4 Overview

29.1653GPPInter-IMS Network to Network Interface (NNI)Release 18TS

Tools: ARFCN - Frequency Conversion for 5G NR/LTE/UMTS/GSM

Interconnection between two different IM CN subsystems shall be guaranteed in order to support end-to-end service interoperability. For this purpose, Inter-IMS Network to Network Interface (II-NNI) between two IM CN subsystem networks is adopted, according to the assumptions coming from 3GPP TS 23.002 [3] and 3GPP TS 23.228 [4].

NOTE: The end-to-end service interoperability within one IM CN subsystem over the Mi, Mm, Mw and I2 reference points not passing over the Mx reference point as defined in 3GPP TS 23.228 [4] is outside the scope of this document.

Aiming to support the delivery of IMS services between two separated IM CN subsystems, protocol interconnection has to occur:

– at a control plane level, in order that IMS procedures can be supported. In this case the adopted reference point is the Ici; and

– at a user plane level, where media streams are exchanged over the Izi reference point.

IP multimedia sessions are managed by SIP. The transport mechanism for both SIP session signalling and media transport is IPv4 (IETF RFC 791 [2]) or IPv6 (IETF RFC 2460 [7]). The 3GPP profile of SIP defining the usage of SIP within the IM CN subsystem is specified in 3GPP TS 24.229 [5]. Example call flows are provided in 3GPP TR 24.930 [6].

The general interconnection model is shown in figure 4.1.

Figure 4.1: Interconnection Model for IM CN subsystems

The II-NNI traversal scenarios in this document are covered in figure 4.2, figure 4.3 and figure 4.4.

NOTE 1: Any II-NNI can contain intermediate transit network(s). The intermediate transit network can either be an IMS transit network with or without a transit function as specified in 3GPP TS 24.229 [5] or a non-IMS transit network using standards outside the scope of 3GPP.

NOTE 2: IMS home network A and IMS home network B represent the IMS home network on originating side and terminating side respectively.

NOTE 3: IMS visited network X and IMS visited network Y represent the IMS visited network on originating side and terminating side respectively.

Figure 4.2: II-NNI traversal scenarios when P-CSCF is located in visited network and when home routeing is used

NOTE 2: IMS home network A and IMS home network B represent the IMS home network on originating side and terminating side respectively.

NOTE 3: IMS visited network X and IMS visited network Y represent the IMS visited network on originating side and terminating side respectively.

Figure 4.3: II-NNI traversal scenarios when P-CSCF is located in visited network and when the roaming architecture for voice over IMS with local breakout is used

NOTE 1: Originating IMS network O represents the IMS network to which the originating UE is attached to, and terminating IMS network T represents the IMS network which accomodates a PSAP.

NOTE 2: Originating IMS network O can be a visited IMS network or a home IMS network.

NOTE 3: The E-CSCF is located in the originating IMS network O.

NOTE 4: Any II-NNI between originating IMS network O and terminating IMS network T does not use any specific capabilities for roaming II-NNI, and is treated as non-roaming II-NNI.

Figure 4.4: IMS emergency session traversal scenario on non-roaming II-NNI

The possible functional entities involved in the signalling plane interconnection (IBCF, I-CSCF, P-CSCF, ATCF, S-CSCF, E-CSCF, BGCF, MSC Server enhanced for ICS, MSC server enhanced for SRVCC, MSC server enhanced for dual radio and TRF) and in the user plane interconnection (TrGW) are specified in 3GPP TS 24.229 [5], in 3GPP TS 24.292 [121], 3GPP TS 29.292 [130], 3GPP TS 29.162 [8] and in 3GPP TS 24.237 [131].

IP Version interworking is described within 3GPP TS 29.162 [8].

Examples of usage of the Inter-IMS Network to Network Interface (II-NNI) for roaming scenarios are described in 3GPP TR 29.949 [192].