Editing IP Multimedia Subsystem (section)

== History ==
*IMS was defined by an industry forum called 3G.IP, formed in 1999. 3G.IP developed the initial IMS architecture, which was brought to the 3rd Generation Partnership Project ([[3GPP]]), as part of their standardization work for [[3G]] mobile phone systems in [[Universal Mobile Telecommunications System|UMTS]] networks. It first appeared in Release 5 ([[Evolution to 3G|evolution from 2G to 3G networks]]), when SIP-based multimedia was added. Support for the older [[GSM]] and [[GPRS]] networks was also provided.<ref>{{cite web|url=http://www.3gpp.org/ftp/Information/WORK_PLAN/Description_Releases/|title=3GPP Release Descriptions|work=3GPP}}</ref>
* [[3rd Generation Partnership Project 2|3GPP2]] (a different organization from 3GPP) based their CDMA2000 Multimedia Domain (MMD) on 3GPP IMS, adding support for [[CDMA2000]].
* 3GPP release 6 added interworking with [[Wireless LAN|WLAN]], inter-operability between IMS using different IP-connectivity networks, routing group identities, multiple registration and forking, presence, speech recognition and speech-enabled services ([[Push to talk]]).
* 3GPP release 7 added support for [[Fixed phone|fixed]] networks by working together with [[TISPAN]] release R1.1, the function of AGCF (access gateway control function) and PES ([[Public switched telephone network|PSTN]] emulation service) are introduced to the wire-line network for the sake of inheritance of services which can be provided in PSTN network. AGCF works as a bridge interconnecting the IMS networks and the Megaco/H.248 networks. Megaco/H.248 networks offers the possibility to connect terminals of the old legacy networks to the new generation of networks based on IP networks. AGCF acts a SIP User agent towards the IMS and performs the role of P-CSCF. SIP User Agent functionality is included in the AGCF, and not on the customer device but in the network itself. Also added voice call continuity between circuit switching and packet switching domain ([[Voice call continuity|VCC]]), fixed broadband connection to the IMS, interworking with non-IMS networks, policy and charging control ([[policy and charging control|PCC]]), emergency sessions. It also added SMS over IP.<ref name=":0">{{Cite web|title=Dispelling LTE Myths|url=https://www.3gpp.org/news-events/1268-Dispelling-LTE-Myths|access-date=2021-04-08|website=www.3gpp.org}}</ref>
* 3GPP release 8 added support for [[3GPP long-term evolution|LTE]] / [[system architecture evolution|SAE]], multimedia session continuity, enhanced emergency sessions, SMS over SGs<ref name=":0" /> and IMS centralized services.
* 3GPP release 9 added support for IMS emergency calls over [[GPRS]] and [[Evolved Packet System|EPS]], enhancements to [[multimedia telephony]], IMS [[media plane]] security, enhancements to services centralization and continuity.
* 3GPP release 10 added support for inter device transfer, enhancements to the single radio voice call continuity (SRVCC), enhancements to IMS emergency sessions.
* 3GPP release 11 added [[Unstructured Supplementary Service Data|USSD]] simulation service, network-provided location information for IMS, [[SMS]] submit and delivery without MSISDN in IMS, and overload control.

Some operators opposed IMS because it was seen as complex and expensive.
In response, a cut-down version of IMS—enough of IMS to support voice and SMS over the LTE network—was defined and standardized in 2010 as [[Voice over LTE]] (VoLTE).<ref>
Ian Poole, editor.
[https://www.electronics-notes.com/articles/connectivity/4g-lte-long-term-evolution/volte-voice-over-lte.php "What is Voice over LTE, VoLTE"].
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