 of IMS includes additional functional entities, from which the most important are the following: Breakout gateway control function (BGCF). This is used for processing user requests for routing from an S-CSCF for the situations when S-CSCF has determined that it cannot use session routing by using DNS. Media gateway controller function (MGCF). This performs signaling translation between SIP and ISUP (ISDN user part, from the SS7 signaling system in PSTN). Multimedia resource function (MRF). This is split into two parts: Multimedia resource function controller (MRFC) is a signaling node that interprets the information coming from the application server and S-CSCF, and further uses such information for control of the media stream in the MRFP. Multimedia resource function processor (MRFP) is a user plane node that provides mixing of media streams. To cover the complete architecture shown in Figure 3.4 one should also note the following IMS entities: Home subscriber server (HSS). This is the main user database in the IMS architecture that contains home subscriber profiles, subscription information, and the user's net- work location (i.e. IP address). Also, HSS provides authentication and authorization of users, based on requests from CSCF entities. Subscriber location function (SLF). This is a resolution function of the IMS that pro- vides information about the HSS which has information on a given IMS user (e.g. upon requests from I-CSCF, S-CSCF, or ASs). Application server. This hosts and executes a given service, and is using SIP for com- munication with S-CSCF. Interconnection border control function (IBCF). This is used as a gateway to external networks, and provides firewall and NAT (network address translation) functionali- ties in the IMS architecture. The IMS is used for different services and different access networks which are all-IP based. It can be used in fixed networks such as xDSL networks, passive optical networks (PONs), QoS-enabled Ethernet, cable networks, as well as mobile networks (e.g. Uni- vers