ure 1.3 shows an example of a segment of a network in which two- and four-wire circuits are traditionally used. Channels A channel defines a logical conversation path. It is the frequency band, time slot, or wavelength (also referred to as lambda, 2) allocated to a single conversation. In the context of telecommunications, a channel is a child of the digital age because digi- tal facilities enable multiple channels, greatly increasing the carrying capacity of an individual circuit. Because we are becoming more digitalized all the time, people often refer to the number of channels rather than the number of circuits. Lines and Trunks Lines and trunks are basically the same thing, but they're used in different situa- tions. A line is a connection configured to support a normal calling load generated Transmission Lines Telephone Set International Telephone Set Local Local Gateway Data Terminal Exchange Exchange Exchange Exchange Data Terminal Subscriber Loop Intertoll Trunks Subscriber Loop (2-wire transmission) (4-wire transmission) (2-wire transmission) Toll Connecting Trunk Toll Connecting Trunk (2-wire transmission) (4-wire transmission) City A = Amplifier/Repeater City B Figure 1.3 Using two-wire and four-wire circuits by one individual. A trunk is a circuit configured to support the calling loads gener- ated by a group of users; it is the transmission facility that ties together switching systems. A switching system is a device that connects two transmission lines. There are two major categories of switching systems: CPE switches-The most prevalent form of switch in the customer pre- mises equipment (CPE) environment is the private branch exchange (PBX), which is called a private automatic branch exchange (PABX) in some parts of the world. A PBX is used to establish a connection between two points. It establishes connections between telephones that are internal to the organi- zation, and it establishes connections between internal extensions and the outside world (i.e., the PSTN). Network switches-A hierarchy o