IMS, also known as IP Multimedia Subsystem is an IP-based multimedia and telephony core network technology, which was introduced by 3GPP and 3GPP2 standards and is based on IETF Internet protocols. IMS is based on a set of specifications describing the Next Generation Networking (NGN) architecture involving the implementation of IP based telephony and multimedia services. It contains the specification of a framework and a complete architecture enabling the integration of video, voice, data and mobile network technology on top of an IP-based infrastructure (Amirth, n.d.).
IMS is access technology independent since it supports IP-to-IP session over wired IP, wireless (i.e., 802.11,802.15, CDMA, etc), and packet data along with GSM/EDGE/UMTS and other packet data applications.

 IMS Architecture

In IMS, the networking infrastructure is subdivided into individual functions with standardized interfaces (reference point) between each of them. Every reference point defines both the operating functions and the protocol over the. Figures 1 and 2 show the IMS architecture overview.
The IMS architecture is split into three main layers containing a number of individual entities.

Figure 1: IMS architecture
Figure 2 presents a graphical overview of the IMS core entities. The combination of legacy mobile signaling networks, other IP multimedia networks, and the PSTN entities form the external interface functions. The combination of BGCF, CFCF, MGCF, and MRCF form the Sequencing and Control Functions. The combination of HSS and SLF form the Storage and Reference Functions. UE covers the User Interface Functions and the combination of MGW and MRFP form the Media Processing Functions. The interfaces between BGCF and MGCF and CDCF are of Ml type. The interface between MGCF and CSCF is Mg and the interface between WGW and MGCF is of M1 type.

Figure 2: IMS core network 
The traffic types are of two major categories; Data/Bearer and signaling/control types. The PSTN link between PSTN and MGW entities contain CSD (TDM) traffic and the Data/bearer, such as Mb interface, contains IPv6 traffic. The rest of the interfaces contain signaling and control traffic, including: MEGACO (H.248), ISUP, SIP, QoS (COPS), MAP/TCAP, and other types of traffic.

Transport and Endpoint Layer

This layer is involved in setting up sessions and providing bearer services and initiating and terminating SIP signaling procedures. This layer is also responsible for providing the media gateways to convert the VoIP data to the PSTN TDM format.

Session Control Layer

This layer includes Call Session Control Function (CSCF), which provides the routing for the SIP signaling messages and the endpoints for the registration. The routing functionality enables the SIP signaling to be routed to the correct application servers. Through communicating with the transport and endpoint layer, the CSCF is able to guarantee QoS.

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