Public Switched Telephone Network (PSTN) In order to understand the issues in replacing legacy PSTN systems with VoIP, it is important to understand the operational characteristics, and limitations, of each.
The PSTN can be characterized by thinking of it as dumb devices driven by an intelligent network. In the PSTN, signals are sent between telephony switches to set up and terminate calls and indicate the status of terminals involved in the calls. These signals are
carried over a separate data network known as Common Channel Signaling (CCS—represented by the black lines in the diagram below). The protocol used by CCS is Signaling System 7 (SS7).
The entire system is called the Intelligent Network (IN). Signaling Control Points (SCPs) are databases that provide information necessary for advanced call-processing capabilities. For example, an SCP would be queried to determine the routing number associated with a dialed toll free 800/888 number or to validate the personal identification number (PIN) of a calling card user.
Within the next few years, the existing PSTN will slowly be replaced by public packet networks. During this transition, the PSTN will continue to be heavily used because of the millions of users and non-IP devices still connected to it. In addition, the PSTN
supports a variety of voice services through the signaling control points as illustrated above. An Internet telephony device needing to connect with one of these services must use PSTN signaling for the foreseeable future.