2.3. IMS reference points

This section explains how the previously described network entities are connected to each other and what protocol is used; moreover, the IMS architecture is depicted (Figure 2.11). You will also find an overview of SIP-based reference points (i.e., where SIP is used and the main procedures involved). However, you will realize that the level of description of SIP-based reference points is not so deep as with Diameter-based reference points. The reason for this division is that several chapters in this book are dedicated to SIP and SDP procedures where such descriptions are given in detail.

Figure 2.11. IMS architecture.

For the sake of clarity, it is impossible to include everything in one figure; so, please note the following:

  • Figure 2.11 does not showcharging-related functions or reference points (see Section 3.11. for more details).

  • The figure does not show different types of ASs (see Section 2.2.3 for more details).

  • The figure does not show the user-plane connections between different IMS networks and the AS.

  • The figure does not show the SEG at the Mm, Mk, Mw reference points.

  • The dotted line between the entities indicates a direct link.

  • IMS Service Control (ISC), Cx, Dx, Mm, Mwterminate at both the S-CSCF and the I-CSCF.

2.3.1. Gm reference point

The Gm reference point connects the UE to the IMS. It is used to transport all SIP signalling messages between ...

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