name: PSTN
full_form: Public Switched Telephone Network
context: While SDES is suitable as an end-to-end solution, where only the endpoints encrypt/decrypt the media streams, it is usable in a straightforward way also as an end-to-middle solution. The classical end-to-middle scenario is a call between an IMS endpoint and a PSTN endpoint. Here, the media gateway (plus its controller) can perform the media plane security procedures on behalf of the PSTN endpoint. In this approach, the network chooses a key for protecting the media sent by the PSTN endpoint on the IP based call leg and inserts the key into the SDP body sent to the IMS endpoint. This key, as well as the key provided by the IMS endpoint, are passed to the media gateway which performs encryption/decryption between the PSTN call leg and the IP based call leg. Figure 13 illustrates the principle of this procedure, for a voice call from an IMS endpoint to a PSTN endpoint. Note that the picture is an abstraction focussing on the SDP offer and answer, not on the SIP messages. It does not show
description: The Public Switched Telephone Network (PSTN) refers to the global network of public circuit-switched telephone networks, which provide basic telecommunication services such as voice calls and facsimile transmissions.
explanation: Based on the context provided from 3GPP TS 33.828, section 7.3.5, PSTN is classified as a Network due to its role in facilitating communication between different endpoints, including IMS and PSTN endpoints, through media gateways and controllers.
section_title: 7.3.5	SDES in end-to-middle scenarios
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