The public-switched telephone network (PSTN) consists of transmission components, switching components, and facilities for maintenance equipment, billing systems, and other internal components. Transmission components (links) define the cable or wireless infrastructure for transmitting signals. Switching components (nodes) include transmitters and receivers for setting up voice circuits.

The existing telecommunication systems in the Unites States consist largely of copper twisted-pair wiring in the local loop (the wiring from the phone company to homes and offices) and fiber-optic cable or microwave systems for backbone trunks and long-distance lines. The local loop still uses analog transmission methods for voice calls.

Figure T-5 illustrates service areas and facilities. LECs (local exchange carriers) operate within specific franchised service areas (basically service monopolies) called LATAs (local access and transport areas). LATAs were defined during the split-up of AT&T. The border of a LATA defines where local service ends and long-distance service begins. A LATA is associated with one or more telephone area code. LECs may be one of the incumbent carriers and a competitive access provider. The local carrier typically has several switching offices (called central offices, or COs) within the same LATA.

Figure T-5 (see book, page 1226)

An IXC (interexchange carrier) is any long-distance provider, such as AT&T, MCI, or US Sprint, that provides services between the LECs. The LECs are required to provide a point of presence (an interface) for the IXCs. Communication facilities employed by the IXCs include fiber-optic cable, ground-based microwave towers, and satellite-based microwave systems.

The telephone system was originally designed as a hierarchy of switches that set up calls across COs, across LATAs, or across long-distance connections. This hierarchy is pictured in Figure T-6.

Figure T-6 (see book, page 1227)

The hierarchy can be traced back to the first phone systems. In the late 1800s, when telephones were first introduced, people would buy a pair of phones and run a wire between the phones. Soon, cities were enmeshed in telephone cables running in all directions. Savvy entrepreneurs like Alexander Graham Bell built telephone companies so customers could run their wires to a single location and let operators connect them with other phones via a manually operated switching system. At first, customers could only connect with other customers at the switch, but soon, trunk lines were established between phone companies and everybody could call everybody else in the same local area. This grew into the hierarchy of switches that eventually extended to outlying areas and other cities. See "Network Core Technologies" for a description of how this system developed into an optical system.

Note in Figure T-6 that calls within the same CO do not need to be switched any further up the hierarchy than the local CO. A call attached to another CO within the same LATA may go directly to that CO if a line exists, or go through a tandem or long-distance carrier. InterLATA calls are handled by IXCs such as AT&T or MCI. The calls circuits are set up to the IXC's point of presence within the LATA, then out across the long-distance lines and into a point of presence at the other end. The point of presence may be within the local carriers CO or a separate building that may be just next door.

While this system can be explained as a hierarchical structure, today, almost all switching offices are now interconnected to avoid congestion. In addition, a nonhierarchical dynamic routing system is used, as explained later. In Figure T-6, the dotted lines indicate the addition of these trunks. Still, hierarchical terminology is used to describe switching equipment.

The original switches were manually configured by operators. Later, mechanical switches were developed, such as the rotary wiper switches that moved through contacts were arranged in a circle. ESS (electronic switching systems) started to appear in the 1960s. At first, the switches were electromechanical, with the electronic components being added to reduce the number of mechanical parts. Eventually, all-electric switches were developed using solid-state components and no mechanical parts. Today, switches are programmable digital systems that support unique services such as call forwarding and caller ID, as described later under "IN (Intelligent Network)."

A Class 5 switch is an end office switch that is located at a CO. Customers are attached to these switches. They provide POTS (plain old telephone service), local numbering, emergency services, and other services. COs connect to tandem office, and the Class 5 switches interconnect with Class 4 switches in the tandem office. Tandem switches provide connections among COs and connections into higher-level switches. There is little differentiation among Class 1 through Class 3 switches. They are often called regional, sectional, or intercity switched, respectively.

In the 1980s, AT&T replaced the static hierarchical network scheme with a dynamic routing scheme called DNHR (Dynamic Non-Hierarchical Routing). DNHR is similar to IP routing. A paper by Greg Trangmoe (see related entries page) describes DNHR.

The telephone system now uses digital signaling except in the local loop. When an analog voice signal reaches a central office, it is digitized and multiplexed into a digital trunk line that connects to another CO or a tandem office.

Analog voice is digitized using PCM (pulse-code modulation), a sampling technique in which the voltage level of an analog signal is read 8,000 times per second. This 8,000-Hz sampling rate is two times the highest frequency of the range used to carry voice (300 to 4,000 Hz) and produces a relatively good digital representation. Each voltage level sample is converted to an 8-bit value that is transmitted across the line. A single digitized voice call requires 64,000 bits/sec of bandwidth (8,000 samples/sec × 8 bits/sample).

The carriers use TDM (time division multiplexing) to transmit multiple voice calls over a single line. A single call has a data rate of 64 Kbits/sec as just described. This is called a DS-0 in NADH (North American Digital Hierarchy). A total of 24 DS-0s are multiplexed into a T1 circuit. A T3 line, which is called a DS-3, consists of 28 T1 lines or 672 DS-0 channels. At higher levels, DS signal are multiplexed into SONET neworks, which use the OC (optical carrier) scheme. See "NADH (North American Digital Hierarchy)" and "OC (Optical Carrier)."

DLC is a system that lets the telephone companies extend telephone services to outlying areas. Picture a small town with a single telephone company central office. All the copper wires for all the phones extend back to this central office. Now, suppose a subdivision is built outside town. To provide service, the telephone company installs a digital loop carrier system near the subdivision. All the subscribers in the subdivision connect to the DLC system, which itself is connected back to the central office via a trunk line (T1/E1) or fiber-optic connection.

With DLC, it is not necessary to run copper cable for every subscriber back to the central office. A DLC basically terminates the copper loop in local neighborhoods. The outlying DLC systems may be either remote offices that house DLC equipment to support entire neighborhoods or they may be small remote terminals, which are typically installed in office buildings and support approximately 100 subscribers.

DLC poses problems for CLECs that want to reach remote customers and offer DSL services. The CLEC must run a cable out to the remote terminal in order to access the copper loops. They may have trouble establishing a presence at the site and ILECs have not been accommodating to their needs.

The bottom of the hierarchy in Figure T-6 seems to indicate that all lines from COs terminate at a single phone. In fact, the phone company extends multichannel digital lines (T1 and T3 lines) into businesses that have multiple phones. The business sets up a PBX (private branch exchange) that essentially provides an extension of the telephone company's switching system into the local business. The telephone company can then route all calls for phones within a business to the PBX and rely on the PBX to distribute those calls. Centrex is a PBX that the carrier maintains at its own facility. Subscribers lease Centrex services rather than buy their own PBX. In either case, a digital trunk extends from the carrier to the customer site. See "PBX (Private Branch Exchange)" and "Centrex (CENTRal Exchange)."