Structure of the PSTN

At the very beginning of the telephony age, telephones were sold in pairs; for a call to be made, the two telephones involved had to be connected directly. So, in addition to the grounding wire, if there were 20 telephones you wanted to call (or that might call you), each would be connected to your telephone by a separate wire. At certain point, it was clear that a better long-term solution was needed, and such a solution came in the form of the first Bell Company switching office in New Haven, Connecticut. The office had a switching board, operated by human operators, to which the telephones were connected. An operator’s job was to answer the call of a calling party, inquire as to the name of the called party, and then establish the call by connecting with a wire two sockets that belonged to the calling and called party, respectively. After the call was completed, the operator would disconnect the circuit by pulling the wire from the sockets. Note that no telephone numbers were involved (or needed). Telephone numbers became a necessity later, when the first automatic switch was built. The automaton was purely mechanical—it could find necessary sockets only by counting; thus, the telephones (and their respective sockets in the switch) were identified by these telephone numbers. Later, the switches had to be interconnected with other switches, and the first telephone network—the Bell System in the United States—came to life. Other telephone networks were built in pretty much the same way.

Many things have happened since the first network appeared—and we are going to address these things—but the structure of the PSTN in terms of its main components remained unchanged as far as the establishment of the end-to-end voice path is concerned. The components are:

  • Station equipment [or customer premises equipment (CPE)]. Located on the customer’s premises, its primary functions are to transmit and receive signals between the customer and the network. These types of equipment range from residential telephones to sophisticated enterprise private branch exchange systems (PBXs).

  • Transmission facilities. These provide the communications paths, which consist of transmission media (atmosphere, paired cable, coaxial cable, light guide cable, and so on) and various types of equipment to amplify or regenerate signals.

  • Switching systems. These interconnect the transmission facilities at various key locations and route traffic through the network. (They have been called switching offices since the times of the first Connecticut office.)

  • Operations, administration, and management (OA&M) systems. These provide administration, maintenance, and network management functions to the network.

Until relatively recent times, switching boards remained in use in relatively small organizations (such as hotels, hospitals, or companies of several dozen employees), but finally were replaced by customer premises switches called private branch exchanges (PBXs). The PBX, then, is a nontrivial, most sophisticated example of station equipment. On the other end of the spectrum is the ordinary single-line telephone set. In addition to transmitting and receiving the user information (such as conversation), the station equipment is responsible for addressing (that is, the task of specifying to the network the destination of the call) as well as carrying other forms of signaling [idle or busy status, alerting (that is, ringing), and so on].

As Figure 1demonstrates, the station equipment is connected to switches. The telephones are connected to local switches (also interchangeably called local offices, central offices, end offices, or Class 5 switches) by means of local loop circuits or channels carried over local loop transmission facilities. The circuits that interconnect switches are called trunks. Trunks are carried over interoffice transmission facilities. The local offices are, in turn, interconnected to toll offices (called tandem offices in this case). Finally, we should note that in all this terminology the word office is interchangeable with exchange, and, of course, switch. It is very difficult to say which word is more widely used.

Figure 1: Local and tandem offices.

The trunks are grouped; it is often convenient to refer to trunk groups, which are assigned specific identifiers, rather than individual trunks. Grouping is especially convenient for the purposes of network management or assignment to transmission facilities. (A trunk is a logical abstraction rather than a physical medium; a trunk leaving a switch can be mapped to a fiber-optic cable on the first part of its way to the next switch, microwave for the second part, and four copper wires for the third part.)

In the original Bell System, there were five levels in the switching hierarchy; this number has dropped to three due to technological development of nonhierarchical routing (NHR) in the long-distance network. NHR was not adopted by the local carriers, however, so they retained the two levels—local and tandem—of switching hierarchy.

Local switches in the United States are grouped into local access and transport areas (LATAs). You can find a current map of LATAs at A LATA may have many offices (on the order of 100), including tandem offices. Service within LATAs is typically provided by local exchange carriers (LECs). Some LECs have existed for a considerable time (such as original Bell Operating Companies, created in 1984 as a result of breakup of the Bell system), and so are called incumbent LECs (ILECs); others appeared fairly recently, and are called competitive LECs (CLECs). Inter-LATA traffic is carried by inter-exchange carriers (IXCs). The IXCs are connected to central or tandem offices by means of points of presence (POPs).

Figure 1 depicts an interconnection of an IXC with one particular LATA. The IXC switches form the IXC network, in which routing is typically nonhierarchical. Presently, IXCs are providing local service, too; however, since their early days IXCs have had direct trunks to PBXs of large companies to whom they provided services like virtual private networks (VPNs).[3] We should mention that IXCs in the United States can (and do) interconnect with the overseas long-distance service providers by means of complex gateways that perform call signaling translations, but the IXCs in the United States are typically not directly interconnected with each other.

Figure 2: The interconnection of LATAs and IXCs.

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