f network switches has evolved over time, and the appropriate switch is called into action, depending on which two points the switches connect. For example, in Figure 1.4 the CPE is on the left-hand side. Each individual single-line instrument rep- resents a subscriber line. (Again, the fact that it's called a line means that it's a circuit configured to carry the calling load of just one user.) Above the single-line instrument is a business enterprise with a PBX. The con- nection from this PBX to the PSTN occurs over a trunk specifically config- ured to carry the calling load of multiple users. Beyond the PBX are multiple end users that are attached to that PBX. Each end user's connec- tion is referred to as a station line, again emphasizing that the line is carry- ing the calling load of one user. Chapter 1 Telecommunications Technology Fundamentals Local Loop Backbone, or Core Network Individual Local Interexchange Trunks User Station Exchange Trunk International Gateway: for Lines, or routing calls between countries Extensions Subscriber Line Toll/Transit Switch (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 1.4 Lines, trunks, and switches The customer environment attaches to the PSTN, and the first point of access is the local exchange, which is also referred to as a Class 5 office or a central office. The traditional (electromechanical and crossbar) local exchange switch can handle one or more exchanges, with each exchange capable of handling up to 10,000 sub- scriber lines, numbered 0000 to 9999. Electronic switches, available since 1980, can handle up to 50,000 subscribers. The only kind of call that a local exchange can complete on its own, without touching any of the other switches in the net- work, is to another number in that same local exchange. (Local exchanges are dis- cussed in detail i