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It is able to map and realise all communication services between the different parties during switching and transfer. IMS systems are primarily used in mobile networks but also in [fixed-line networks](/gb/get-started/cloud-telephony/lexicon/knowledge-base-detail/the-fixed-line-network). ### IMS standardization for mobile and fixed-line networks The first specification of IMS is from 2003 and was created by the 3rd Generation Partnership Project committee (3GPP). The original area of application of the IP Multimedia Subsystem was the service network of the 3G UMTS mobile network. IMS is now also used in 4G LTE networks. The standard describes how multimedia connections and services are controlled by the IMS core network. Services can be provided over open interfaces via application server. The standard specifies transport, security mechanisms and signaling for this purpose. Whilst IMS was only defined for mobile networks, the standard was soon expanded to fixed-line networks. ### Fundamental requirements for the IP Multimedia Subsystem Key requirements for a IP Multimedia Subsystem are linking classic mobile or fixed-line networks with IP-based networks and providing additional services. This includes IP-based real time services such as voice, video-telephony, machine-to-machine communication, as well as games. Thanks to cross-provider function and defined IMS standards new services and products can be established within a short amount of time. All communication between the subscribers uses packet-switched connections which are able to guarantee defined [qualities of service (QoS)](/gb/get-started/cloud-telephony/lexicon/knowledge-base-detail/qos) end-to-end. IMS allows services to operate independently from the access network and can be charged according to service. ### SIP as an important part of an IP Multimedia Subsystem One key component of IMS is the Session Initiation Protocol, or SIP for short, primarily known as a protocol for signaling and establishing a connection in [Voice-over-IP telephony (VoIP telephony)](/gb/get-started/cloud-telephony/lexicon/knowledge-base-detail/voice-over-ip). In the IP Multimedia Subsystem, [SIP](/gb/get-started/cloud-telephony/lexicon/knowledge-base-detail/sip) governs communication between the individual network elements, as well as between the various users. ### 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. 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