3GPP has defined IMS for supporting converged multimedia services. Radisys Media Servers deliver the Multimedia Resource Function (MRF) component of the IMS architecture.
Radisys media servers support two control interface options for IMS MRF deployments:
SIP
Radisys media servers support SIP for MRF control as defined for the 3GPP Mr and Mr’ interface. The media control interface includes supplementary languages like VoiceXML or Media Server Markup Language (MSML – RFC 5707) for feature rich MRF control.
H.248
Radisys media servers also support H.248 for MRFP control as defined for the 3GPP Mp interface. The Mp interface is sometimes used by Media Gateway Control Function (MGCF) to control media processing on RadiSys media server when deployed as an MRFP.
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The 3rd Generation Partnership Project (3GPP) standards body developed IP Multimedia Subsystem (IMS) to facilitate converged IP-based services, while providing a migration from mobile GSM networks. The objective was to enable the delivery of rich services to any device and across any access network – essentially services to everyone, everywhere. The standard promotes cost-effective infrastructure by ensuring components (hardware and software) are compatible and interoperable, regardless of the manufacturer.
Within the 3GPP standards, a role has been clearly defined for the Media Resource Function (MRF), to provide real-time IP media processing functions such as media play, record, collect digits and mix audio/video against IP media streams. An MRF is sometimes further decomposed into a Media Resource Function Processor (MRFP), which terminates and processes the media streams, and the Media Resource Function Controller (MRFC), which selects and controls MRFPs.
The 3GPP has defined the Mb interface as the bearer traffic terminated on the MRF, or more specifically the MRFP. Bearer traffic includes the growing volume of Real Time Protocol (RTP) packets, along with associated Real Time Control Protocol (RTCP) information in the bearer plane.
In a 4G/LTE network, the Mb packet streams interface with the Packet Gateway (PGW), which provides connectivity between the user equipment and external packet data networks, manages quality of service (QoS) and provides deep packet inspection (DPI).
The PGW also performs routing, allocates IP addresses, provides access for non-3GPP access networks and enforces policy. For legacy networks, the Mb packet streams interface with the IMS Media Gateway (IMS-GW), which converts packet-based bearer traffic (e.g., RTP) for backwards compatibility with circuit-switched bearer paths (e.g., T1′s).
The MRF is an important and powerful IMS network element, however it requires command and control by other elements in the IMS architecture, such as Application Servers or Call State Control Functions (CSCFs).
Session Initiation Protocol (SIP) is the predominant standard for controlling MRF equipment in an IMS architecture. The Serving Call Session Control Function (S-CSCF) is a SIP server that is the central node for session control, basic call control, and policy enforcement in an IMS, which uses the Mr (SIP) interface to control MRF media processing functions.
When a value-added service (VAS), enhanced service, or more complex call processing is required, the CSCF will hand-off call control to an Application Server (AS), which would use the Mr’ (SIP) interface along with the Cr interface (media control requests) to control the MRF in order to create the desired service, such as ringback tones, multimedia streaming, or multimedia conferencing.
