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The features of a PSTN are: * Subscribers can be connected by entering telephone numbers * The existing connections are primarily used to transmit speech information * After hanging up the connection is closed and the resources used become available to other subscribers The term public switched telephone network is primarily used for public [landlines](/gb/get-started/cloud-telephony/lexicon/knowledge-base-detail/dedicated-line). With traditional analogue landlines speech is transmitted analogue, a [ISDN](/gb/get-started/cloud-telephony/lexicon/knowledge-base-detail/isdn) network transmits speech information digitally. ### The traditional structure of the Public Switched Telephone Network The traditional public switched telephone network has a strictly hierarchic architecture and a star structure. The individual subscriber lines are connected to a local exchange, which in turn communicates with trunk exchanges, main and central exchanges. Lines within a local exchange typically have the same area code. To call a number outside the local exchange the area code must be added. International calls require dialling the [country code](/gb/get-started/cloud-telephony/lexicon/knowledge-base-detail/national-international-phone-numbers). These start with a double zero. ### Replacing the Public Switched Telephone Network with Voice over IP telephony (VoIP) Since the introduction of [Voice over IP telephony (VoIP)](/gb/get-started/cloud-telephony/lexicon/knowledge-base-detail/voice-over-ip), traditional public switched telephone networks are slowly being replaced. Voice over IP uses the internet for speech communication and transmits the voice data as individual data packets. Unlike public switched telephone networks, there is no static connection between the two subscribers, instead data is dynamically routed over the data network using a virtual connection. Since each data packet has a destination address, the packets can even be routed differently over the network. On the other end they are reassembled in the correct order and converted back into analogue speech. ### Related Articles ### Voice over Internet Protocol (VoIP) **What is VoIP?** The **Voice over Internet Protocol,** also called IP telephony, is a method of communication for making calls over a broadband Internet connection as an alternative to regular calls made over an analogue phone using the [PSTN](/gb/get-started/cloud-telephony/lexicon/knowledge-base-detail/pstn). These VoIP calls can happen entirely over the Internet but where required, gateways also link to the PSTN. This means that VoIP users can still connect to anyone with a telephone number including local, mobile, and international numbers. VoIP services can be accessed from a computer with a so-called [softphone,](/gb/get-started/cloud-telephony/lexicon/knowledge-base-detail/softphones) from an IP phone, from a smartphone or from a conventional landline phone that is connected to a VoIP adapter. [Voice over Internet Protocol (VoIP)](/gb/get-started/cloud-telephony/lexicon/knowledge-base-detail/voice-over-ip) ### Integrated Services Digital Network (ISDN) In telecommunications technology, the abbreviation ISDN stands for the technical term "Integrated Services Digital Network" and refers to a digital standard for telephone networks. As the name implies, various types of communication services can be transmitted through this kind of network. In addition to voice calls, ISDN allows packet-switched or circuit-switched data transmission and fax transmission. In some countries, ISDN technology has taken over from analogue telephone networks, integrating all types of services into a common network for the first time. This digitisation has also enabled digital ISDN phone lines to be made available to end users. In comparison to an analogue connection, an ISDN subscriber line offers a whole range of advantages. The basic ISDN line allows multiple phones or communication [devices](https://web-production.nfon.com/gb/products/devices), with different numbers ([MSNs](/gb/get-started/cloud-telephony/lexicon/knowledge-base-detail/msn)), to operate and use two parallel connections simultaneously. In addition, the ISDN connection also offers higher data transfer rates to an analogue connection using a modem, for example, when accessing the Internet. To utilise the ISDN line, end users need special ISDN-capable phones or faxes. The use of so-called [A/B converters](/gb/get-started/cloud-telephony/lexicon/knowledge-base-detail/a-b-interface), linking analogue devices to a digital connection, is common. Meanwhile, other techniques such as DSL or cable connections are used for connecting to the internet. 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