page_title: IP Multimedia Subsystem
section_title: Architecture
hierarchical_section_title: IP Multimedia Subsystem / Architecture
caption_attribution_description: English: IP Multimedia Subsystem
context_page_description: The IP Multimedia Subsystem or IP Multimedia Core Network Subsystem is an architectural framework for delivering IP multimedia services. Historically, mobile phones have provided voice call services over a circuit-switched-style network, rather than strictly over an IP packet-switched network. Alternative methods of delivering voice or other multimedia services have become available on smartphones, but they have not become standardized across the industry. IMS is an architectural framework to provide such standardization.
  IMS was originally designed by the wireless standards body 3rd Generation Partnership Project, as a part of the vision for evolving mobile networks beyond GSM. Its original formulation represented an approach for delivering Internet services over GPRS. This vision was later updated by 3GPP, 3GPP2 and ETSI TISPAN by requiring support of networks other than GPRS, such as Wireless LAN, CDMA2000 and fixed lines.
  IMS uses IETF protocols wherever possible, e.g., the Session Initiation Protocol.
context_section_description: Each of the functions in the diagram is explained below.
  The IP multimedia core network subsystem is a collection of different functions, linked by standardized interfaces, which grouped form one IMS administrative network. A function is not a node (hardware box): An implementer is free to combine two functions in one node, or to split a single function into two or more nodes. Each node can also be present multiple times in a single network, for dimensioning, load balancing or organizational issues.
caption: 3GPP / TISPAN IMS architectural overview – HSS in IMS layer (as by standard)
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