PSTN - Public Switched Telephone Network

PSTN is a circuit-switched network based on analog technology. Among numerous WAN interconnection technologies, interconnection via PSTN requires the lowest communication cost, but its data transmission quality and speed are also the poorest, and the utilization rate of PSTN network resources is relatively low. Access capabilities via PSTN include: - Dial-up Internet/Intranet/LAN access; - Network interconnection between two or more LANs; - Interconnection with other WAN technologies. Despite the various issues associated with data transmission over PSTN, it remains an irreplaceable networking medium (technology). In particular, the xDSL technology and products developed by Bellcore based on PSTN have expanded the development and application space of PSTN, enabling networking speeds between 9 Mbps and 52 Mbps. Technologies employed by PSTN: PSTN provides an analog dedicated channel, with channels connected via several telephone switches. When two hosts or router devices need to connect via PSTN, modems must be used at the network access side (i.e., the subscriber loop side) of both ends to perform analog-to-digital and digital-to-analog signal conversion. From the perspective of the OSI seven-layer model, PSTN can be regarded as a simple extension of the physical layer, without providing users with services such as flow control or error control. Moreover, since PSTN operates in a circuit-switched manner, a path, from establishment to release, has its entire bandwidth available only to the devices at both ends of the path, even if there is no data to transmit between them. Therefore, this circuit-switched approach cannot achieve full utilization of network bandwidth.

In today's public network architecture, a simple analog telephone call between two parties is no longer simple. First, and most importantly, there are actually only two "network clouds" in the PSTN: one carries signaling, and the other carries voice-bandwidth data. Part of the problem with merging services in the current network architecture is that the signaling for basic local access services is hardware-based, primarily tailored for voice telephony, with some basic considerations for point-to-point voice signaling over ISDN lines, mainly targeting those who can afford it. This is a serious limitation and a good reason for implementing changes. Second, bringing interactive multimedia applications, such as distance learning, into the home presents difficulties. Insufficient signaling transmission and bandwidth are both serious limitations. The convenience of on-demand entertainment and online shopping is also a driving force behind network modernization. In new telephone services, inexpensive full-color, full-motion videoconferencing with whiteboard capability is also not far off. The side effects of all these new features may cost us a great deal, but they indeed bring us tremendous benefits, namely enabling us to exchange information conveniently in ways we have never experienced before.