IMS (IP Multimedia Subsystem) is a network architecture originally specified by the 3GPP (3rd Generation Partnership Project) wireless standards body designed for supporting multimedia services. Since then IMS has been expanded to include the majority of mobile wireless technologies as well as fixed broadband access technologies. The major benefit of IMS is that it offers a common approach to establishing multimedia connections across mobile and fixed broadband networks.
The intention is to converge on a protocol suite allowing interoperability among networks independent of the access to the end user, including DSL, cable, and various wireless technologies.
IMS-based femtocell architectures leverage the benefits of an IMS network, which include excellent scalability and an infrastructure to build advanced services.
In an IMS femtocell architecture one of the key aspects is where the SIP user agent is placed. The illustration below shows a SIP user agent residing in the femto access point (FAP).
When a mobile station is in the proximity of the femtocell, it communicates with the FAP using the standard air interface just as it does with a macrocell. The SIP user agent in the FAP registers and places SIP sessions for services on behalf of the mobile station utilizing the IMS network via the femtocell gateway. The femto gateway interfaces with the mobile core network to establish multimedia connections between subscribers attached in a femtocell and subscribers attached to the mobile core network.
An alternative to having the SIP user agent reside in the FAP is for it to be implemented in the femto gatewayas shown in the figure below;
In this approach the FAP encapsulates the control plane signaling received from the mobile station into an IPsec tunnel. Real-time voice and video signals are encoded using RTP at the FA. The RTP can also be encapsulated into an IPsec tunnel between the FAP and the femto gateway for added security preventing eavesdropping.
Watch the video below for an explanation of IMS;
