within a city), toll offices (for routing calls to or from other cities), and international gateways (for routing calls to or from other countries). Primary network intelligence is contained in the Class 4 switches (i.e., toll offices switches) Chapter 4 The PSTN and Class 5 switches (i.e., local exchange switches). The Class 4 switches provide long-distance switching and network features, and the Class 5 switches provide the local switching and telephony features that subscribers subscribe to. Figure 4.3 shows where the types of telephone exchanges are located. The Local Exchange The local exchange (also called the Class 5 office or central office) is where communications common carriers terminate customer lines and locate the switching equipment that interconnects those lines. The local exchange represents the local network. Every subscriber line location in a local exchange is assigned a number, generally 7 digits (in the United States) or 8 digits (in many other countries). The first 3 (or 4) digits represent the exchange and identify the local exchange switch that serves a particular telephone. The last 4 digits identify the individual line number, which is a circuit that is physically connected from the local exchange to the subscriber. Some areas have gone to 10-digit dialing for local calls, meaning the area code must be included even when dialing a local number. The traditional local exchange switch can handle one or more exchanges, and each exchange is capable of handling up to 10,000 subscriber lines, numbered 