 queries from the I-CSCF or appli- cation server. Policy controllers-A policy controller is a server that performs the pol- icy decision function (PDF) for QoS and security. It checks the HSS data- base and uses COPS to control policy enforcement points embedded in other network elements. Border gateway control function (BGCF)-The BGCF handles control/ signaling messages to other IMS domains. Multimedia resource function controller (MRFC)-The MRFC inter- prets information coming from an application server and S-CSCF and controls the multimedia resource function processor (MRFP) accord- ingly. It also generates CDRs. Multimedia gateway controller function (MGCF)-The MGCF, also known as a softswitch or call agent, communicates with the CSCF and controls the connections for media channels in an IMS media gateway. The MGCF performs protocol conversion between ISUP (ISDN, User Part) and the IMS call control protocols. (ISUP is a part of SS7 that is used to set up calls in the PSTN.) Transport- or access-layer components: Multimedia resource function processor (MRFP)-The MRFP provides a wide range of functions for multimedia resources, including provision of resources to be controlled by the MRFC, mixing of incoming media streams, sourcing media streams (for multimedia announcements), and processing of media streams. Media gateway (MGW)-The MGW functions as a translation device between different networks (e.g., by performing the conversions between VoIP and TDM-based voice on the PSTN). A related signaling gateway (SGW) may be included for exchanging control messages with the PSTN's SS7 control network. MGWs are controlled by the MGCF, and they com- municate with one another using protocols such as MGCP or Megaco. The History and Future of IMS The base IMS functionality was first defined in the 3GPP Release 5 (R5) standards, finalized in March 2003. This standard was optimized for use by GSM UMTS wire- less networks. The Next-Generation Network Infrastructure The second phase of IMS standards development ended in Septe