ent, the maintenance and the release of the connections in the IMS-MGW entity. A connection represents an association between an end-point related to the interface with the PSTN or PLMN networks and an end-point related to the interface with the IP network. The MGCF entity performs the translation between SIP (Session Initiation Protocol) signaling used in the IMS network and telephone Interconnections signaling at the interconnection between IMS or CS networks, using the following protocols: - ISUP (ISDN User Part), for interconnection in TDM mode, with PSTN and PLMN networks; - BICC (Bearer Independent Call Control), for interconnection in IP mode with the PLMN networks corresponding to the Release 4; - SIP-I (SIP with encapsulated ISUP), for interconnection in IP mode with the PLMN networks corresponding to the Release 4. H.248 signaling is used by the MGCF entity to control the IMS-MGW interconnection gateway, whose deployment is mandatory for the interconnection in TDM mode (case of ISUP signaling) and optional for the interconnection in IP mode (the case of BICC or SIP-I signaling). The IMS-MGW performs the conversion of protocols relating to the media flows between the two end-points. It contains the treatments carried out on the media flows, such as transcoding (modification of the type of codec between the two end-points), echo cancellation and transmission of tones and announcements. 6.1.2. Protocol architecture 6.1.2.1. ISUP signaling The protocol ISUP defines the procedures used to establish, manage and release the circuits which transport the voice signals. Initial address message (IAM) is transmitted between digital switches to notify the request for establishment of a communication. It contains the telephone numbers of the caller and the callee; Address complete message (ACM) is returned by the destination switch to indicate that the ringing has been activated for the callee. Answer message (ANM) is transmitted by the destination switch to indicate that the callee has picked up the telephone. Relea