Public Switched Telephone Network (PSTN)
Public Switched Telephone Network also known as POTS (Plain Old Telephone Service) is a wired phone system over which landline telephone calls made. The PSTN relies on circuit switching. Telephone invented 1876 when initially sold in pairs. First switching office opened 1878 then second-level switching offices, then third-level, etc. The Phones connected to end office (local CO) via local loop. COs connected to toll offices via turn & Calls moved from one trunk to another via switching office.
PSTN can divide 3 categories:
- Local Exchange Network
- Inter Exchange Network
- International Network
Hierarchical Networks (PSTN Hierarchy): The PSTN hierarchy is implemented differently in the different countries. The following sections provides an overview of the general PSTN hierarchy and its related terminology.
The PSTN is generally divided into three categories:
- Local Exchange Network: The Local Exchange Network consists of the digital switching nodes(EOs) that provide network access to the subscriber. The Local Exchange terminate both lines and trunks, providing the subscriber access to the PSTN. A Tandem Office often connects End Offices with a local area, but they can also be connected directly. The primary purpose of a Local Tandem is to provide interconnection between End Offices in a localized geographic region.
- Inter Exchange Network: The Inter Exchange network is comprised of digital switching nodes that provide the connection between Local Exchange network. Because they are points of high traffic aggregation and they cover larger geographical distances, high-speed transports are typically used between transit switches.
- International Network: The International network consists of digital switching nodes, which are located in each country and act as international gateways to destinations outside of their respective countries. These gateways adhere to the ITU international standards to ensure interoperability between national networks.
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