The IMS Server is responsible for call/session control provided by the IMS in the subscriber’s home network. The IMS Server manages SIP sessions, provides features/services, co-ordinates with other network elements for session control, and allocates media resources.

IP Multimedia Subsystem (IMS) is a generic architecture for offering multimedia and voice over IP services, defined by 3rd Generation Partnership Project (3GPP). IMS is access independent as it supports multiple access types including GSM, WCDMA, CDMA2000, WLAN, Wire line broadband and other packet data applications. IMS will make Internet technologies, such as web browsing, e-mail, instant messaging and video conferencing available to everyone from any location. It is also intended to allow operators to introduce new services, such as web browsing, WAP and MMS, at the top level of their packet-switched networks.

IP Multimedia Subsystem is standardized reference architecture. IMS consists of session control, connection control and an applications services framework along with subscriber and services data. It enables new converged voice and data services, while allowing for the interoperability of these converged services between internet and cellular subscribers. IMS uses open standard IP protocols, defined by the IETF. So users will be able to execute all their services when roaming as well as from their home networks. So, a multimedia session between two IMS users, between an IMS user and a user on the Internet, and between two users on the Internet is established using exactly the same protocol. Moreover, the interfaces for service developers are also based on IP protocols.

Some of the possible applications where IMS can be used are:
Presence services
Full Duplex Video Telephony
Instant messaging
Unified messaging
Multimedia advertising
Multiparty gaming
Videostreaming
Web/Audio/Video Conferencing
Push-to services, such as push-to-talk, push-to-view, push-to-video

Effectively, IMS provides a unified architecture that supports a wide range of IP-based services over both packet- and circuit-switched networks, employing a range of different wireless and fixed access technologies. A user could, for example, pay for and download a video clip to a chosen mobile or fixed device and subsequently use some of this material to create a multimedia message for delivery to friends on many different networks. A single IMS presence-and-availability engine could track a user’s presence and availability across mobile, fixed, and broadband networks, or a user could maintain a single integrated contact list for all types of communications.

A key point of IMS is that it is intended as an open-systems architecture: Services are created and delivered by a wide range of highly distributed systems (real-time and non-real-time, possibly owned by different parties) cooperating with each other. It is a different approach to the more traditional telco architecture of a set of specific network elements implemented as a single telco-controlled infrastructure.

Home subscriber server HSS
HSS is the master database containing all users and subscription data, which the different network core functional entities (CSCF) access when handling subscribers. HSS contains user data that is downloaded to S-CSCF and stores temporary data with the location of the S-CSCF where the user is currently registered.

Policy decision function (PDF)
PDF is the logic function that implements the decision concerning the policy to be applied, making use of one of the standard QoS mechanisms on the IP connectivity layer.

Signaling gateway
Provides the conversion of signalling in both direction on the transport level between SS7 and signalling transport based on IP (e.g. ISUP/SS7 and ISUP/SCTP/IP).

Multimedia Resource Function Controller (MRFC)
The MFRC controls the media stream resources in the Mulitmedia Resource Function Processor (MRFP). It includes the media stream resources needed to provide tones, announcements, and conferencing).

Media Gateway Control Function (MGCF)
The MGCF provides the signalling inter-working functions between the IMS and the PSTN/PLMN network. The MGCF controls a set of Media Gateways (MGWs) in the form of H.248 signalling. With H.248, the MGCF is able to control establishment of bearer resources for sessions that require inter-working for bearer between the PSTN/PLMN and the IMS.

The MGCF will allow the support of a Virtual Media Gateway functionality, allowing a physical MG to be logically split into several virtual MGs.

The major IMS network elements include

Access Gateway (AG): This network element provides an interface between the radio network (Access Network) and the IP-based network.

Access Network (AN): This is the radio portion of the network.

Breakout Gateway Control Function (BGCF): Controls resources allocation to IP sessions

Call Session Control Function (CSCF): Provides control and routing function for IP sessions.

Foreign Agent (FA): Advertises itself to mobile stations in serving area. Provides registration information to Home Agent. Forwards packets from mobile to Home Agent.

Home Agent (HA): Tracks current Foreign Agent serving the mobile. Forwards packets to current FA.

Home Subscriber Server (HSS): Can take the place of a HLR in all-IP network. Contains AAA function and other databases

Media Gateway (MGW): Provides interface for bearer traffic between IP and PSTN

Media Gateway Control Function (MGCF): Provides signaling interoperability between IP and PSTN domains – SIP to ISUP and vice versa

Policy Decision Function (PDF): As IP networks, unlike TDM networks, assign network bandwidth and resources in real-time, the PDF’s role is to assign resources according to demand and QoS requirements.

Position Determining Entity (PDE): While some mobiles can determine position independently, the PDE can provide assistance by way of location determination algorithms

SIP Application Server: Represents a platform for SIP application development and operation.