0000 to 9999. In large metropolitan areas, it is common to find one local exchange Local Loop Backbone, or Core Network Local Individual Exchange Trunk Interexchange Trunks User Station Lines, or International Gateway: for routing calls between countries Extensions Toll/Transit Switch Subscriber Line (Class 4): for routing calls to or from other cities Local Residential Exchange Trunk Service Tandem/Junction Exchange: for routing calls between local Subscriber Line exchanges within the city Local Exchange, Tandem Trunks Class 5 Switch, Central Office Figure 4.3 Types of telephone exchanges The PSTN Infrastructure building that houses more than one local exchange switch and for each switch to handle five or more exchanges. These offices are sometimes referred to as multi- entity buildings. The Tandem Office The tandem office, or junction network, is an exchange used primarily as a switching point for traffic between local exchanges in a metro- politan area. It is an office used to interconnect the local end offices over tandem trunks in a densely settled exchange area where it is not economical for a tele- phone company to provide direct interconnection between all end offices. The tan- dem office completes all calls between the end offices but is not directly connected to subscribers. The Toll Office The toll office (also called the trunk exchange or transit switch) is a telephone company switching center where channels and toll message circuits terminate-in other words, where national long-distance connections are made. This is usually one particular exchange in a city, but larger cities may have several exchanges where toll message circuits terminate. The International Gateway The international gateway is the point to and from which international services are available in each country. Protocol conversion may take place in the gateway; in ITU terminology, this is called a centre de transit (CT). CT1 and CT2 international exchanges connect only international circuits. CT2 exchanges switch traffic between regiona