Public Switched Telephone Network
Public Land Mobile Networks need to connect to the Public Switched Telephone Network (PSTN) in order to route calls.
The PSTN is the world's collection of interconnected voice-oriented public telephone networks, in much the same way that the Internet is the concentration of the world's public IP-based packet-switched networks. It is both commercially- and government-owned. This aggregation of circuit-switching telephone networks has evolved greatly from the days of Alexander Graham Bell, and in the late 20th century became almost entirely digital in nature — except for the final link from the central (local) telephone office to the user (the local loop). It also extends into mobile as well as fixed telephones.
The PSTN also furnishes much of the Internet's long-distance infrastructure and, for the majority of users, the access network as well. Because Internet Service Providers (ISPs) pay the long-distance carriers for access to their infrastructure, and share the circuits among many users through packet switching, the end Internet user avoids having to pay usage tolls to anyone other than their ISP.
Many observers believe that the long-term future of the PSTN is to be just one application of the Internet — however, the Internet has some way to go before this transition can be made. For example, the Quality of Service (QoS) guarantee is one aspect that needs to be improved for Voice over IP (VOIP) technology.
The PSTN is largely governed by technical standards created by the ITU-T, and uses E.163/E.164 addresses (usually called telephone numbers) for addressing. A number of large private telephone networks are not connected to the PSTN, and are used for military purposes (such as the Defense Switched Network). There are also private networks run by large companies which are linked to the PSTN, but only through controlled gateways such as private branch exchanges.
Read more about this topic: Public Land Mobile Network
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