Public Switched Telephone Networks (PSTNs)

Public telephone networks are unrestricted dialing telephone networks that are available for public use to interconnect communication devices. Public telephone networks within countries and regions are a standard integrated system of transmission and switching facilities, signaling processors, and associated operations support systems that allow communication devices to communicate with each other when they operate.

Figure 1 shows a basic overview of the Public Switched Telephone Network (PSTN) as deployed in a typical metropolitan area. PSTN customers connect to the end-office (EO) for telecommunications services. The EO processes the customer service request locally or passes it off to the appropriate end or tandem office. As Different levels of switches interconnect the parts of the PSTN system, lower-level switches are used to connect end-users (telephones) directly to other end-users in a specific geographic area. Higher-level switches are used to interconnect lower level switches.


Figure 1: Public Switched Telephone Network (PSTN)
Switches send control messages to each other through a separate control-signaling network called signaling system number 7 (SS7). The SS7 network is composed of signaling transfer points (STPs) and service control point (SCP) databases. A STP is used to route packets of control

messages through the network. SCP’s are databases that are used by the network to process or reroute calls through the network (such as 800 number toll free call routing). SS7 also provides for the newer features such as incoming call identification and automatic call rerouting used by some service companies that provide 24/7, worldwide dial-in support.

Overview
Public telephone networks include local loops (access lines), switching systems, numbering plans, and are coordinated by network management systems. Post, telephone, and telegraph (PTT) and local exchange carriers (LEC’s) are the established telephone network operators or companies that provide local telecommunications services. For most countries, PTTs are government operated telephone systems. In the United States, LEC’s are granted franchises to provide telephone services to certain geographical areas as mandated by the Federal Communication Commission (FCC). Recently, deregulation and privatization of telecommunication systems worldwide have allowed the creation of a new competing local exchange carriers (CLECs). CLEC’s provide similar services as LEC’s and PTTs. In some cases, CLECs provide services by leasing existing lines from incumbent local exchange carriers (ILECs) and reselling services on these lines. In other cases, CLECs install new communication lines or provide connection by wireless service.