IMS Application Level Gateway (IMS-ALG) – IMS Access Gateway (IMS-AGW); Iq Interface; Stage 3
The present document describes the protocol to be used on the IMS Application Level Gateway (ALG) – IMS Access Gateway (IMS-AGW) interface. The basis for this protocol is the H.248 protocol as specified in ITU-T. The IMS architecture is described in 3GPP TS 23.228 [2]. The underlying reference model and stage 2 information is described in Annex G of 3GPP TS 23.228 [2] and in 3GPP TS 23.334 [23].
This specification describes the application of H.248 on the Iq interface (see Figure 1). Required extensions use the H.248 standard extension mechanism. In addition certain aspects of the base protocol H.248 are not needed for this interface and thus excluded by this profile.
Figure 1: Reference model for IMS access
The reference model for the IMS-ALG and the IMS-AGW supporting the ATCF/ATGW function is shown in Figure 1a below.
Figure 1a: Reference model for IMS-ALG/IMS-AGW with ATCF/ATGW function
See 3GPP TS 23.237 [38] clause 5.2 for a comprehensive description of the reference model.
The reference model for the P-CSCF enhanced for WebRTC (eP-CSCF) and the IMS-AGW enhanced for WebRTC (eIMS-AGW) to support WebRTC client access to IMS is shown in Figure 1b as below, see 3GPP TS 23.228 [2] Annex U for a comprehensive description of the reference model.
Figure 1b: Reference Architecture for eP-CSCF/eIMS-AGW supporting WebRTC access to IMS
NOTE: The presence of dashed elements in the figure depends on the configuration.
PCC functional elements are present only for EPC access with QoS.
The corresponding PCC elements for fixed access are also optionally supported but not shown.
The NAT in figure 1b is meant for non-cellular access to IMS.